AQUO. Achieve QUieter Oceans by shipping noise footprint reduction. FP7 - Collaborative Project n WP 3: Measurements. Task T3.

Transcription

1 AQUO Achieve QUieter Oceans by shipping noise footprint reduction FP7 - Collaborative Project n WP 3: Measurements Task T3.1 European URN Standard Measurement Deliverable Reference Issue Dissemination status yes no D 3.1 PU File name D3.1 European URN Standard Measurement _Rev2.Doc Name Organisation Function Date Issued by Alfonso Moreno TSI Task leader 21/04/2014 Checked by Publio Beltrán TSI WP leader 21/04/2014 Approved by Christian Audoly DCNS Coordinator 23/062014

2 REVISION HISTORY ISSUE DATE NAME ORGANISATION MODIFICATIONS Rev Alfonso Moreno Raúl Salinas Publio Beltrán TSI Draft document for comments Rev Publio Beltrán TSI Verification Christian Audoly DCNS Minor modifications in Annex A 2

3 LIST OF CONTRIBUTORS CHAPTER SECTION NAME ORGANISATION Chapter 1 Chapter 2 Chapter Christian Audoly Raúl Salinas Publio Beltrán Raúl Salinas Alfonso Moreno Raúl Salinas Publio Beltrán Ángela López Alfonso Moreno Raúl Salinas Publio Beltrán Alfonso Moreno Raúl Salinas Alfonso Moreno Álvaro Pérez DCNS TSI TSI TSI TSI TSI TSI 3.2 Christian Audoly DCNS Raúl Salinas Publio Beltrán Alfonso Moreno Raúl Salinas Publio Beltrán David Cordero Óscar Zaba Alfonso Moreno Christian Audoly Celine Rousset TSI TSI DCNS Jan Hallender SSPA Chapter All Raúl Salinas Álvaro Perez David Cordero Alfonso Moreno Raúl Salinas Publio Beltrán Álvaro Pérez David Cordero Ángela López Alfonso Moreno Christian Audoly Jan Hallender Eric Baudin TSI TSI TSI TSI TSI TSI DCNS SSPA BV 3

4 CHAPTER SECTION NAME ORGANISATION Chapter 5 Annex A All All Raúl Salinas Álvaro Pérez Publio Beltrán Alfonso Moreno Christian Audoly Celine Rousset TSI DCNS 4

5 CONTENTS SUMMARY INTRODUCTION AIM AND SCOPE METHODOLOGY NEEDS DEFINITIONS REVIEW OF EXISTING STANDARDS AND PROCEDURES KEY PARAMETERS LIST OF EXISTING STANDARDS AND PROCEDURES CONSIDERED STANAG ANSI-ASA S /PART 1 AND ISO DOCUMENTS IN PROGRESS DNV SILENT CLASS NOTATION SYNTHESIS OF THE EXISTING REFERENCE DOCUMENTS INITIAL CONSIDERATIONS HOW THE KEY PARAMETERS ARE TACKLED IN THE STANDARDS THE EXISTING UNCERTAINTIES IN THE STANDARDS CONCLUSIONS AND RECOMMENDATIONS PARAMETER EVALUATION AND IDENTIFICATION OF DEFICIENCIES IN THE CURRENT MEASUREMENT PROCEDURES DEVICE

6 ENVIRONMENT VESSEL PARTICULARS POST-PROCESSING PROPOSAL FOR A NEW URN MEASUREMENT PROCEDURE INTRODUCTION SUMMARY OF THE REQUIREMENTS INSTRUMENTATION HYDROPHONES, SIGNAL CONDITIONING AND RECORDER DISTANCE MEASUREMENT SYSTEM TEST SITE TEST SITE REQUIREMENTS SEA SURFACE CONDITIONS HYDROPHONE DEPLOYMENT GRADE A GRADE A GRADE B GRADE B TRANSMISSION LOSS MODELLING ACTIVITIES SIMPLE LAW URN MEASUREMENT BACKGROUND NOISE

7 VALIDATION OF THE MEASUREMENT LOCATION CONFIGURATION OF THE DIFFERENT RUNS REPEATABILITY CHECKING UNCERTAINTY CHECKING VESSEL CONDITIONS TEST SEQUENCE POST-PROCESSING ACOUSTIC CENTRE DATA WINDOW BACKGROUND NOISE SPECIFIC POST-PROCESSING NARROW BAND ANALYSIS DIRECTIVITY UNCERTAINTY AND REPEATABILITY STUDY THEORETICAL STUDY EXPERIMENTAL STUDY CONCLUSIONS REFERENCES A. ANNEX A - EUROPEAN WATERS ENVIRONMENTAL DATA A.1 GENERAL DATA A.2 ACOUSTICAL DATA A.3 PROPAGATION LOSSES

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9 SUMMARY This study has been realized in the scope of AQUO, a collaborative research project supported by the 7 th Framework Programme through Grant Agreement N , whose final goal of AQUO project is to provide to policy makers practical guidelines to mitigate underwater noise footprint due to shipping, in order to prevent adverse consequences to marine life. The present document is the deliverable D3.1 project European URN Standard Measurement. The URN (Underwater Radiated Noise) is the physical quantity that allows quantifying the underwater noise emission of a ship or a maritime system. Thus the availability of reliable measurement procedures is needed in order to determine properly the underwater noise field created by a vessel or more generally ship traffic in a maritime area of interest. These are already some standards for the ship URN measurement, but they suffer some limitations: - the requirement on measurement parameters and of the quantification of uncertainties and repeatability is not sufficient, as only few justification documents are available, - they address mainly measurements in deep waters, whereas many European maritime areas are in shallow waters. The purpose of the present study is to fill this gap. The report is organized in different sections: The introduction, section 1, reminds the objectives of the study and expresses the needs. General and specific terminology useful for this study is defined in section 2. In section 3, a review of the existing standards and procedures for ship URN measurements is done and their limitations are identified. Then, the main part of this section consists in identifying the key parameters driving the uncertainty and repeatability of the measurement. An in-depth analysis is performed, giving examples and also by quantifying, as far as possible, the impact of each parameter. The parameters are of different types: frequency range of interest and disturbances such as cable vibrations, signal processing issues, determination of accurate distance between the ship to be measured and the sensors, influence of underwater acoustic propagation and reflection of sound waves on sea surface and sea bottom, etc. 9

10 In section 4, a proposal for a new procedure for ship URN measurements is given. Two grades are defined: Grade A for engineering purposes, with high accuracy and repeatability, and Grade B for comparison to noise limits, with medium accuracy and repeatability. Furthermore, these two grades are split into grades A1/B1 and A2/B2, for use in shallow waters and deep waters, respectively. The main evolutions by comparison to existing procedures are: the recommendation to deploy the sensors from sea bottom in case of shallow water measurement, the use of specific methods for determining the actual distance between ship and sensors and Grade A. One key issue is the determination of sound waves propagation loss between the noise source (i.e. the ship) and the sensors, which is done by using adequate predictive models for Grade A, instead of simple laws (such as spherical spreading). Section 5 consists in the validation of the proposed procedure. This has been done first by using numerical simulation, allowing verifying the impact of different parameters on the accuracy. Then a real experiment has been performed on a ship. Results show a very good repeatability, confirming the robustness of the proposed procedure. In conclusion, this study has allowed to study in detail the effect of different key parameters on the uncertainty and repeatability of URN measurement of ships, both in deep and shallow waters. One of the most important parameters is the accurate determination of sensors locations and estimate of sound propagation loss. A new procedure is defined, split into two grades and two variants for deep and shallow waters. The results of the study prove that the needs expressed for accuracy and repeatability can be achieved. It is thought that the work carried out here is a significant contribution to the improvement of ship URN measurement techniques, which can be used to build a new standard or to contribute to the work done in the scope of the international standardization organizations. 10

11 1. INTRODUCTION 1.1. Aim and scope This study has been realized in the scope of AQUO, a collaborative research project supported by the 7th Framework Programme through Grand Agreement N , whose final goal of AQUO project is to provide to policy makers practical guidelines to mitigate underwater noise footprint due to shipping, in order to prevent adverse consequences to marine life. The present document is the deliverable D3.1 project European URN Standard Measurement. Underwater radiated noise (URN) is the physical quantity that represents the underwater noise emitted by a ship or a naval system, due to acoustic radiation of the hull excited by internal machinery, to the propeller, or to other noise sources such as flow noise. For a long time, URN has been a matter of concern mainly for military vessels, in order to limit the risk of detection by adverse passive sonar systems. In the civilian domain, acoustic requirements on research vessels have been imposed for a few decades. Nowadays, the need for URN mitigation is foreseen also to any type of vessel in the context of environmental concerns regarding ship traffic, for the preservation of fish resources and biodiversity of marine fauna. Then, appropriate standards have to be defined for the measurement of ship URN levels in accordance with the needs. Different measurement procedures for Underwater Radiated Noise are already available but suffer from some limitations. The aim of the present study is the development of European standard measurement methods for ship URN ology The approach used to perform the study is as follows: Identification of the needs with regards to URN control, measurement and assessment of the environmental impact in terms of underwater noise emitted by ships. Analysis of the current standards and measurement procedures to identify the advantages and possible deficiencies. Exhaustive review of all the parameters affecting the measurement with special emphasis on the assessment uncertainty associated to them. Initial proposal for the procedure, according to the needs; Validation of proposed procedures through experiments at sea (not detailed in this deliverable); Development of a final proposal. Provide experimental and numerical support for the uncertainty and repeatability of the measurement procedure. 11

12 Finally, with the aim to describe properly the approach to cover the scope of the present document, the Figure 1.1 shows the flow chart of the different activities considered, by decomposing the activity into different subtasks. Figure Flow chart of the activities considered Needs The aim is the development of a European standard measurement method for ship URN source levels that can be used in both, shallow and deep water. The case of shallow waters is an important point, as in a large proportion of European maritime areas, no deep waters are available. This standard proposal, which is expected to fulfil the needs of industry and end-users, can be summarized as follow: Accurate source identification for engineering purposes. 12

13 Assessment of the underwater environmental impact of the vessel in terms of underwater noise. Besides, the possibility of estimating the underwater noise signature of the vessel without disturbing its route has been considered. Having in mind these needs the following grades have been defined: A-Applications. Engineering purposes: For accurate source identification and assessment of the URN signature of the vessel for further comparison with limits: o o Grade A1 (Measurement Uncertainty 4dB, Repeatability 1.5dB, Frequency range: 10Hz-50KHz): For sources identification and accurate quantification. The measurement procedure and configuration of the device are meant to allow narrow band analysis over the range 10Hz-50KHz. This grade is conceived for achieving high accuracy and repeatability in the identification of the sources below 20 Hz, aiming at building silent vessels. It also focuses on measurements in shallow water. In spite of the fact that important noise sources are sometimes below 10Hz, it is unfeasible to measure them due to technical reasons (as we will detail later). Grade A2 (Measurement Uncertainty 4dB, Repeatability 1.5dB, Frequency range: 20Hz-50KHz): It covers similar needs than the previous grade but it is conceived for measurements in deep water. Due to technical limitations in the measurement device used in this kind of measurements, there is an increase of the lower limit of the frequency range. This is why, this grade is recommendable only for vessels whose main harmonics are foreseen to be above 20Hz. B-Applications. Comparison with limits: Procurement of the URN signature of the vessel for traffic management: o o Grade B1 (Measurement Uncertainty 7dB, Repeatability 2dB, Frequency range 20Hz-50KHz): This grade is conceived to obtain an indicator (signature) of the underwater environmental impact of the vessel. This grade is conceived for measurements in shallow water. Grade B2 (Measurement Uncertainty 6dB, Repeatability 2dB, Frequency range 20Hz-50KHz): It tries to cover similar needs as the previous grade but it is conceived for measurements in deep water. C-Applications Ocean monitoring: It is conceived to obtain rough estimates of underwater noise pollution of the vessel without disturbing its route. Those estimates can be later used also for traffic management, deriving URN patterns, etc. This topic is outside the scope of the research carried out in this study. 13

14 2. DEFINITIONS Acoustic centre: Position on the ship where it is assumed that all the noise sources are co-located as a single point source Arithmetical average: Average expressed in db of different and comparable signals referred in db under the same reference value. Background noise: Noise from all sources (biotic and abiotic) other than ship being measured. Beam aspect: Direction to either side of the ship under test. Calibrated underwater source: Device transforming an electrical signal into an underwater sound whose frequency and level are known if the frequency and level of the electrical signal are known. Closest point of approach: The point with the minimum horizontal distance (during a test run) between the acoustic centre of ship under test and the hydrophones. Data window(s): Different sets of data for which an averaged spectrum will be computed. Data window length: Distance covered by the acoustic centre of the vessel during a data window. Dipole source: Type of model of the vessel as noise source that assumes that the source is made up of two points sources in opposite phase symmetrically positioned with respect to the sea surface. Directivity enveloping spectrum: The spectrum resulting from the envelope of the spectra corrected to 1m of a given side taken at different angles between the acoustic centre of the vessel and the sea surface above the hydrophones. Dynamic range: It is the ratio between the maximum and the minimum signal the system can distinguish. End data location (FINEX): Position of the acoustic centre of vessel under test where data recording is finished. Experimental field calibration: Procedure applied in situ to evaluate the actual transmission loss of the test site. Far field: Field where the ship under test can be considered to behave as a point source collocated for all frequencies with a low error. Floating configuration: Deployment configuration where the hydrophones are suspended from a floating buoy and not anchored on the bottom. 14

15 Frequency range: It is determined by the minimum and maximum frequency where the measurement system shall record the noise level with an uncertainty lower than required. Frequency resolution: Minimum required change of frequency between two spectral components so they can be distinguished in the narrow band spectrum of the time signal recorded by the measurement device. Frequency response: Sensitivity of the measurement chain along the frequency. Hydrophone bandwidth: It is given by the frequency range where the frequency response of the hydrophone does not vary more than a certain quantity. Hydrophone directivity: It will be determined by the hydrophone sensitivity for a given frequency and angle. Hydrophone sensitivity: It is the magnitude of the noise signal required to produce a specified output signal. It is expressed in db (ref 1V/µPa). Insert voltage calibration: Known, calibrated and traceable stimulus in the form of an electrical input injected at the input (or other stage) of a measurement system in order to ascertain that the system is, in fact, responding properly to known stimulus. Measurement Uncertainty (MU): Expected difference between the measured resulting signature source level and the true signature source level stated in decibels for a given measurement system and procedure, in each one-third octave band. Measurement Repeatability (MR): Expected difference between the resulting signature source levels, stated in decibels, in one-third octave bands, taken from a same vessel at different times, under the same vessel and environmental conditions in terms of what can be quantified, following the same measurement procedure and using the same measurement equipment. Monopole source: Type of model of the vessel as noise source that assumes that the source is made up of one point source collocated for all frequencies. Narrow band component: It is a clear frequency line in the spectrum of the signal measured by the noise recording system, commonly related to the machinery or the propeller of the vessel. Nominal distance to CPA (dn cpa ): It is the distance between the hydrophones and the CPA of the closest run to the hydrophones. Omni-directional hydrophone: Underwater sound pressure transducer that responds equally to sound from all directions. Power average: Average expressed in db of different and comparable signals referred in a linear scale. Range average transmission loss: It is the transmission loss along the horizontal distance for a given frequency and receiver depth resulting from the logarithmical 15

16 interpolation of the numerical values of the transmission loss obtained from the corresponding numerical model at the receiver depth and different horizontal distances. Range-source depth average transmission loss: It is the arithmetic average of the range average transmission loss of different source depths for a given frequency and receiver position. Receiver depth-average sound pressure level: It is the power average of the underwater sound pressure level of the hydrophones placed at different depths. Resulting signature source level: Final underwater sound pressure level after removing the effects of sea bottom and sea surface and referred to a distance of 1m adjusted using the proper transmission loss. It is also referred to as a source level or signature. It is stated in decibels (db) whose reference is 1µPa/ Hz. Self-generated noise: It refers to the artificial noise recorded by the hydrophones due to wave induced motion, electronic noise and any spurious signal caused by the intrinsically physical properties of the measurement device. Ship-draught after: Distance between the vessel s waterline and the lowest point of the vessel measured in the perpendicular of the stern. Ship-draught forward: Distance between the vessel s waterline and the lowest point of the vessel measured in the perpendicular of the bow. Ship-length: It is the maximum length of the vessel's hull measured parallel to the waterline. Signal plus noise to noise ratio (SNR): It is the difference stated in db between the signal measured during a run (signal plus noise) and the background noise measured during the trial. Sound celerity profile: Sound speed along the water depth for a certain place and a given time. Source depth average transmission loss: It is the arithmetic average of the transmission loss of different source depth for a given frequency and horizontal distance. Source-receiver depth average sound pressure level: It is the arithmetic average of the receiver depth-average sound pressure level measured for different source depths. Source-receiver depth averaged transmission loss: The difference, stated in db for a given frequency and horizontal distance, between the source level generated by an emitter at different depths and the source-receiver depth average sound pressure level measured by the column of the hydrophones. Start data location (COMEX): Position of the acoustic centre of vessel under test where data recording is started. 16

17 Test site: Location where the underwater noise measurements are performed. Tilt angle: Angle between the vertical axis and the line created by the cable supporting the hydrophones. Transmission loss: Difference, expressed in db, between the source level at 1m produced by an emitter placed at a given depth and the underwater sound pressure level measured by a receiver at a given horizontal distance from the emitter and vertical distance from the sea surface. Transmitting voltage response (TVR): The output sound intensity level generated at 1m range by the transducer per 1V of input Voltage. Underwater noise true signature source level: Underwater sound level stated in decibels represented in one-third octave bands of a point noise monopole source whose free far field noise is equivalent to the free far field noise of the vessel, i.e. no interaction with the sea surface or sea bottom are considered. 17

18 3. REVIEW OF EXISTING STANDARDS AND PROCEDURES 3.1. Key parameters Once the requested applications of the standard have been defined, it is necessary to identify the variables that can affect the measurement procedures and devices. In order to be able to define proper methodologies and equipment for the applications, the parameters to be taken into account have been classified as follow: Device (D) Environment (E) Vessel constraints (V) Post-Processing (PP) 1 Frequency Range of the device Shallow / Deep Water Vessel Speed Bandwidth 2 Frequency Resolution of the device Mirror effect Vessel Length AC (Acoustic center) Definition/ location 3 Relative distance measurement equipment Sea State Propeller & Machinery sources type and working conditions Data adquisition Window 4 Tilt. Propagation Model Vessel load conditions Background Noise Correction 5 Hydrophone movement Celerity Profile (Temperature & Salinity) Family vessel Directivity 6 Distance to the CPA Currents Availability Number of runs 7 Number of hydrophones Seabed type 8 Cost Background noise 9 Handling 10 Independency (Auxiliary vessel, Previous underwater hull works, Diver.) 11 Directivity 12 Accuracy of the instrumentation. 13 Dynamic range 14 Electrical noise Table Parameters that can influence the measurement procedures and devices.- The following step will consist on defining requirements for each parameter in order to determine procedures and devices for A-Applications and B-Application purposes. 18

19 3.2. List of existing standards and procedures considered NATO Standardization Agreement N 1136 (STANAG), S tandards for use when measuring and reporting radiated noise characteristics of surface ships, submarines, helicopters, etc. in relation to sonar detection and torpedo risk, May 29, 1995 (reference [1]). AMERICAN NATIONAL STANDARD - Quantities and Procedures for Description and Measurement of Underwater Sound from Ships Part 1: General Requirements. ANSI-ASA S /Part1 (reference [2]). ISO/DPAS "Acoustics Quantities and procedures for description and measurement of underwater sound from ships Part 1: General requirements". ISO/DIS Ships and marine technology Marine environment protection Measurement and reporting of underwater sound radiated from merchant ships. DNV. Part 6, Chapter 24 - Silent Class Notation. Rules for Classification of Ships. s.l. : DNV, January 2010 (reference [9]) STANAG 1136 The scope is to standardize measurement and report of the radiated noise characteristics of surface ships, submarines, helicopters, etc, in relation to sonar detection and torpedo acquisition risk. The initial goal was to fulfil needs for NATO Navies (North Atlantic Treaty Organization), to characterize the risk for military vessels of being detected by adverse systems using underwater acoustics means. However, the document is NATO Unclassified, so that it is made available and can be used for other purposes. The contents of this document are mainly: Terminology units, such as frequencies and noise levels, reference distance, etc; The definition of the format for reporting the measurement (this part is detailed in an appendix); Guidelines to conduct the trial and to process the data. The standard deals both with narrowband and one-third octave band analysis. For the measurement, one or several hydrophones are considered, and it is recommended to perform the trial in deep waters. For some applications, measurement can be done in relatively shallow waters, and a keel aspect hydrophone is considered. Note also that this STANAG applies both to surface ships and submarines (or underwater vehicles). For the analysis in section 1.3, we shall consider only the case of surface ships. 19

20 ANSI-ASA S /Part 1 and ISO documents in progress The scope is the description of general measurement systems, procedures, and methodologies used for the measurement of underwater sound pressure levels from ships at a prescribed operating condition. It contains methodology for the reporting of one-third octave band sound pressure levels. The context for the production of this standard is the need for reduction of underwater noise impact on marine life due to man activity, by an appropriate characterization of ship underwater radiated noise at sea. This it is in close relationship with the aim of AQUO project. The contents of this document are mainly: Terms and definitions; Instrumentation; Measurement requirements and procedure; Post-processing; Measurement uncertainty; Reporting example (table of contents for an URN report). An important issue in this standard is the definition of three grades (A, B, and C) corresponding to different levels of accuracy and/or completeness of information. The main purpose is measurement of one-third octave noise levels. However, narrowband analysis is considered in grades A and B for deeper analysis. Grade C method, which is intended for survey, uses only one hydrophone (Figure 3.1). Grades A (precision method) and B (engineering method) use three hydrophones, as shown on Figure 3.2. The minimum measurement distance is 100 m, or the ship length if it is greater than 100 m. Figure Hydrophone location for Grade C method 20

21 Figure Hydrophone location for Grade A and B methods The hydrophones are fixed on a vertical line deployed under a surface buoy, or using a bottom anchor (Figure 3.3). Figure Examples of hydrophone deployment arrangements Using ANSI-ASA S as a start, two ISO Technical Committees have prepared draft standards for measurement of ship URN in deep waters: ISO Committee 8-2 (Marine technology and protection of environment): ISO ISO Committee 43-3 (Underwater Acoustics): ISO

22 The situation is not very clear, as two committees have been working on the same topic. The latest orientation followed is that two ISO documents will be derived from ANSI-ASA S12.64: ISO close to Grade C, and ISO covering the merging of Grades A and B. These two documents are starting the approval process in In all cases, these documents apply to deep waters measurements only (bottom reflection and non standard underwater propagation are not considered), the water depth being typically greater than ship length and 100 metres. Work is in progress or planned in ISO Technical Committees to address the issue of ship URN measurements shallow waters DNV Silent Class Notation. The main goal of this document, issued by a classification society, is to define URN limits for some classes of ships. Associated to these requirements, a measurement procedure is defined. Section 1 includes an introduction, general definitions and the contents of the documents to be produced for a ship URN measurement. For the Silent Class Notation, the following classes of ships are considered (section 2 of the document): seismic survey vessels; fishery ships; research vessels; environmental (any vessel demonstrating a controlled environmental noise emission). The limit noise curves are expressed in one-third octave band levels. Narrowband measurements are optional. Section 3 provides general requirements and guidance for the measurement. Appendix A describes the procedure. As shown on Figure 3.4, the method uses a single hydrophone placed on sea bed, with a typical CPA distance comprised between 150 m and 250 m. Then, the measurement is carried out in relatively shallow waters. However, the document requires a minimum 30 m water depth under the keel. The measurement hydrophone must be at a maximum distance of 0.2 above bottom. A -5 db correction factor is applied to account for acoustic wave reflection on seabed. 22

23 Figure DNV document measuring situation 3.3. Synthesis of the existing reference documents Initial considerations. The Standards and regulations under study, listed in paragraph 3.2, are the most commonly used in Europe by the scientific community to carry out underwater noise measurements. The analysis to identify and evaluate the uncertainties revealed that the strict revision, standard by standard, of the detected deficiencies would be too tedious and useless because most of the listed standards are based on similar assumptions, thus with several common points in their contents. The main problem is the reliability of the assumptions, which can lead to underestimated or even neglected uncertainties. In general, no document is available regarding these justifications and the estimation of uncertainties. The analysis of the standards has detected some similarities among them, which has permitted to concentrate the analysis on three standards, STANAG [1], ANSI/ASA [2] and the Silent Class notation of the DNV [9], making more practical and friendly to the reader the presentation of the results. The most relevant difference among them is that the STANAG and ANSI/ASA standard have been developed to perform deep water measurements while DNV class notation is defined for shallow water. This fact is important to weight properly the relevance of the reflections and mirror effect as well as the configuration of the recommended devices, mainly floating configurations for deep water and bottom anchored for shallow water. Finally, it is important to pay attention to the global uncertainty and repeatability of each standard. These two parameters show the quality of the different standards under study. Moreover, they will be used to help us to understand the applicability and usefulness of each methodology. 23

24 How the Key Parameters are tackled in the standards. Once the most significant Key parameters have been listed in the previous paragraphs, it is now important to spend time presenting the image of how the standards under study tackle all the parameters affecting the underwater noise measurement. The following tables summarize, by representative data, the definition of each parameter in the standards under study. For those parameters without clear definition in the standards, a question mark has been included to identify the lack. This exercise permits, at first glance, the identification from a qualitative point of view of the number of issues undefined that can also generate uncertainties. Moreover, the fact that some explicit information exists in the standard related to some parameters does not guarantee that their corresponding uncertainty was completely annulled. Device group Focusing on the definition of an easy procedure to review all this information, a subdivision by standards is proposed. In general terms all the matters related to cost, handling and independency to external support means in the device group have not been considered. It is not the matter of this paragraph to assess the applicability and cost-effective of the different devices and procedures to the industry. a. STANAG standard. Regarding the device, this standard does not deal with all the key parameters identified within this topic. Beyond the general comments included as introduction for the device paragraph, some lack remains present in terms of hydrophone movement and tilt suffered by the floating configuration. Besides, the aspects regarding the instrumentation and their capabilities are not specified which implies lack of standardisation of the uncertainty because of this matter. b. ANSI/ASA standard. As happens in the STANAG standard some issues remain undefined in terms of accuracy required for the measurement of the relative distance between the targeted vessel and the hydrophones as well as the uncertainty induced by movements in the hanging line. The complete definition of the instrumentation is also partially defined, anyway a general indication about uncertainty levels are mentioned to ensure the compliance with the general uncertainty and repeatability of the standard definition. c. DNV Class Notation. Due to the fact that this standard has been defined to measure in shallow water, the proposed configuration of the device is bottom anchored which does not suffer from the issues related to tilt and hydrophone movements. On the other hand the instrumentation remains no completely defined as the accuracy of the instrumentation as well as the selfnoise have not been mentioned, leaving these aspects open. 24

25 Device Nº Parameters STANAG ANSI/ASA DNV 1 Frequency Range kHz G-A: 10 to 50 khz G-B: 20 to 25 khz G-C: 50 to 10 khz G-A: Less than 1 Hz 2 Frequency Resolution 1Hz & 3% G-B: Not defined G-C:No Narrow band Analysis 3 2% distance to CPA Relative measurement +/- 1dB distances 2% distance to CPA 5% distance to CPA +/- 5m 4 Tilt (for floating devices)? Max 5º Bottom 5 Hydrophone movement Track + Beam? Bottom By vessel Families No Narrow band Analysis 6 Distance to the CPA >100 m Greater 100m or 1xLoa +/- 10% 150m to 250m 7 Number of hydrophones 2 (100 m/ 30 m keel) G-A: 3 G-B: 3 1 G-C: 1 8 Cost??? 9 Handling??? 10 Independency??? 11 Directivity 1 to 3 db?? 12 Accuracy of the instrumentation? Aprox 1.3 db? 13 Dynamic range?? more than 90 db 14 Electrical noise Mentioned?? Table 3.2.-Device Group.- Environment group. At this stage of the document it is important to note that one of the most important and uncontrolled parameter is the propagation model. As seen previously, sea surface and sea bottom will have a considerable impact on the propagation of the sound, especially for shallow waters and low-medium frequencies. Moreover the medium is dissipative, especially for high frequencies. All of this makes the transmission loss vary with frequency, receiver and source depth and sea depth. However, the three standards simplify this problem defining a simple equation only function of distance and constant for the whole frequency range to make these corrections. The dependency on how each frequency propagates is directly linked to the depth water in the measurement location, which is the reason why the propagation law assumed by DNV is more similar to the cylindrical propagation. Besides, other parameters affecting the transmission loss as sound celerity profile have not been considered. Finally, the minimum distance between vessel and hydrophones from which the vessel can be considered as a point source, remains not well empirically supported. a. STANAG standard. The following assertion included in the standard definition, The depth of water must be sufficient to ensure that the level of bottom reflection is insignificant, does not remove the uncertainty related to this aspect because it does not specify from what depth this assertion holds. There is also a lack of how to proceed in terms of sea state, currents and sea bed. 25

26 b. ANSI/ASA standard. The most important uncontrolled aspect in this case is the currents that can affect the noise measurements in the hydrophones. The effect of this parameter can be limited by the use of a specific control procedure. General assertions, included in the standard have to be also considered: The measurement methods mitigate the variability caused by Lloyd s Mirror surface image coherence effects, but do not exclude a possible influence of bottom reflections. c. DNV Class Notation. Due to the proposed bottom anchored device in this case, the currents are not considered as problematic issue oppositely to the correction of 5 db considered to account for bottom reflection in all the cases. Environment Nº Parameters STANAG ANSI/ASA DNV 1 Shallow / Deep Water 0.6* (gh) G-A: Greater 300 m or 3xLoa G-B: Greater 150 m or 1,5xLoa G-C: Greater 75 m or 1xLoa 2 Mirror effect Mentioned Mentioned but not separately studied? 3 Sea State? 100 m and above: Wind Speed less than 20 Knots. Small vessels not specified 4 Propagation Model 20Log 20Log 18 Log More than 30m or 0.64V^2 for high speed vessels Max Beaufort 4/Sea State 3 5 Celerity Profile??? 6 Currents??? 7 Seabed type? Nothing-Deep water Not perfectly flat (correction -5 db) 8 Background noise Defined Defined Defined Vessel group. Table 3.3.-Environment Group.- In general terms for the three standards, this group can be considered as the worst defined. The matter of fact is that the achievement of the fix image of the vessel under study is absolutely necessary to be able to ensure a minimum level of test repeatability. Moreover, some standards require to perform several runs then to average them, so it is really important to define what vessel conditions must be under control. In particular, the proper definition of all the possible underwater sources of the vessel, including the machinery on board, as well as the accurate load conditions should be requested to define how the vessel is working as speaker. This is one of the major lacks regarding the requirement definition in the standards. Despite of this fact, no supplementary uncertainties are included because of this. The adjustment of the requirements depending on the final purpose of the measurement, the vessel typology, vessel availability and other parameters is not considered in the standards excepts for ANSI/ASA, for which is partially achieved through grades. 26

27 Vessel constraints Nº Parameters STANAG ANSI/ASA DNV 1 Vessel Speed Through Water? (through water or ground) Through water & Ground 2 Vessel Length Mentioned Without restriction? 3 Propeller & Machinery Mentioned Mentioned Mentioned 4 Vessel load To be conditions described? Normal Load Range 5 Family vessel Military Underway surface vessels Acoustic/Seismic/Fishery/Research/Environment 6 Availability? Three grades of accuracy (A,B & C)? Table 3.4.-Vessel group.- Post Processing group. Once all the data have been collected, different post processing strategies can deliver different results for the same data. In this sense, it is important not only to define univocally the way to proceed but also it is crucial to review if all the particularities of the vessel have been really taken into account. For example, depending on the directivity of the vessel the highest radiation may occur at different angle from around 90º beam angle, so window symmetric around the closest point of approach does not necessarily represent the worst condition. Such effect has not been properly addressed by the current standards. Vessel constraints Nº Parameters STANAG ANSI/ASA DNV 1 Bandwidth G-A: 1/3 o.b & Narrow Narrow & G-B: 1/3 o.b & Narrow 1/3 o.b G-C: 1/3 o.b & Opt.Narrow 1/3 o.b (Narrow Optional) 2 Determined During Test AC (Acoustic centre) Keel/ Hull Half way E.R and Propeller Definition/ location Half way E.R and Propeller 0,7xR propeller 3 Data acquisition window? +/- 30º Half or vessel length depending on speed 4 Background Noise Correction? Defined Defined 5 Directivity Defined?? G-A: Number of runs 2/speed G-B: 3 2/speed in opposite directions G-C: 1 Table 3.5.-Post Processing Group The existing uncertainties in the standards. The first issue that has to be clarified is the concept of global uncertainty as well as repeatability. The only standard that tackles these essential concepts is the ANSI/ASA. Unfortunately, the poor treatment of these global parameters in the standards produces a high unawareness about the quality of the measurement itself as well as their stability when a new set of measurements are performed. This general approach, always complicated because many uncertainties exist, is unavoidable and much more important than particular uncertainties corresponding with specific parameters. 27

28 As it has been mentioned at the beginning of this paragraph, the aim is not to review and assess the uncertainties of the current standards through a quantitative point of view but following a qualitative approach to highlight the current scenario in terms of quality of the results of the underwater noise measurements. Moreover, this analysis will permit to continue with the ongoing work aimed at defining a new European Standard for the Underwater Measurement Procedure. With the aim to deal with the analysis in a proper way, some Key Groups have been defined regrouping Key Parameters according to their common effects in the same area of interest. Some of the parameters included in the four groups introduce uncertainties to the procedure itself while others are only useful to define and assess the measurement conditions, validity of the measurement, measurements methodology definition and cost/applicability to the current shipbuilding industry. Anyway the uncompleted definition of some criteria can add a supplementary uncertainty not foreseen. The following paragraphs try to present in an easy way the final uncertainties affectation present in the different parameters of the current standards under study. Uncertainties due to undefined criteria. Those parameters with red crosses are not sufficiently well defined and produce insufficient uncertainty assessment. The most relevant parameters which are not well determined are those that can invalidate the measurement. Uncertainties of the defined criteria. The following table tries to summarise the affectation produced by those parameters that add uncertainty by themselves due to an inexistent or unsuitable definition in the standards. As it can be seen in Table 3.7, many issues are adding uncertainties to the full-scale measurement tests. Therefore it is not so realistic to deliver accurate results without dealing with complementary analysis of the uncertainty and repeatability obtained during the measurement campaign. Following the expertise of the authors involved in the creation of this document, the most important challenge that has to be accomplished is the better knowledge about the transmission loss. As an example of that the Figure 3.5 shows the source level spectra obtained for the same vessel and same conditions corrected from three different distances. As can be seen the difference between 50Hz and 200Hz are up to 10dB, which is a clear proof of the level of uncertainty introduced by the current measurement procedures. This is why, as it is normally done in the noise and vibration engineering field, we have to attack firstly the major source of uncertainty. 28

29 Description Key Group Key parameters STANAG ASA DNV Basic requirements Frequency range of the device Frequency resolution of the device Frequency range Frequency resolution of the device Distance to the CPA Distance to the CPA Measurement conditions Sea depth Sea depth Sea state Sea state Background noise Parameters that can invalidate the measurement Signal to noise ratio Hydrophone movement Electronic noise Parameters that improve the repeatability Number of runs Number of measurements Number of hydrophones Table 3.6.-Uncertainties due to undefined criteria.- URN source level taken at different ranges Presure db (ref 1 1 m - 1/3 Octave Band (1 Hz) Hydrophone A 30m Hydrophone B 30m 130 Hydrophone A 72m Hydrophone B 72m 120 Hydrophone A 82m Hydrophone B 82m Frequency HZ Figure URN source level obtained at different ranges.-.- In-depth studies regarding the uncertainties definitions for each parameter have been developed in previous paragraphs. Specific numerical and experimental tests have been defined to get a final approach to the whole uncertainty and repeatability of the new developed URN procedure. In general terms, several laboratory and full-scale measurement have been scheduled to solve, group by group, the deficiencies detected in the current measurement standards. 29

30 Description Key Group Key parameters STANAG ASA DNV Distance Hydrophone line tilt measurement accuracy Relative measurement distances Hydrophone Directivity Accuracy of the instrumentation Noise recording accuracy Dynamic range Electronic noise Hydrophone movement Parameters introducing uncertainties Transmission loss Currents Mirror effect Sound propagation Sea bed reflections Celerity profile Vessel length Vessel speed Vessel Propeller and machinery conditions Load conditions Currents Acoustic centre Post-processing Data Window definition Directivity of the vessel Bandwidth Table 3.7.-Parameters inducing Uncertainties Conclusions and recommendations The uncertainty and repeatability of the measurement procedures as a whole have been identified as crucial to know the quality of the data obtained. Otherwise, we will not be able to know the accuracy of the results, which is fundamental information in all experimental activities as well as the stability of the data obtained along the time. 30

31 The only current standard that deals with this request is the ANSI/ASA [2] standard. Anyway, according to the qualitative results shown in the previous paragraphs of this document, it could be interesting to review in detail with ANSI/ASA team the proposed uncertainties and repeatability values that seems to be a bit optimistic in some cases. In this way, two assertions included in the standard ANSI/ASA [2] - 7 have to be recovered: The estimates (the overall measurement uncertainty & repeatability for the measurements) given above are provided as representative values for guidance and should not be considered to be exact. But at the time: With careful implementation of the methodology described in this standard, the overall uncertainties described above are achievable. ANSI/ASA [2] is in our opinion the most relevant standard in use to carry out properly underwater measurements and it have to be considered as the best starting point for future improved standards. One first conclusion is that some specific on-site tests have to be included in the procedure to finalize the measurements with enough information to be able to guarantee the uncertainty and repeatability levels certifying the quality of the final signature delivered Parameter evaluation and identification of deficiencies in the current measurement procedures Device 1 Frequency range and disturbances: The frequency range required will depend on the purpose of the measurement and which frequency components need to be captured. So, the device and procedure must allow to measure within this frequency range with uncertainty and repeatability according to what is specified. This requirement will strongly affect the configuration of the device and therefore, the problems and phenomena causing these limitations must be understood, the root causes identified, and it must be properly quantified. Globally, the frequency range will be limited by the signal plus noise to noise ratio achieved and the frequency response of the measurement device. The first one will depend on the arrangement of the system, the sea state and the source pattern of the vessel, whereas the second one will depend exclusively on the technical characteristics of the measurement device. In the low frequency one of the main problems arising is the parasitic signals due to: The movement of the hydrophones. The cable vibrations. The mechanical chafing between components. 31

32 The change of depth (consequently changes on hydrostatic pressure) caused by the hydrophone movement and the waves. These phenomena produce in the hydrophones what is known as self noise. They do not generate uncertainties on its own but mainly generates background noise in the measurement that may mask the actual sound received from the vessel and thereby limiting the frequency range for a given distance to her. The Figure 3.6 shows the existence of these phenomena in a real measurement performed with a floating configuration. In this figure the slight difference between background noise and the measurement of the two hydrophones for a certain run highlights the problems in the low frequency. In fact, for this particular measurement the results should be discarded below 50Hz. Hydrophone 20m (white) Hydrophone 40m (red) Figure Comparison between background noise and real measurement.- Figure Comparison of background noise level of different configurations under sea state 2.- Moreover, in the Figure 3.7 shows the importance of the arrangement of the measurement device. This graph shows the background noise for a given sea state of a bottom supported configuration and of a floating configuration. Below there is a description of the main causes generating excessive background noise in the low frequency. 32

33 Movement of the hydrophones: The movement of the hydrophone will be studied in the item 5, but as a summary, the major source for this phenomenon is the wave motion of the sea surface and the currents existing into the sea. Decoupling the hydrophones from the sea surface movement is a way of reducing the effect of sea state in floating configurations, whereas performing the measurement in the absence of strong currents can prevent from excessive self-generating noise in the hydrophone affected. Cable vibrations: The cable vibrations are another important source of self noise in the low frequency range for hanging measurement systems (floating configuration). These vibrations are induced by vortices appearing behind the cables by the effect of the water flow. There are two main types of motions to be considered: gross oscillatory motion at frequencies lower than 5Hz controlled by the effect of the gravitational forces and hydrodynamic forces around the buoy, and higher frequency vibratory motion of the cable controlled by the cable tension and the hydrodynamic forces affecting the cable. In the reference [12] a comparison between the noise records of floating hydrophones and a bottom supported hydrophone without any cable (see Figure 3.8 left) are presented (Figure 3.8 right). The noise spectrum levels of the three hydrophones are almost the same in the frequency range 64Hz to 1000Hz. However in the 10Hz to 63Hz frequency band the noise spectrum level measured by the bottom supported hydrophone were 3dB to 25dB lower than the levels measured by the floating ones. Figure 3.8.-Averaged underwater noise spectrum level of the three hydrophones.- Moreover, this difference is remarkable in the range between 10Hz and 25Hz. Figure 3.9 shows the recorded amplitude for the 20Hz and 100Hz components at different current speeds to compute the current speed dependency at the mentioned noise frequency range. This results in a neglected correlation for the non-cabled hydrophone put onto the seabed, and a large correlation for the cabled hydrophones. Besides a larger correlation for 20Hz noise component appears showing that the effect is more prominent for lower frequencies. The fact that there is no dependency between noise spectrum level and current speed for the bottom supported hydrophone may be a proof that part of the increase in the noise level for low frequencies is due either to vortex-induced vibration on the cable or to flow induced noise. 33

34 Figure Underwater noise level at selected frequency versus current velocity.- Solutions to this problem already exist. Some of them are based on the disruption of the characteristic vortex flow field. These solutions are attached along the entire length of the cable and tend to break up the coherence of the vortices by causing a variable separation point. The vibrations are also reduced if the boundary layer is induced to turbulent since the pressure gradient is then reduced. Other solutions consist on standing-off the hydrophones from the supporting ropes by flexible materials to reduce the influence of strumming. Mechanical chafing between components: It is important to take care of the coupling of the different elements (hydrophones, cable, ) because mechanical contact induces vibration which generates parasitic signals and sound. In order to avoid undesirable effects, flexible plastic supports are used for the hydrophones to minimise mechanical contact as well as no metal parts in the deployment to avoid metal-on-metal contact. With sufficient care this phenomenon will not be an important source of self noise so it does not need to be estimated. Water surface motion: The change of depth of the hydrophones due to water surface motion caused by waves generates hydrostatic pressure fluctuation that is picked up by the hydrophones. The Figure 3.10 (reference [13]) shows the distance between two hydrophones with different nominal depths to the sea surface. As can be seen, mean variation of up to 1 metre was recorded for this case, which means a difference of 0.1 bar on the hydrostatic pressure, generating a sound level of about 200dB in the hydrophones. On the other side, the frequency components of this phenomenon are really low. As can be seen in the Figure 3.11 (reference [14]) the main components are below 1Hz. Anyway this phenomenon could limit the frequency range and force the use of high-pass filters to eliminate this signal that may overload the hydrophone invalidating the measurement. 34

35 Figure Distance of the hydrophones to the sea surface along time.- Figure Energy spectrum of the sea state.- Cut-off frequencies: One effect in shallow water channels is that they do not allow the propagation of low frequency signals. Because of the depth, the low frequency noise components are trapped between the sea surface and sea bottom causing that sound waves do not propagate below a cut-off frequency so they cannot be present in the noise field beyond ranges equivalent to a few water depths. Figure 3.12 (reference [16]) shows the cutoff frequency as a function of depth for a shallow water channel whose sea bottom has certain features (one infinitively rigid, the other soft enough). One obvious effect of this phenomenon is that it limits the minimum sea depth required and the maximum distance of the measurement device to the target vessel depending on the lowest frequency to be caught. In particular, we can see that it is impossible to measure signals below 10Hz in shallow waters. Considering a homogeneous water column of depth and sound speed overlying an homogeneous bottom of sound speed the cut-off frequency is given by: Factors limiting the highest frequency: As said before the highest frequency will be determined by: 35

36 The frequency sample of the measurement system. The hydrophone characteristics. Figure The lower cut-off frequency as a function of depth for a certain shallow water channel.- On the one side the frequency sample must be at least more than twice the highest limit of the frequency range required (Nyquist limit), and on the other side the hydrophones should be suitable and must present a flat frequency response, according to the uncertainty requirements, within this frequency range. After what have been seen in this point, the following actions shall be taken in the definition of the measurement procedure: In order to be able to measure in the low frequency range the use of bottom supported configurations for the measurement device should be required, as long as it is technically feasible. This is true for measurement in shallow waters. In deep waters, deploying a measurement device with this configuration will be complicated and extremely difficult, so the procedure should contemplate the use of floating configurations with a system to decouple the sensors from the sea movement. Besides, after what has been seen with regard to the cut-off frequency phenomenon, at least 60m water depth should be required. Finally, the technical characteristics of the measurement equipment as well as the frequency sample acquisition should be defined in accordance with the frequency range required. 2 Frequency resolution. Depending on the aim of the measurement certain frequency accuracy will be required. For instance, if the main aim of the measurement is the source identification, high frequency resolution will be required for narrow band spectra analysis whereas if the aim is to obtain the sound levels for traffic management an analysis in one-third octave bands will be enough. This characteristic does not introduce any uncertainty but it is a specific requirement of the measurement. In order to achieve certain frequency accuracy the duration of the sample and the following post-processing (window selection) must be suitable to ensure that components 36

37 of the spectrum whose difference in frequency is larger than the frequency resolution required can be distinguished. In order to have a frequency accuracy of f a the duration of the sample must be larger than: Two effects should be taken into account: Doppler effect: It affects the frequency accuracy, especially for high frequency components and/or high speed vessels. Sometimes this effect can prevent from obtaining some spectral lines. Wrong distance correction due to the fact that the distance over the time sample used to obtain the spectra is not constant, thus, uncertainties in the source energy level appears (they are presumed to be low, however in the current standards the distance used along the whole data window is the same so the error due to this phenomenon is larger) Doppler effect: The Doppler Effect is the change in frequency of a wave for an observer with relative movement to its source. Considering that the relative speed of the source in the medium is constant and equal to the frequency of the signal that the hydrophones will record is going to be related to the actual frequency of the signal by: Where is the sound speed in the sea water (approximately 1500m/s), the actual frequency of the sound and the frequency measured by the hydrophones. In the Table 3.8 the frequency range observed by a measurement device is calculated according to a given vessel speed and data window angle, for different frequencies. Data Window Angle = +/-30º Vessel speed = 11knots Vessel speed= 22knots Actual frequency Observed frequency Observed frequency Actual frequency range range 25 Hz 24.95Hz-25.05Hz 25Hz 24.9Hz-25.1Hz 50 Hz 49.9Hz-50.1Hz 50 Hz 49.8Hz-50.2Hz 100 Hz 99.8Hz-100.2Hz 100 Hz 99.6Hz-100.4Hz 500 Hz 499Hz-501Hz 500 Hz 498.1Hz-501.9Hz Table Maximum source speed required to achieve a given frequency accuracy.- As can be seen in this table the effect of vessel speed begins to be important for frequencies higher than 100 Hz. In order to understand better this phenomenon the actual situation of the vessel with respect to the hydrophones in the current standards are shown in the Figure 3.13 which causes the variation in distance (approximately) shown in the Figure As can be seen in these figures, the source speed to be taken into account is lesser than the vessel speed and its maximum value occurs in the extremes of the Data Window Length (DWL). 37

38 Figure Actual position of the vessel and the hydrophone line.- Figure Distance between the vessel and the hydrophone line for a given run.- Figure Illustration of the Doppler effect before CPA.- Therefore the relative velocity of the vessel regarding the hydrophone is not constant, and it changes from negative to positive when the acoustic centre of the vessel is in the closest 38

39 point of approach (CPA). To illustrate this effect an example is shown in the Figures 3.15 and The Figure 3.15 shows the source spectrum around a discrete component for a given vessel. The sample used to obtain it corresponds with the two seconds period around the CPA. The Figure 3.16 shows the source spectrum obtained using the sample corresponding to the two seconds period starting five seconds before the vessel reaches the CPA. A frequency shift for this discrete component of the spectrum is observed. Figure Illustration of the Doppler effect after CPA.- The Figure 3.17 (reference [19]) shows a very clear spectral line modulated due to the Doppler effect. Figure Spectral line modulated due to Doppler effect.- Because of the actual speed profile of the vessel, what really happens is that when the averaging is done along the DWL, and considering only the effect in a single tune, an energy spread of this tune is produced. This distortion will be more important as frequency, vessel speed and data window angle increases as Table 3.8 shows. This may be important 39

40 because the spectral line could be hidden in the continuous spectrum if the power of this line is spread over a broad frequency integration band. Therefore the Doppler Effect shall be taken into account. The simplest way of avoiding its impact on the measurement is to define a variable frequency resolution with the frequency: the higher the frequency, the wider the frequency resolution. Wrong distance correction: Other effect that must be studied is the variation in range throughout the passage of the vessel through the data window used to compute the spectra which is related to the frequency accuracy. The influence of that in the accuracy of the measurement is presumed to be low, but in any case, it is tried to be quantified below. The Table 3.9 shows the maximum error made with different range variations (in terms of percentage) assuming a spherical transmission loss law ( ). Maximum Range variation Maximum error in the source level 1% 0.08dB 5% 0.42dB 10% 0.82dB 15% 1.21dB Table Maximum error expected in the source level due to range variation during the measurement window.- On the other hand, the range variation during each single sample will depend on the distance to the closest point of approach (CPA), the duration of the time window used as well as the speed of the vessel during the run (Figure 3.18). The Table 3.9 shows some values of this mathematical relationship in order to see the importance of this effect. Taking this distances is proposed. 2s data window d 2 d 1 d cpa ANSI/ASA considers this same distance along the DW Hydrophones Figure Relationship between the distance to the closest point of approach and the distance at the end of the measurement window.- As can be seen in the Table 3.10 small range variations are expected, therefore there should be small errors in the source level due to this effect. However special care should be taken when the distance to the closest point of approach is short or when the vessel speed is high. Anyway the uncertainties introduced by this phenomenon are expected to be really low so no countermeasures shall be taken. 40

41 Finally, current standards only use the distance to the CPA along the whole data window which is composed by a larger data window than the length of the samples used to perform a narrow band analysis as shown in Figure 3.18 (even though different distance could be used among the different samples used to compute the spectra). This phenomenon is similar to the one previously explained but since the time window could be much larger the error is higher and could be relevant (see Data Acquisition Window definition). Speed = 11knots Time window = 2s (0.5Hz) Speed = 22knots Distance to CPA Range variation Distance to CPA Range variation 300m 0.01% 300m 0.06% 100m 0.1% 100m 0.6% 50m 0.6% 50m 2.4% Speed = 11knots Time window =8s (0.125Hz) Speed = 22knots Distance to CPA Range variation Distance to CPA Range variation 300m 0.2% 300m 1.11% 100m 2.5% 100m 9.7% 50m 9.7% 50m 34.8% Table Range variation for different cases.- On the other hand, no special countermeasures shall be taken to avoid the fact that the same distance correction must be used for the smallest data window required to compute the spectrum for a given frequency resolution as the error because of this is presumed to be negligible. 3 Relative distance measurement equipment. A correct measurement of distance between the vessel and the hydrophones should be achieved in order to properly estimate the propagation loss transfer function. Two important sources of uncertainties associated to this parameter can be distinguished when a GPS or similar system is used: The uncertainty due to the measurement of the distance between the location of the GPS receiver and the actual location of the hydrophone/s in the sea. The uncertainty due to the fact that despite of being able to idealize the vessel as a point source and to estimate its location along the length of the vessel, it will be probably highly difficult to estimate its location over the breadth of the vessel. In the current measurement procedure standards the side of the hull is the reference taken from which the distance to the hydrophones is measured, which is sometimes ambiguous since the hull of the vessel is not straight. The uncertainties associated to the first item, when a GPS system is used, are made up of two factors: 41

42 The uncertainty in the distance between the buoy and the vessel supply where the GPS antenna receiver is located (Figure 3.19). This will depend on the accuracy of the GPS used. Commonly, a Differential GPS achieves an accuracy higher than 3m for most of European waters, whereas simple GPS systems achieves an accuracy about 10m. The lack of awareness of the actual position of the hydrophones relative to the buoy. Currently, it is generally assumed that hydrophones are placed vertically in relation to the buoy. Some measurement standards claims that the tilt angle of the hanging line does not need to be taken into account if it does not exceed 5º, otherwise it should be considered but the standard does not specify how. Anyway, this phenomenon will be studied in the next item. Figure Diagram of the support vessel and the surface buoy.- For the second item, the further the vessel passes from the hydrophone, the less important this error will be. In any case, this source of error is considered to be low compared to other sources of error. The tilt angle and the accuracy of the GPS system should be defined according to the required accuracy for the measurement procedure and the value of other sources of uncertainties. In order to do this, numerical studies to quantify the uncertainty of the source level obtained for a given tilt angle have been performed in this task. Moreover, the use of other methods to measure the distance which do not suffer from the drawbacks of the GPS system, specially the tilt angle, should be analysed. 4 Hydrophone line tilt. Due to the sea currents the hydrophone line is not totally straight but a drift angle appears which mainly affects the measurement of the distance between the target vessel and the hydrophones (it also may have undesirable effects on other parameters). 42

43 Figure Measurement layout.- The uncertainties caused by this phenomenon have been already studied in reference [13] for the layout shown in Figure In order to estimate the hydrophone line tilt during the measurement, the distance between the hydrophones, the sea surface and the sea bottom were measured. The results are shown in Figure It can be seen that there is a medium deviation from the nominal depth of the hydrophones (see Table 3.11) caused by the hydrophone line tilt. The small variations in the graphs of the Figure 3.22 are due to waves in the sea surface and the drifting of the measurement device. According to these values the graph of the Figure 3.22 shows the maximum error (up to 3dB) what may be made depending on the distance to the closest point of approach. As it can be seen the error decreases as the distance increases. Figure Distance of the hydrophones to the sea surface and sea bottom.- 43

44 Nominal (m) Derived from data (m) Vertical position Average Standard deviation H H Horizontal position Average Standard deviation H1 H Table Comparison between actual and theoretical position (reference [13]).- However this error may be higher if the actual transmission loss is taken into account instead of the one obtained from assuming a spherical transmission. As an example, the Figure 3.25 shows a computation of the transmission loss for a given environment. It can be seen that a difference of a few meters can determine if the hydrophone is in a shadow zone or not which could imply differences up to 5 db just due to this phenomenon. There are three ways to reduce this uncertainty: Increasing the distance to the CPA as much as possible. Other undesirable effects may appear, such as low signal-to-noise ratio. Measuring the angle tilt during the measurement. For this case, the uncertainties will be reduced to the ones caused by the variation of this angle (given by the standard deviation in table 3.11) which will be much smaller than the previous one. However it could be not sufficient if more than one hydrophone is deployed because the shape of the line into the sea is not known. Using another distance measurement system obtaining directly the distance between the source and the receiver, such as methods based on the Doppler Effect. Due to the importance of this factor its influence in the noise source level measured has been internally studied in this task. Figure Error in db versus distance to CPA (reference [13]).- 44

45 5 Hydrophone movement. Figure Transmission loss at 500Hz numerically computed.- One of the main problems in some of the current devices in use is the movement of the hydrophone in the sea. In fact the American National Standard (reference [2]) mentions that special measures should be taken to isolate the hydrophone from the movement of the waves in order to avoid self generating noise in the receivers. As it can be seen in the Figure 3.11 the spectrum content of the wave motion are concentrated in the very low frequency, however mechanisms are present in the system for the generation of high frequency accelerations given only forcing a much lower frequency. Three types of mechanisms that generate this type of noise can be distinguished: Noise generated by snap loadings. Hydrophone acceleration induced noise. Flow induced noise. Snap loads occur when one or more segments of the cable go slack and then retension due to wave induced motions. This phenomenon generates an impulsive load with broadband frequency content. One of the possible solutions is to increase the sinker weight to add extra tensional level to the cable to avoid that goes slack. The Figure 3.25 shows the acceleration time signal of the subsurface buoy of the Figure The Figure 3.26 shows the band pass filtered subsurface float acceleration where the phenomenon described above can be seen. Finally the Figure 3.27 shows the time signal recorded by one of the hydrophone of the system of the Figure 3.24, which has very large spikes corresponding to the occurrence of snap loads. 45

46 Figure Measurement configuration with a subsurface float.- However according to reference [18] this source of self generated sound is much lower than the other two mentioned above. Most of hydrophones are sensitive to accelerations, so they record relevant signals when they have acceleration. Figure 3.28 (reference [18]) shows an acceleration equivalent noise level diagram computed using the acceleration recorded by an accelerometer and the known acceleration response of the hydrophones. Figure Acceleration time signal of the subsurface float.- Figure Bandpass filtered subsurface float acceleration.- 46

47 Figure Time signal recorded by one hydrophone.- On the other side the noise level measured by the same hydrophone is shown in the Figure The total noise of the hydrophone is much higher than the component due to hydrophone acceleration induced noise, which means that the flow induced noise is the major source of self generating noise caused by the movement of the hydrophones. However, the levels for low frequencies and high vertical speed of the acceleration induced noise are high enough to mask the noise that we want to measure so it must also be removed. Finally, flow noise results for the pressure fluctuation generated by the flow around the hydrophone which excites vibrations in it. Taking into account that the noise level expected for these frequencies during the measurement is 85dB, the fact that for a vertical velocity of 0m/s a 90dB noise level was recorded suggests that flow around the hydrophones due to sea currents must be considered as well. All these phenomena generate background noise that can mask the actual noise coming from the source (the vessel) making the measurement impossible for low frequencies. Figure Acceleration equivalent noise level.- As seen in the item Frequency range, the movement of the hydrophones shall be minimized so the bottom supported configuration is recommended. Moreover, the sea state strongly affects this item so the measurement procedure will define a minimum acceptable level in terms of sea state. 47

48 Figure Diagram of the noise level recorded by one hydrophone.- 6 Distance of the hydrophones to the Closest Point of Approach (CPA) The distance of the hydrophones to the CPA is a really important key parameter, what, despite of not introducing uncertainties by itself, will strongly influence factors introducing uncertainties, such as the frequency range where the signal is valid, the transmission loss factor, the importance of the background noise compared to the vessel noise, the duration of the data window, etc. This parameter will be influenced by: The signal-to-noise ratio expected, in particular, the level of noise emitted by the vessel. Difficulty of the computation of the transmission loss. If a transmission loss of a point source is considered, this distance shall be large enough so the assumption of the vessel behaving as a point source can be realistic. On the one hand, the maximum distance will be limited by the expected signal-to-noise ratio as the further the CPA is the lower the signal-to-noise ratio is expected. Since the background noise must be 10dB lower than the vessel noise to be able to ignore it, the distance to the CPA cannot be larger than a value what depending on the kind of vessel, the transmission loss inherent to the medium and the background noise present in the area, which is mainly determined by the sea state. On the other hand, the acceptable minimum distance will be limited by the hypothesis about the vessel as a source, which means that if the vessel is considered as a point source the measurements must be taken far enough from the vessel. This consideration is assumed in all the current standards because it eliminates from the physic model the complications appearing in the near field where the model of the source as a monopole or dipole point source is no longer valid. There is no clear agreement about the distance where the assumption of far field measurements applies. It is considered that it is necessary to measure at ranges larger than one ship length to reach this condition however, it may be influenced by the depth of the location. If this condition is not complied, there will be bigger uncertainties due to the disagreement with either the physic model (in case a theoretical propagation law is 48

49 considered) or the experimental model (in case a calibrated point source is used to estimate the propagation loss). Even though the distance will be mainly defined according to these two factors, the larger the distance is the better to reduce the undesirable effects on the measurements caused by the hydrophone line tilt and the Doppler Effect. Besides, larger distances allow longer data windows for a given Data Window Angle. However, very large distance also complicates the computation of an accurate transmission loss. If the background noise is low enough along the frequency range, then the distance can be higher. That is why not only the sea state must be appropriate during the measurement (environment) but also the self generated noise of the measurement chain should be reduced as much as possible. The sources of background and self generated noise are studied in other items within this paragraph. Current standards already defined this requirement. The reference [2] and [3] require a distance larger than 100m or once the vessel length, whichever is greater for the maximum grade of accuracy and the reference [9] requires a distance from 150m to 250m. On the other hand, this requirement may imply that, especially for silent vessels, the distance to the vessel is too large since the noise coming from it may be the same order as the background noise, which makes the measurement not valid. For example, the Figure 3.30 shows the spectrum measured for a fishing research vessel where the distance to the closest point of approach was about 42m. It can be seen that the levels are 10 db higher than the background noise for frequencies between 40 and 75 Hz and less than 10dB higher between 30 and 45Hz. If the distance requirement had been in accordance to what is specified in current standards the level measured would have been 7.5dB lower in the best case (considering a transmission loss factor equal to which would have made the measurement not valid due to the excessive background noise. That is why the vessel type should be taken into account somehow in the definition of this requirement. Other effects may appear if the distance to the CPA is too short, such as large spurious signal recorded by the hydrophones caused by the wave pattern produced by the testing vessel or interaction between the vessel and her surroundings. Figure Noise spectrum measured by two hydrophones at a given distance.- 49

50 Therefore, a minimum distance between the hydrophones and the CPA should be defined taking into account the constraints regarding the assumption of the vessel as a point source. Due to the importance of this parameter, the suitability of this requirement has also been experimentally verified. 7 Number of hydrophones The references [2] and [3], for grades of accuracy A and B, deploy more than one hydrophone at different depths and then take the power average of their outputs to obtain the final measurement of one run. The purpose of this is to average as much as possible the effects caused by the Lloyd Mirror effect as well as averaging the error in the transmission loss. Besides the hydrophones shall be positioned vertically in the water column at depths which results from nominal 15º, 30º and 45º from the sea surface at a distance equal to the nominal distance at CPA. However, it is very difficult to obtain an actual distance to the CPA close to the nominal one. Therefore, the hydrophones would be positioned at different angles from the ones specified in the standard, so a more realistic target should be provided, in terms of tolerance for this angle, in case more than one hydrophone are used, or different approaches to take the Lloyd Mirror effect into account should be proposed. Other possible advantage of deploying more than one hydrophone is that the transmission loss can be estimated from the measurements taken by different hydrophones if they are positioned at different ranges although the deployment of this configuration is quite more difficult. This study can also be done fixing different nominal distance to the CPA for each run performed. Moreover the repeatability of the measurement increases as the number of hydrophones increases. Solution: The grades with the most demanding requirements with regard to accuracy should require the use of several hydrophones, whereas B-application whose accuracy requirements are less demanding, will only require the use of one hydrophone, as deploying more than one hydrophone is costly. 8 Cost, Handling & Independence The cost associated to the measurement will strongly influence the scope of it. On the one hand it must be in accordance with the overall cost of the ship building for new vessels, on the other hand, the performance of the test should not imply an excessive time where the vessel is out of service. That means for example that neither the cost of the test can be excessive for small vessels (like fishing vessels) nor it can imply that a large merchant vessel must spend too many days of navigation to go to a proper location to perform the test according to the standard. The overall cost can be split into the next items: Cost associated to the device: This item only includes the cost associated to the instrumentation (hydrophones, amplifiers, recorders, the distance measurement system, etc). It will strongly be affected by the requirements specified for it. The specifications given in the standards [2] and [3] about the instrumentation are not detailed and depend on the grade of accuracy. The main difference is the number of hydrophones, the frequency range, the bandwidth required for them and the distance measurement system accuracy as can be seen in Table

51 ANSI/ASA Grade A Grade B Grade C Number of hydrophones Sensitivity As needed As needed As needed Dynamic range As needed As needed As needed Frequency range 10Hz to 50kHz 20Hz to 25kHz 50Hz to 10kHz Bandwidth Narrow band Narrow band One-third octave Distance ranging uncertainty 2% 2% 5% Table Requirements for the instrumentation in the standards [2] and [3].- The requirements specified in the standard of DNV [9] are given in Table Number of hydrophones 1 DNV Sensitivity Dynamic range Frequency range Bandwidth As needed 90dB Depend on the operational group One-third octave bands Distance ranging error +/-5m Table Requirements for the instrumentation in the standard [9].- It can be seen that the cost of the instrumentation required in the standard of the reference [9] will be similar to the one associated to the grade C of accuracy of the references [2] and [3], whereas the cost of grade A and B will be higher, mainly because of the number of hydrophones and the bandwidth required. Apart from the factors mentioned in the previous tables there are other factors affecting the cost of the device, which are: Necessary instrumentation to measure environmental variables, such as temperature, hydrostatic pressure, sound speed profile, or other kind of parameters such as orientation of the hydrophones. Within this category is also included all the measurements to be performed to evaluate the transmission loss (none of these variables are required to be measured in the current standards). Requirements for the autonomy of the measurement chain which will depend on the time required to perform the test (number of runs, data window period). Cost associated to the deployment of the measurement chain: There are different arrangements to deploy the hydrophones. The Figure 3.31 (references [2] and [9]) shows the main configurations currently in use. 51

52 Figure Different arrangement to deploy the hydrophone.- The diagram on the left shows a hanging line which is the easiest way to deploy the hydrophones although it suffers from a severe movement of the hydrophones. The middle diagram shows a bottom anchored deployment which insulates the hydrophones from the sea surface movement although it does not eliminate all the problems in the low frequency such as the effects of the currents or cable strumming. It is also more expensive and requires the recorder to be into the sea. Finally the figure on the right shows a bottom mounted deployment which requires ways to recover the hydrophones and suffer from other disadvantages as the strong influence of the reflection coming from the seabed, which must be taken into account. Besides it will probably require divers to put the device onto the sea bottom and ways to measure and visualize where it is exactly. Due to all the reasons mentioned above the bottom mounted option implies a higher cost of deployment than if the hydrophones are deployed in a hanging line. In general the deployment cost will be according to the requirements in terms of accuracy of the position of the hydrophones, the necessity of external help (like the use of divers, cranes, supply vessel, etc.) which will be accordingly to the handling and independence of the device and the complexity of the measurement chain (number of hydrophones, etc.) Cost associated to auxiliary measurements in the vessel: In order to assure a proper comparison among different measurements it is necessary to know the vessel conditions, speed, etc. In this item the cost associated to these auxiliary measurements is included and it will be equal to achieve a good accuracy no matter what procedure and device. Personnel cost: This item includes the cost associated to the personnel directly involved in the completion of the test. No big differences are expected. Cost of diversion of the ship: This item includes the cost associated to the navigation of the vessel to the proper location, the vessel crew, the cost necessary to navigate and perform the test and, when it is applicable, the cost associated to the fact that the vessel is out of service during the test. 52

53 Figure Depth map in the Mediterranean sea.- This cost is going to be determined by the kind of vessel, the time spent to perform the test, and by how far the closest proper location is from the shore. The last point is the most important and it will depend on the depth required according to the procedure. The Figure 3.32 and 3.33 show depth maps of the sea around Europe. Figure Depth map in the North sea.- The references [2] and [3] require for grade A of accuracy a minimum water depth of 300m or three times the overall length of the vessel (for some vessel this requirement implies more than 600m depth). As can be seen in the maps the German, the Adriatic and Baltic country shores do not comply with this requirement, which enforce to navigate long distances. 11 Hydrophone directivity. The directivity of the hydrophone influences the uncertainties associated to the measurements. If the directional characteristics are not accounted in the final data postprocessing it should show no more than a certain variation in the whole frequency range of the measurement to assure a certain level of uncertainties. The Figure 3.34 shows a 53

54 directivity diagram for different frequencies of a hydrophone model which in principle is omnidirectional: Figure Directivity characteristics of a hydrophone.- If directional hydrophones are going to be used, specific data post processing to account the directivity should be defined as well as the necessary measurements to know the actual position and orientation of the hydrophones. 12 Accuracy of the instrumentation The frequency response is the output-to-input ratio of the hydrophone. As it was said in the item dealing with frequency range, one of the factors determining the frequency range of the measurement chain is the range where the frequency response is flat, which means the range where the hydrophone outputs the same amount of voltage per amount of acoustic excitation regardless the frequency. The Figure 3.35 shows an example of frequency response of one hydrophone. The frequency range of the hydrophone will be determined according to the acceptable deviation from the flat response which is directly related to the acceptable uncertainty for the isolated hydrophone. Finally this parameter varies with temperature and pressure even though the uncertainties associated to this effect is much lower than others described in this paragraph. Figure Frequency response of the hydrophone type 8103 of B&K.- 54

55 13 Dynamic range Figure Hydrophone sensitivity for different hydrostatic pressure.- It is the ratio between the largest and smallest components that the measurement chain can measure. In underwater acoustics, the smallest signals that the measurement device is able to measure are not only limited by the dynamic range of the measurement chain but also by the background noise. The references [2] and [3] do not specify the dynamic range for the measurement chain, unlike the reference [9]. Even though the background noise will be the most limiting factor determining the ratio between the biggest and smallest signal that can be measured, this may not be true for all kind of vessels (specially the noisy ones) and, consequently a suitable dynamic range may have to be specified in future standards. 14 Electronic noise Electronic noise is a random fluctuation in an electrical signal, characteristic of all circuits. It may affect the signal-to-noise ratio if the electronic noise of the electronic components of the measurement chain is high, however it is presumed low compared to the background noise of the measurement location, except for very high frequencies. Therefore the procedure shall specify a minimum level of performance of the instrumentation with regard to the points of frequency response, dynamic range and electronic noise Environment The effect of environment on underwater detection has been addressed for a long time, mainly for the assessment of the detection range of sonar systems in the military domain. A very abundant literature exists and it is of course not possible to present here in detail all the phenomena and implications. The reader can find more information in some references such as [15], [22] and [23]. However we should note that here we are more interested in short propagation ranges, because URN measurements are carried out at maximum distances of a few hundreds of meters, as military sonar detection ranges can achieve several tens of kilometres. 55

56 The main issues regarding the effect of environment on sound propagation are: Celerity profile and reflection of acoustic waves on sea surface and sea bottom, which affect the propagation loss between the acoustic source and the detection devices. Ambient noise level, which affects the quality of the measurement on the signal to noise ratio point of view. Other environmental effects such as currents, wind, etc, which will disturb the deployment of equipment and produce self noise in the hydrophones, so introducing inaccuracy on the actual distance between the acoustic source and the detection devices. 1 Shallow/deep water. Water depth influences URN in different ways: Hydrodynamics: shallow waters can alter hydrodynamic behaviour of the propeller, and possibly the hull, thus affecting underwater radiated noise; Reflection on sea bottom: refer to item 6 Sound propagation: refer to item 3. Relation with ship speed: Referring to STANAG 1136 [1], the hydrodynamic effect is related to an interaction between the waves produced by the moving vessel and the water layer of height h. A warning speed v w, not to be exceeded during the trial, is defined as:., where v c is the critical speed defined as: For the ISO standard part I [3], shallow effects are not considered, because the measurement is assumed to be done in deep waters. The minimum water depth, for Grade C, which is the less accurate, is 75 m or greater than ship length. Ship speeds up to 50 kts are considered, although it may raise some practical or safety issues. In the DNV document, reference [9], it is required the minimum water depth to be greater than 30 m and 0.64v 2. For comparison, the criterion from the STANAG gives approximately 0.3v 2. In both cases, a squared speed law is used, but the criterion from the DNV document is more stringent. Solution: The depth required by the procedure should also account for this phenomenon. Knowledge of sea bottom profile: If the measurement is not performed in deep waters, a good knowledge of sea bottom profile is necessary. This information can be taken from cartography, if detailed information is available, or using an adequate echo sounder survey. It is recommended to include a map of the area in the measurement report. In addition, care must be taken to sea level variations which can vary significantly along time due to tides. 56

57 2 Mirror effect. Because of the high acoustic impedance contrast between air and water, sea surface behaves like a pressure-release boundary condition, on the acoustical point of view. For the radiation of a sound point source, the problem can be modelled by introducing a second fictive source, opposite in phase, symmetrically above sea surface (Figure 3.37). Assuming sea surface to be a perfect planar surface, the acoustic pressure measured at the receiver is the algebraic sum of the waves coming from the sound source and from its image: Where is source strength, the acoustic wavenumber and the speed of sound. For a pressure-release surface,, then the system behaves as an acoustic dipole with separation, leading to acoustic interferences between direct and reflected paths, producing alternatively constructive and destructive interferences at the measuring point. This is known as the Lloyds mirror effect (reference [15], page 128). The Lloyd mirror effect is expected to depend on: frequency immersion depth of the sound source distance and immersion depth of the receiver(s) size of the sound source sea state (possible variation of sea surface shape due to waves, affecting refection coefficient) avenging effects o narrow band / wide band o time average (corresponding to measurement angle with respects to CPA) Figure Lloyd Mirror effect.- 57

58 Pressure level (db) Direct Total (narrow band) Total (1/3 octave) Frequency (Hz) Figure Mirror effect on the radiation of a point source.- In the limiting case of perfect boundary, the interference effect is illustrated in Figure 3.38, for a noise source immersed at 5 m and a measurement at 100 m distance. When noise level is averaged over 1/3 octave frequency bands, this interference pattern is less visible, except at low frequencies, and a deviation of about 2 db at high frequencies. Two methods can be considered to separate the reflected path to the direct path: time separation, because the reflected signal comes later than the direct path, use of a directive sensor. Unfortunately, these methods cannot be used easily in practice. Time separation is not possible because all methods used in practice require to average signal over time (typically several seconds). Exploitation of directivity would require a large vertical array with numerous hydrophones, which increases cost, requires complex post-processing and in any case the main lobe aperture is not small enough to separate sources at low frequencies. Vertical arrays are sometimes used for military applications for the measurement of very silent vessel such as submarines running at low speeds, in order to increase measurement signal to noise ratio. Coming back to the Lloyd mirror effect, the actual effect in realistic conditions is not as acute except from the low frequency. Different mitigation effects are: Sound radiated from a ship does not come from a single noise source, but from several noise sources distributed on the hull, or from the propeller. As an example, the Figure 3.39 shows the difference between considering a point source at 2m depth and a distributed source whose mean depth is 2m and standard deviation of 1m (reference [24]). It can be seen that the effect of the sea surface for the two hypotheses is the same up to the first peak, then the distributed source shows an increasing attenuation of the higher frequencies peaks and nulls. sea surface is not perfectly smooth, due to the waves (depending on weather conditions) so that the level of the coherent wave scattered by sea surface is significantly lower than the one produced by a perfect mirror; 58

59 frequency average over 1/3 octave bands; time averaging during measurement. In order to decrease the measurement deviation due to Lloyds mirror effect, some procedures or standards require an average on several measurement points. This is the case in particular for the Grades A and B of ISO standard, where the measurement is averaged on three points on a vertical line (Figure 3.40). As the wave interference pattern will not be the same for the three points, the average will be less sensitive. To assess the efficiency of this averaging process, a simulation model has been built. Figure 3.41 shows the comparison of the real URN 1/3 octave-averaged spectra with the simulated measured level with only one hydrophone (Grade C) and three hydrophones. Figure Lloyds Mirror Pattern Interference of a sound point source and a distributed source.- As expected, the grades A and B reduce the Lloyd Mirror effect compared to grade C. To check the influence of the sea surface, the previous calculation is redone with a different value of reflection coefficient R. In this example it is assumed that R remains close to one at low frequencies, up to 100 Hz, then diminishes linearly down to 0.2 at 1500 Hz and constant at higher frequencies. This approach is justified by the fact that for a given wave height, there will be some destructive interference of the acoustic waves reflecting on sea surface, reducing the effective reflection coefficient. In that case, we see that the deviation introduced by the comparison to free-field radiation occurs only for low frequencies (lower than 300 Hz in that example). Figure Measurement points in ANSI-ASA standard, Grades A and B.- 59

60 Figure Lloyd Mirror effect with 1/3 octave band average.- Figure Mirror effect simulation with an effective surface reflection coefficient.- It is also important to take into account that the current standards provide results as the Dipole Source Level (termed affected source level by ANSI/ASA reference [2]) whereas if the Lloyd Mirror effect is accounted in the transmission loss the results obtained correspond to a monopole source. The conversion between a dipole and monopole source level (considering a perfect reflexion surface and a point source) is given by Ainslie (reference [25]) as: Where is the wave number, the depth of the source and θ is the depression angle relative to the surface. The correction to obtain the ANSI affected source level dipole from the monopole source level, including the averaging from the three depression angle is plotted against frequency in Figure 2.45 (reference [17]). 60

61 Figure The correction applied to the monopole Source Level at 4m depth to convert to the ANSI/ASA S12.64 affected source level averaged over depression angles of 15º, 30º and 45º.- 3 Sound propagation into water. General propagation rules: Basic propagation rules in a fluid medium are plane waves, spherical waves and cylindrical waves. Plane waves can be used locally or to describe wave propagation in pipes, but is not relevant for outdoor propagation. Figure 3.44 presents the geometry corresponding to spherical and cylindrical propagation. As a start, we consider that the fluid medium is not dissipative. In that case, for spherical propagation, acoustic pressure decays as the inverse or the radial distance r, so the level N at observation point M as : Figure Coordinates for spherical and cylindrical propagation.- For cylindrical propagation, if the height H of the water layer is small by comparison to the distance R, the pressure field decays as R -1/2, then, instead of R -1 : Influence of sound absorption: In addition the previous loss factors, which are only due to geometrical spreading of waves, some additional loss can be due to dissipative effects related to the presence of particles or chemical components in sea water. Many others have dealt this issue, one reference model for deep waters being Thorp s model: 61

62 Different models are compared on Figure The orders of magnitude of these losses at 200 meters from acoustic sources are: 0.01 db/km at 100 Hz 0.1 db/km at 1 khz 1 db/km at 10 khz 30 db/km at 100 khz. Figure Sound attenuation in sea water.- As a consequence, if we consider a ship URN measurement distance set at 200 meters, propagation loss will be negligible up to 10 khz (lower than 0.2 db), but on the other hand it can exceed 5 db at 100 khz. It should be noted that this effect is not clearly addressed in the existing standards, leading to an underestimate of URN level. Propagation loss in real environment: In real environment, propagation loss will depend also on celerity profile (discussed in item 4), and reflection on boundaries (bottom and sea surface). As a consequence, propagation loss generally does not match exactly either spherical or cylindrical law. In deep waters, propagation field for short distances is approximately spherical. This can be seen on the example presented on Figure A-9. However, it can be seen that for this example, propagation loss at 1 km distance is approximately 57 db, instead of 60 db for spherical decay. In shallow waters, propagation tends to be cylindrical at sufficiently large distance R. This pattern is clearly visible on Figure A-11.In this example, propagation loss at 1 km distance is approximately 54 db, to be compared to 60 db for spherical propagation and 30 db for pure cylindrical propagation. As a consequence, for shallow waters, a possible approach is to consider spherical propagation up to a short distance r 0, then cylindrical propagation for r > r 0 [7]. 62

63 Then the expression of acoustic pressure level at distance r is: For the STANAG and ISO standards, the distance correction factor is assumed to be spherical, i.e.. For the DNV standard, the correction factor is, justified by the fact that measurements in that case occur in shallow waters, at a typical distance of 200 meters. Figure Shallow waters short range propagation experiment (d=60m).- Relatively high errors or deviations can occur if the distance correction factor law is not appropriate. For example, if the actual correction factor is instead of, the URN will be underestimated by, Errors can be even more important in very shallow waters, because propagation is complex and subject to important environmental variations. The study reference [11] gives such an example. Figure 3.48 shows that the correction factor with respects to spherical propagation at 60 meters can exceed 5 db. In addition, it is important to note that it depends on frequency, by about +/-2dB. There are several models, based on different physical principles, to compute the transmission loss with distance. Some standard models are listed below: BELLHOP: ray tracing based model. Kraken: normal model propagation code. KrakenC: same as Kraken but allowing complex value data. RAMGeo: parabolic equation model that uses a split-step Padé algorithm. RAMsGeo: same RAMGeo but incorporating shear. Scooter: wave number integration model. 63

64 OASES: wavenumber integration model. Figure Propagation loss uncertainty due to the unawareness of the parameters of the model in shallow waters (yellow) and deep waters (red).- All these models need the introduction of a number of parameters to model the environment. Most of the time the value of these parameters are not known and they are estimated or taking from bibliography sources. Within the internal studies, a Monte Carlo Analysis was performed to evaluate the uncertainties associated to the unawareness of the parameters of the model (celerity profile, hydrophone depth, source depth, ), showing difference up to +/- 5dB (Figure 3.47). 4 Celerity profile. The value of sea water celerity (i.e. speed of sound) depends significantly upon temperature, depth, and salinity. An approximate formula is: Seawater celerity profile with respects to depth, which can be routinely measures using appropriate equipment, influences greatly sound propagation. The effect is particularly visible at large distance, the reason for what it is a key issue for determination of sonar system performance. Because of the variation of celerity, acoustic rays do not propagate along straight lines, but present some curvature, generally dependent on the gradient of celerity. Furthermore, celerity profile varies generally, with the period of the year, and can even vary during daytime in shallow waters environment. Figure A-6 in Annex A gives examples of celerity profiles. From the examples in Annex A, we can note that: in deep waters in summer, the acoustic rays tend to plunge to sea bottom, in other cases, interferences occur due to reflections on sea bottom and sea surface. A special case occurs when celerity reaches a maximum for a given depth, generally a few tens below sea surface, due to the warmer water layer close to the surface, and a surface propagation duct appears. This effect, explained in reference [7] and represented schematically on Figure 3.48, leads to a situation where some acoustic energy is trapped near sea surface. 64

65 An example of propagation loss computation in such a situation is given on Figure A-12 of Annex A. It can be seen that some minimum distance is needed for the phenomenon to appear. Then, it is not expected to influence greatly URN measurements. Figure Surface propagation duct.- Figure Uncertainty due to changes in the celerity profile.- In order to see if the celerity profile affects significantly the transmission loss at ranges typical of this kind of measurements, Within internal studies a numerical simulation was performed for different water depths (shallow and deep) to compute the standard deviation of the results obtained considering different celerity profiles. The Figure 3.49 shows the results of this study. This figure reveals that the influence of celerity profile on transmission loss is much more important in deep waters than in shallow waters. 5 Currents. The main effect of currents is to displace the equipment used for the measurement, mainly buoys and underwater cables and instruments, by comparison to the expected location. The main consequence is a bias or an inaccuracy of the determination of the actual distance between the hydrophone(s) and the ship to be measured. ANSI/ASA and ISO standards ([2] and [3])addresses this issue by a requirement of accuracy for the determination of that distance, which is 2% for Grades A and B, and 5% for Grade C. Regarding the effect of currents, guidance is given by requiring that the tilt angle of the cable supporting the hydrophone should not exceed 5. A s uggested method to estimate the deviation between actual position and vertical is to measure the static pressure at 65

66 measurement location, giving the actual depth to be compared with the length of the cable. This principle is shown on Figure We can note that this method is not very reliable: the current may vary with depth, acting on the cable in such a way that the tilt angle is not constant. For example with a parabolic shape the approximate relationship between the lateral deviation L and the depth variation h is (3.h. h) 1/2 instead of (2.h. h) 1/2 for a straight cable tilt. In addition, the error on measurement distance will be greater if the lateral deviation direction is perpendicular, rather that parallel to ship course. The STANAG requires some accuracy for determination of measurement distance but does not address particularly the effect of currents. The DNV document does not address this topic because the measurement hydrophone is placed on sea bottom. The hydrophone is placed in a cage, sufficiently large not to be displaced by currents. The effect of the current on the vessel working conditions should be also accounted since this could have a great impact on the underwater radiated noise of the vessel. This effect is studied in the item regarding the vessel. Finally, currents increase the self-generated noise of the hydrophones in the low frequency range as seen in paragraph 3. 6 Seabed effect. Figure Effect of current on measurement equipment.- Reflection of acoustic waves on sea bottom will modify the acoustic pressure and introduce some deviation or uncertainty on ship URN measurement. If water depth is sufficient (typically greater than 100 m and greater than measurement distance), the travel distance of the acoustic wave reflected on sea bottom is about two times the direct wave travel distance R (Figure 3.53). If it is assumed that seabed reflects totally the incoming wave, which is a rare limit case, the maximum error due to bottom reflection will be, in narrowband: 66

67 In practice, bottom losses for acoustic reflection on seabed, if grazing angle is not too small, is at least 6 db, then the error will be reduced to 0.75 db. A shallow waters configuration, close to the one defined in DNV standard, is also shown on Figure Acoustic path N 3 contribution is expected to be small because of multiple reflection, and in any case it is taken into account in the propagation loss factor discussed in its respective item. In the case where the acoustic sensor is on sea bottom, acoustic interference occurs between acoustic paths N 1 and N 2. Using modelling from reference [22], Figure 3.52 gives the reflection coefficient of a perfect plane seabed composed of mud, sand and clay, then the total pressure, along frequency, sensed by a hydrophone put at 20cm from bottom on a sandy seabed. For this measurement configuration, the grazing angle is expected to be about 30 or smaller. Conse quently, the expected reflection coefficient is close to one (0 db), except in the case of muddy seabed. In the case of a more reflective seabed, such as sand, interference can occur, and total pressure oscillates between a maximum of about +5 db with respects to direct path, and lower values when destructive interference occurs. This is probably why the DNV document introduces a -5 db correction factor to account for seabed effect. However, we can sea that this correction factor is not reliable in all cases, because it depends on seabed characteristics and frequency. Figure Effect of seabed on acoustic propagation paths.- In reality, sea bottom is not perfectly smooth. In case of strong rugosity, the wave impinging on sea floor is scattered in all directions, as represented on Figure In that case, the specular reflection of the impinging wave is lower, and the interference effect shown previously is mitigated. This is probably the reason why the DNV document recommends the sea floor not to be too flat. In the case of the STANAG standard, there is a possibility to put a sensor on sea bottom, for ship URN measurement at keel aspect. However, in this case, the grazing angle is close to 90, then the reflection coefficient and consequent ly the interference effects are lower than in the previous case.. 67

68 Figure Sea bottom roughness.- Other important effect of the seabed in shallow waters is the bad transmission of lower frequencies. This effect was already explained in the item frequency range. 7 Background noise. The presence of background noise affects the signal to noise ratio, and so the accuracy of the measurement. For that reason, all existing standards require to make a measurement without the ship to be measured, then with the ship running in the acoustic range. If we discard electronic noise and self hydrophone noise, of the measuring and processing equipment which should be appropriately low, ambient noise at sea is due to different components: turbulences (very low frequencies) sea noise related to sea state and wind remote shipping noise thermal noise (very high frequencies). A reference model for ambient noise level at sea in deep waters is Wenz model represented on Figure 3.53 (see also reference [15] page 210). This model introduces as an input the sea state level (which can be related to wind speed) and a shipping traffic index Tracom (low, medium, heavy, very heavy). Before selecting a measurement site, it is also possible to get some on-site ambient noise measurement data, such as data shown on Figure A-7 in Annex A. To validate a ship URN measurement, the following signal to noise ratio (in db) is first calculated for all frequencies of interest: If S/N is greater than 10 db, the measurement is valid and the influence of background noise is considered to be negligible. 68

69 If S/N is between 3 db and 10 db, the URN can be corrected using the following formula: Noise level (db ref. µpa^2/hz) Turbulence Tracom 1 Tracom 4 Tracom 7 Sea state 1 Sea state 3 Sea state 6 Thermal Frequency (Hz) Figure Wenz model for ambient noise at sea.- If S/N is lower than 3 db, the measurement is not valid and the corresponding data must be discarded. If possible, the disturbing noise source should be investigated or removed, if possible, or the experiment postponed in order to benefit from better environmental conditions. Conclusion As this topic must be dealt globally, the guidelines for the definition of the measurement procedure drawn from this study are analysed at the end of the point Environment. The effect of the environment on the measurement is confined to two items: Transmission loss. Background noise. Regarding the transmission loss, this paragraph has shown that: The effect of the Mirror effect is important for low frequencies. Moreover, this phenomenon is one of the causes of the poor signal to noise ratio of the vast majority of the vessels in the very low frequency range. The water depth affects significantly its value. It is not constant along the frequency span. The sea bottom characteristics shall be accounted in the case of shallow waters or receivers located close to the sea bottom in measurements of deep waters. The celerity profile has a non-negligible effect on the transmission loss for the case of measurements in deep waters. Due to the amount of parameters affecting the transmission loss, the procedure should consider a different and more accurate approach to compute it. Initially, a procedure for 69

70 obtaining experimentally the transmission loss of the test site was defined and tested in real measurements. However, the tests did not give good results. Therefore, a numerical approach must be followed to obtain a more accurate transmission loss for A-Applications. It has been seen in internal studies performed that some input parameters shall be measured or estimated. As we have seen here, the most important ones are: Sea bottom characteristics for shallow waters. Celerity profile for deep waters. Moreover, due to the high uncertainty with regard to the characteristics of the sea bottom, it is recommended not to deploy a receiver close to it. Regarding the background noise, it is clear that the measurement should be performed with a proper sea state Vessel particulars The URN strongly depends on the vessel particulars like the ship type, speed and loading condition. The questions are how these parameters will influence the measurement, especially the measurement uncertainty, and if the current measurement standards account for these parameters. 1 Vessel speed: The influence of vessel speed on other parameters is not commented in the existing standards. DNV Silent-E class notation prescribes 11 kn as quiet cruise condition if the overall vessel length is larger than 50 m (8 kn if LOA 50 m). The same applies to the SILENV Green label proposal. The noise radiated by a ship with a fixed pitch propeller (FPP) does generally increase with vessel speed. The noise from a ship propeller will depend whether it is cavitating or not. The lowest speed at which cavitation occurs is called the Cavitation Inception Speed (CIS). Naval vessels and quiet research vessels are designed to be free from cavitation below a certain operating speed. Naval vessels are often designed to have as high CIS as possible, typically around kn. However, all propellers will cavitate above a certain speed (if the installed power is enough), no matter how well designed the hull and propeller are. Below the CIS, other noise sources grow in importance. Thus, for naval vessels and quiet research vessels it becomes important to reduce e.g. the non-cavitating propeller noise. The CIS can be improved by increasing the blade area and by reducing the loading at the blade tips and hub. Unfortunately, both these measures cause reduced propeller efficiency. Merchant ships are designed primarily for optimum fuel efficiency and do thus have some amount of cavitation when operating at design speed. The CIS for a merchant ship is likely to be 10 kn or lower [Leaper & Renilson, 2012]. In the literature [Ross, 1976; Arveson & Vendittis, 2000], one can found models that suggest a linear relationship between spectrum levels in db and the log of the speed for a cavitating propeller. A ship with a controllable pitch propeller (CPP) could become noisier with decreasing speed [Wittekind 2009]. This is particularly the case if they have a shaft generator and operates at constant rpm. In this case, the speed is adjusted by the propeller pitch. This gives poor 70

71 hydrodynamic conditions that lead to increasing face cavitation and tip vortex cavitation with increasing divergence from design pitch. In summary, the dominating source and the frequency characteristics of the sources changes with speed. 2 Vessel length: In the current standards the vessel length influences the sea depth required. These requirements will be analysed in terms of accuracy and measurement cost implications. Moreover the vessel length will influence other phenomena such as the Mirror effect observed. In the existing standards, direct relationship with ship length is not mentioned. It is expected that ship length has indirect influence: a bigger ship is expected to have a larger draft, then reducing water depth under keel; a longer ship will influence the time window for data acquisition and can introduce a bias on the acoustic pressure levels coming from widely separated noise sources on the hull. The vessel length does also influence the acoustic centre location (distance between propeller and machinery location). The propulsive power needed increases with the ship length. [Ross, 1976] suggest a linear relationship between spectrum levels in db and the log of the ship length (proportional to 20 log L). 3 Propeller and machinery source type and working conditions The primary working condition should be the ones that propeller and machinery combination is designed for. The DNV and SILENV procedures propose 85% MCR which usually corresponds to design speed for a merchant ship. Secondary working conditions are other typical operating conditions, e.g. passage in an area with restricted speed, which are offdesign working conditions. The cavitation characteristics are determined by the propeller loading which is characterised by a certain combination of advance ratio (J) and cavitation number (σ): Where p 0 is the local pressure, p v is the vapour pressure of water, ρ is the density of water, V A is the inflow velocity to the propeller, n is the shaft speed and D is the propeller diameter. The parameters influencing the advance ratio and cavitation number are mainly: draft, speed, power and shaft speed. Thus, to be able to reproduce a certain condition, one must have control over these parameters. Environmental parameters such as wind, waves and current may violate the possibility to repeat an exact combination of speed, power and shaft speed. 71

72 In field, one has to accept some deviation from at least one of these. If a deviation in these parameters will significantly change the cavitation characteristics and hence the URN will vary very much from case to case. For a given propeller, the inception limits for different of cavitation can be plotted as a function of advance ratio and cavitation number, Figure The design point for a naval propeller is usually in the non cavitating area while a commercial propeller typically has tip vortex cavitation and some amount of sheet cavitation on the suction side. Going from design draft to ballast draft decreases the cavitation number, i.e. one moves downwards in the graph which normally increases the amount of cavitation. If the point of operation is close to one of these inception limits, the cavitation characteristics might be sensitive to a deviation in parameters. Figure Inception limits for different types of cavitation.- The number of propeller blades and the typical rpm of the machinery type affect the frequency range of the radiated noise. For example, a water-jet unit has typically a higher number of rotor blades and a higher rpm than a conventional propeller designed for a ship of equal size. Thus, the fundamental blade rate frequency will be considerably higher for the water-jet vessel. Different types of engines (two-stroke, four-stroke, low speed, medium speed, steam turbine, gas turbine, etc.) have different frequency characteristics. 4 Silent vessel: It is a specific characteristic of certain kind of vessel that strongly influences the measurement. The sound level of the vessel will be an important factor determining the maximum distance where the device can be achieving a good signal-to-noise ratio. Moreover it will influence the magnitude of other parameters such as the Mirror effect of the reflections coming from the seabed. 5 Availability The current procedures require certain conditions for the measurement location. For certain vessels this requirement implies going far from the coast which means several days of unavailability of the vessel. For a commercial vessel it is almost out of the question to taking it out of operation in order to go to a designated test area. The DNV procedure seems to be well suited to available European conditions while the ISO procedure is very hard to achieve close to European coasts. The ISO procedure is maybe too influenced by US Navy procedures. 72

73 Post-processing 1 Bandwidth Once the data are collected the way of reporting the results will depend on the scope of the measurement. The standards [2], [3] and [9] specify that the results must be reported using one-third octave bands and the use of narrow bands is optional whereas the reference [1] does not require a specific bandwidth. The election of the way to report the results for the standard to be developed in this document must depend on the required application defined in paragraph 2.1. For A- applications narrow band representation is necessary because the aim of this application is the identification of the sources, although it is true that the narrow band analysis is more relevant for low than high frequencies since tonal components are more likely to appear in this frequency range. On the other hand, for B-application one-third octave band will be enough because its aim is to evaluate the source level of the vessel. For some purposes is convenient to use the power spectral density per unit bandwidth and the most widely accepted method to convert the spectrum level expressed in acoustic pressure to a spectrum level in terms of power is to divide the in-band noise pressure by the square root of the bandwidth. The assumption behind this method is that the sound energy is uniformly distributed across the measuring band. 2 Acoustic centre definition/location. The current standards assume that the vessel is a point source and therefore, they idealise that the sound comes from an acoustic centre with the same location for all frequencies. This point is important since the distance between the vessel and the hydrophones are measured from it. In the reference [9] the acoustic centre is assumed to be at 0.7 propeller radius when the blade is pointing upwards. In this case the predominant source is assumed to be the propeller. However, depending on the machinery characteristics this assumption may not be true. On the other side the references [2] and [3] propose two ways of estimating the acoustic centre depending on the grade of accuracy. For grade B and C the location of this point is assumed to be halfway between the centre of the engine and the propeller and for grade A it is estimated experimentally using the time signal recorded by the hydrophones. However there is no specification on how to do it but it is a possible solution to the problem. It proposes that this point can be defined by the maximum broadband hydrophone output during each run. Nevertheless such thing is sometimes difficult to determine for some kind of vessels because the signal may have parasitic signal or the vessel may not be noisy enough, which make this task difficult and subjected to uncertainties. In fact, as seen in the Table 3.14, the results of this approach are not consistent. Indeed, this table shows the location of the acoustic centre for the same vessel using the records of different runs. As can be seen there is any coherence in the results. 73

74 Run Distance to CPA Distance from AC to GPS Portside 1 96m -37.8m Portside 2 92m -10m Portside 3 105m -48.6m Portside 4 185m 48.6m Portside 5 207m 0m Table Results of the experimental location of the acoustic centre.- Besides, the idealisation of the existence of an acoustic centre implies to allocate in the same place all the different sources of the vessel (propeller, machinery, etc.). Anyway for sufficiently large distance (see the item distance of the hydrophones to the CPA) the error made assuming this hypothesis is presumed to be low compared with other sources of uncertainties. However the idealization of the existence of an acoustic centre is not appropriate if the measurements are taken in the near field since the interaction among the different sources are important in the noise transmission at short ranges. 3 Data acquisition window definition. The data acquisition window determines the amount of data used to compute the source spectrum of the vessel. The longer it is the larger the computed average will be, however the fact that the average is larger does not necessary imply a better repeatability as other factors such as the directivity of the source may strongly affect if they are not properly considered. Figure Definition of the data acquisition window in the references [2] and [3].- The references [2] and [3] define the data window fixing the data window angle to ±30º (Figure 3.55) but the distance to the CPA is not fixed and it will depend on the length of the vessel and, even though this distance were fixed, it would be really difficult to achieve it in a real test. That definition leads to different data window period which, as can be seen in Figure 3.55, implies different sound levels obtained in the post-processing depending on the distance to the CPA. Such factor introduces uncertainties if no proper distance corrections are taken due to the variation in the distance, as it can be seen in the Table 3.15 considering a variation equal to 15%. However in order to properly take distance correction it is important to have good estimation of transmission loss, otherwise the distance correction may introduce more uncertainties than if it is not used. 74

75 Figure 3.56a.- Difference in the data window period depending on the distance to the CPA.- Hydrophones 30º Hydrophones DW1 DW2 Figure 3.56b.- Difference in the data window period depending on the distance to the CPA.- The speed of the vessel during the test is also important because it is going to determine the data window period, therefore the number of terms in the average. The reference [9] defines the averaging time considering the vessel speed and length. According to that, the higher the vessel speed the shorter the time window period and the fewer terms in the average could be used for the same frequency accuracy. Besides, this definition usually results in shorter averaging time than the definition provided by [2] and [3]. However If everything is properly controlled more terms in the average implies better repeatability. Therefore, in order to assure the same grade of repeatability the procedure must be defined to achieve similar data window periods for different tests. Nevertheless different vessels will have different operational speeds so the distance to the CPA is the only parameter that can change (under the limitations explained in the above item) to achieve the previous condition. Finally, the directivity of the source may need being taken into account for long data window since it may affect the final average. A way of accounting this is to define data windows at different angles. 75

76 4 Background noise correction. This item was explained from a physical point of view in the paragraph The measurement should be taken far from other sources so the background noise can be much lower than the vessel noise measured by the hydrophones. Besides the sea state must be suitable to assure, among other things, low background noise. The references [2] and [3] specify that if the background noise is 10dB lower than the signal measured by the hydrophones during the run no correction shall be made and if the difference is between 3dB and 10dB it should be applied the following correction. Where, L s+n/n is the signal-plus-noise-to-noise-ratio. L s+n (h i ) is the sound pressure level, in db, recorded by the hydrophone h i when the vessel is performing one run. L n (h i ) is the sound pressure level recorded by the hydrophone h i during the record of the background noise. L(h i,r j,f) is the sound pressure level by the hydrophone h i after applying the background noise correction. However if the difference between the vessel and background noise is less than 3dB the results must be discarded. The reference [9] proposes a similar specification but it recommends that the measurements must be discarded when the background noise is not 5dB lower than the vessel plus background noise. This way to proceed has several drawbacks. Background noise should be sufficiently stationary so the results of the correction applied can have enough accuracy. As a matter of fact, the background noise used for the correction is the one measured previous or after the runs, and it may be different from the actual one during the run. In this way, in order to have a given accuracy such difference cannot be higher than a certain value, which can be higher as the higher the signal plus background noise to background noise ratio is. Figure 3.57 shows how stationary the background noise should be to achieve 1dB of accuracy. 76

77 Applicability of background noise correction Variation of background noise (db) 6 5,5 5 4,5 4 3,5 3 2,5 2 1,5 1 0, Signal plus noise to noise ratio (db) Figure Variation of background noise maximum to achieve 1dB of accuracy in the background noise correction.- 5 Directivity. When the vessel is modelled as a point source it is observed that this source has directivity which depends on the value of the frequency. Generally the vessel emits less at bow aspect than at beam aspect due to the shadow effect caused by the hull. Moreover differences are appreciated when the measures are taken at keel aspect. The Figure 3.58 shows an example of a directivity diagram of a vessel for a given frequency. The directivity of the source may affect the extraction of the averaged source spectrum at 1m depending on the duration of the data window. Furthermore, in order to fully characterize the vessel as a source this directivity should be obtained and the new standards should take it into account and provide methods to compute it, however the current standards do not explicitly consider this phenomenon. merchant5: Source levels for 125 Hz Figure Directivity diagram of a vessel for a frequency of 125Hz.- In the current state-of-the-art there are several methods to compute these kinds of diagrams. However, all of them require the following points: Computing accurately the angle and range between the vessel and the hydrophones. Computing accurately the transmission loss for each third-octave band covered in the analysis. No spherical or cylindrical transmission loss can be applied because induce 77

78 large errors becoming more important in the process of obtaining the directivity diagrams. Having the suitable distance to the CPA in order to achieve a good signal-to-noise ratio to be able of computing source spectrum averages for angles far from 90 or 270º. Depending on the speed of the vessel it may be necessary to perform a Doppler Effect correction; otherwise the spectrum lines may be hidden. It requires an identification of the tonal components. If the directivity along the vertical angle is required it is necessary to take measurements at different depths or distances to the CPA. 6 Number of runs Taking several underwater radiated noise measurements under the same conditions and then taking the average as the final result is a way of reducing the uncertainties associated to the measurement and improving its repeatability. That is why the number of runs should be fixed for a given accuracy grade as the references [2] and [3] do for each side of the ship and as the reference [9] does with the two measurement taken, one per each side of the ship. The average of the different runs is usually arithmetically averaged in db. Moreover, performing runs at different distances is a way of reducing the uncertainty with regard to the transmission loss. This novel approach is not followed by current standards, and the studies performed in this task show its high potential. Besides, taking several measurements for different runs may give us a way to evaluate the repeatability and uncertainty of our measurement for the actual conditions and to establish a reject criterion for the runs according to their deviation from the average. 78

79 4. PROPOSAL FOR A NEW URN MEASUREMENT PROCEDURE 4.1. Introduction This paragraph describes the measurement procedure developed in the Task 3.1 of the AQUO project. This procedure is the result of the studies performed in the previous paragraph as well as the experimental validation of the modifications of this procedure with regard to the existing ones. These experimental verifications were performed within internal studies of the project. In particular, the following items were experimentally verified: Measurement uncertainty and repeatability. In fact this study is also shown in the paragraph 4.9 of this document. The number of runs. The goodness of performing runs at different distances. Distance to consider that the vessel behaves as a point source. Analysis of the influence of the directivity of the vessel to define properly the data window. Analysis of the repeatability achieved by different types of post-processing. Transmission loss. Suitability of the procedure to obtain experimentally the transmission loss of the test site with portable equipment. Transmission loss. Suitability of the numerical models, its sensitivity to the uncertainty of the input parameters required and experimental assessment of the methodology by computing the repeatability achieved. Numerical verification of the mathematical formula used to compute the transmission loss for B-Applications. Sea state required. Background noise for different measurement device configurations subject to different sea states. Assessment of the variation of the background noise during a trial. Number and position of the hydrophones by means of an assessment of the repeatability achieved. Experimental assessment of the accuracy of the distance measurement when a GPS system is used. Experimental verification of the technical feasibility of an alternative way of measuring the distance using the Doppler Effect. Definition of the acoustic centre. Definition of the limits of the repeatability checking defined in the paragraph Definition of the limits of the uncertainty checking defined in the paragraph Study of the different sources of uncertainties: o Hydrophone line tilt. o Influence of celerity profile. o Influence of the source depth. o Influence of the sea bottom in deep and shallow waters. o Sensitivity analysis of the numerical models. This standard procedure is applicable to surface vessels whose operating at speeds lower than 50 knots. Besides, it lies on the assumption that the vessel behaves as a point source, which requires the measurements to be conducted in the far field and then, normalized to 1m 79

80 distance from the centre plane of the vessel for further comparison. This standard provides ways of obtaining the vessel signature source level at beam aspect of the equivalent monopole source where the effects of the free surface and sea bottom are taken into account Summary of the requirements The following table summarizes the requirements for each application and grade within the scope of this document: Application A B Grade A1 A2 B1 B2 Global Measurement Uncertainty Measurement Repeatability 4dB 4dB 7dB 6dB 1.5dB 1.5dB 2dB 2dB Minimum Frequency resolution 0.5Hz if f<50hz 1%f if f>50hz One-third octave band Frequency range 10Hz-50KHz (*) 10Hz-50KHz (*)(**) 20Hz-50KHz (*) 20Hz-50KHz (*)(**) Test site Minimum sea depth (***) 50 m 0.3v 2 under the keel. 3 x ship draught aft under the keel. 200m or 2 x ship 50 m 0.3v 2 under the keel. 3 x ship draught aft under the keel. 100m or 1 x ship length. Sea state Sea state 3 Sea state 2 Sea state 3 Sea state 2 General characteristics Flat seabed. homogeneous, not seabed features Noise recording Configuration Bottom mounted Floating Bottom mounted Floating Minimum number of hydrophones Hydrophones depth See Figure 7.1a See Figure 7.1b See Figure 7.2a See Figure 7.2b Hydrophone sensitivity uncertainty Hydrophone directivity Dynamic range Hydrophone calibration System verification <3dB <3dB <4dB <4dB <2dB 80dB Every two (2) years Every three (3) years. Prior to the measurements by means of insert voltage calibration, calibrated sources,etc... 80

81 Application A B Grade A1 A2 B1 B2 Distance measurement Distance uncertainty between the vessel and surface buoy +/-2m +/-10m Maximum tilt angle 10º Field calibration Modelling / Simple law Modelling Modelling Simple law Simple law Runs and vessel characteristics Nominal distance to CPA 200m or 1 x ship length whichever is larger. Number of runs 5 3 Vessel Speed (***) Log speed Conditions to keep constant (****) Power provided by the main engines Rotational speed, pitch and load of any propeller or thrusters Load conditions Fore and aft draught Machinery working during the test Post-processing Acoustic centre location Halfway between the main engines and the propeller or Halfway between the main engines and the propeller. Data window length 100m or 1 x ship length whichever is larger. Data window angle -45º to 45º each 5º for each side. Narrow band analysis Required Optional Directivity analysis Optional (*) Upper frequency recommended 100KHz. (*) Due to the measurement device configuration used in deep waters, practical frequency range for this grade will go from 20/30Hz to 50KHz. (***) Deviation to this requirement should be explicitly indicated. (****) According to contract specifications. 81

82 4.3. Instrumentation Hydrophones, signal conditioning and recorder The hydrophones shall have a flat frequency response over the frequency range required for the application and shall be omnidirectional. In particular the hydrophones shall be linear with a maximum variation of: +/-3dB for grade A. +/-4dB for grade B. And their directivity diagram shall not present a variation higher than 2dB. Moreover, the hydrophones should have proper sensitivity, bandwidth and dynamic range to measure the ship under test. The required performance shall be obtained with the full cable length that will be used during the test. The recorder and signal conditioning shall be capable of recording data from the hydrophones without loss of accuracy. The full measurement system shall be laboratory calibrated every 24 months according to ANSI S1.20 or IEC for all required one-third octave bands for A-applications and every 36 months for B-applications. They will also be verified in the field prior to, and daily throughout, the measurements series using either insert voltage methods for all one-third octave bands, capacitance metre, Megger or a single frequency device, such as pistophone Distance measurement system A distance measurement system is required to determine, continuously throughout the acquisition of the data, the distance between the hydrophones and the acoustic centre of the vessel. The distance can be measured in two ways. The first one consists on measuring the horizontal distance between the vessel and the buoy located above the hydrophones. In this case, the position of the vessel and the buoy surface shall be recorded every second with an error of +/-2m for A-applications and +/-10 for B-applications. The first one implies the use of differential GPS whereas the latter one requires the use of standard GPS. For bottom supported configurations the buoy swing shall be lower than 10m. For bottom supported and suspended configuration the tilt angle shall be lower than 10º. If the hydrophone cable tilt angle exceeds this quantity, it can be reduced attaching weight to end of the cable. The first method is subject to many uncertainties so for grade A it is recommended to employ the second way of measuring the distance. This method consists in measuring directly the distance between the hydrophone and the vessel to avoid the uncertainty related to the tilt angle. In this case, the GPS cannot be used as the hydrophones are located into the sea, therefore other systems based on the Doppler effect, radar, etc. must be used. If this is the case, the maximum error shall be lower than 5% of the distance to the CPA. 82

83 4.4. Test site Test site requirements The place where the measurements will be performed shall be far from ship lanes or areas with dense maritime traffic so the measurement is not affected by external ships. The sea bottom shall be as flat as possible and neither sloping seabed nor seabed features shall be present in the test site as much as possible. The sea bottom shall have the same characteristics over the location where the runs will be carried out. It is recommended to measure in the absence of currents as far as possible since they may increment the level of background noise recorded by the hydrophone. The sea depth of the location shall be higher than 50m for grades A1 and B1. For grade A2, it shall require 200m or twice the ship length, whichever is larger. Grade B2 requires a see depth of 100m or once ship length whichever is greater. Besides and in any case, the depth of the location shall be enough so as to have under the keel 0.3v 2 (being v the vessel speed in m/s) or three times the ship draught after, whichever is greater, to assure a representative hydrodynamic performance of the vessel Sea surface conditions The sea surface conditions may strongly affect the measurements so they must comply with some requirements. Rough seas may increment the background noise in the area limiting the frequency range free of background noise corrections. Moreover it may cause added self generating noise in the hydrophones since the hydrophone line will be excited. Finally, the repeatability of the surface vessel s source level will be affected since rough seas cause added instability of the ship under test and its propulsion system. For all these reasons the sea state shall be 3 or lower if bottom supported hydrophones are used (grade A1 and B1) and the length of the ship under test is larger than 50m, otherwise the sea state should be 2 or lower Hydrophone deployment Grade A1 The hydrophones shall be arranged vertically in the water column and located to measure the beam aspect of the vessel under test. At least three omni-directional hydrophones shall be deployed in a bottom supported line as shown in Figure 4.1a. The grade A1 requires the deepest hydrophone to be at least 3m above the sea bottom to avoid undesirable effects due to the pulls to the base. The hydrophones shall be separated 15/20m from each other and always the closest hydrophone to the sea surface shall be more than 15m away from it. Measures shall be taken to mitigate the effect of cable strum. 83

84 Grade A2 Figure 4.1.-Hydrophone deployment for grade A1 and A2.- The hydrophones shall be arranged vertically in the water column and located to measure the beam aspect of the vessel under test. At least three omni-directional hydrophones shall be deployed, suspended from a buoy which shall be uncoupled somehow from the sea surface as shown in Figure 4.1b. The closest hydrophone to the sea surface shall be more than 40m away from the sea surface, and the hydrophones shall be separated more than 30m from each other. Finally, the deepest hydrophone shall be located not deeper than half the sea depth to minimize the effects of the sea bottom Grade B1 At least one hydrophone shall be deployed not rigidly mounted on the base and shall be approximately half depth from the sea bottom (see Figure 4.2a) Grade B2 At least one hydrophone shall be located at 50m depth from a floating buoy, which shall be uncoupled somehow from the sea surface as shown in Figure 4.2b. 84

85 Figure 4.2.-Hydrophone deployment for grade B1 and B Transmission loss In order to express the noise level measured at a reference distance for further comparison a correction shall be applied taking into account how to the sound propagates in the test location (transmission loss). This procedure envisages two possibilities to compute the transmission loss: Modelling activities: By means of the use of mathematical models that require input data such as sound speed profile, bathymetry, sea bottom characteristics, etc. Simple law: It is a mathematical formula Modelling activities Grade A1 and A2 will compute the transmission loss by means of the appropriate numerical models. These models require the input of the following data: Sound speed profile, water density and salinity during the sea trials: This is more important for deep waters than for shallow waters, which is why measurements of celerity profiles is required for grade A2. Anyway, due to its not negligible importance in shallow waters, measurement of the sound speed profile during the sea trials is recommended for grade A1. Bathymetry of the test site, sea bottom density and acoustic characteristics: The bathymetry and the acoustical properties of the sea bottom should be obtained from the Total Sediment Thickness of the World's Oceans & Marginal Seas2 database or a similar one. Unless information that is more specific is available, the sea bottom will be considered flat with only one semi-infinite layer with acoustical properties equivalent to the actual seabed structure of the test site. This information is much more relevant for grade A1 than for grade A2. 85

86 Source depth: The numerical models shall be able to handle individual point sources. Due to its importance in the results of the models, several source depths must be considered according to the following: o o For vessels whose ship draught is below 8m two different source depths must be considered. The depth of the first one shall be 1/3 of the actual aft draught, and the depth of the second one shall be 2/3 of the actual aft draught of the vessel. For vessels whose ship draught is above 8m three different source depths must be considered at 1/4, 1/2, and 3/4 of the actual draught of the vessel. These data shall be measured with enough accuracy. The extension of the area that shall be modelled to compute the transmission loss shall cover enough distance for the computation of the URN signature and directivity diagram of the vessel. The transmission loss shall be computed at each hydrophone depth every one (1) meter from 50m to the maximum distance required for the computation of the URN signature or the directivity diagram. For each point, source depth, and centre frequency of all the one-third octave bands within the frequency range, the transmission loss at the position of the receivers (hydrophones) will be computed. Then, for each point and centre frequency of all the one-third octave bands the source average transmission loss shall be computed arithmetically averaging the transmission loss obtained from all the source depths considered for each point and frequency. Where, is the source depth average transmission loss in db, at horizontal distance,frequency and for the hydrophone. is the depth of each source depth considered. is the number of source depths considered. Once it has been done for all the points of the grid, the values obtained for each frequency and hydrophone shall be used to perform a logarithmical regression analysis to obtain a range-source depth average transmission loss. This accounts the fact that, on the one hand, the vessel is a moving point source and on the other hand, the transmission loss computed previously is for a given frequency and it shall be used for the whole one-third octave band. Where, 86

87 is the range-source depth average transmission loss for a given frequency and hydrophone. is the horizontal distance between the source and the receiver. and are the parameters obtained from the logarithmical regression analysis. Two models will be used, one for low frequencies (<1KHz) and other for high frequencies (>1KHz). The following models are recommended: Low frequencies: Scooter + Fields. High frequencies: Bounce + Bellhop Simple law.the following mathematical relationship should be applied to obtain the transmission loss: Water depth lower than 100m. o For frequencies below 100Hz :. o For frequencies above 1000Hz:. o For frequencies between 100Hz and 1000Hz: where will vary linearly with the frequency being 19 for 100Hz and 20 for 1000Hz. Water depth above 100m: Where o For all frequencies: is the distance from the acoustic centre of the vessel to the hydrophone. is the transmission loss without accounting the mirror effect. Due to the fact that the aim of this procedure is to obtain the source level considering the vessel as a monopole source a low frequency correction will be made to account for the Mirror effect. Therefore, the final transmission loss to apply during the post-processing is obtained according to the following equation: Where, is the frequency. 87

88 is the sound speed in water (1515m/s). is the source depth and equal to 2/3 of the aft draught of the ship during the measurement. is 20º URN measurement Background noise The background noise shall be recorded at least before and after the runs. If possible, it is recommended to record the background noise after each run. Each record shall be at least two minutes long and the vessel shall be far enough or with its machinery completely stopped during the record Validation of the measurement location Some actions shall be taken to validate the measurement location before performing the required measurements according to this procedure. They are the following: Sea state: Sea state shall be Beaufort 3 or lower when the hydrophone bottom supported configuration is used and the ship length is larger than 50m. Otherwise, the sea state shall be Beaufort 2 or lower. Background noise: If the background noise measured by the hydrophone(s) is not 10dB lower for a reasonable quantity of one-third octave bands than the expected signal at the location of the hydrophones the test location in that moment is not valid Configuration of the different runs The vessel under test shall transit a straight line course to achieve the required distance at CPA for this run. Recording of data shall be performed from approximately the minimum of 800m or 4 minutes before the front of the vessel reaches the CPA to the minimum of 800m or 4 minutes after the whole vessel has passed the CPA. Before the vessel reaches the starting point of the record, she shall have achieved the required run conditions, and unless required by the test plan, vessel speed, machinery conditions, etc. shall be constant. Depending of the application, the configuration of the runs is different: A-applications: Six runs per each side and vessel conditions shall be performed at distances to CPA according to the Figure 4.3 with a margin of +/-10m. The nominal distance to CPA (dn cpa ) shall be 200m or 1 x ship length, whichever is greater. For some very silent vessels, this requirement may cause problems in terms of the SNR. In these cases, a shorter dn cpa can be considered taking into account that the closer the distance, the less realistic the assumption that the vessel is as a point source is. 88

89 The direction of the course of each run shall not vary more than 5º among them to minimize the differences in the vessel hydrodynamic performance due to the effects of the currents. B-Applications: Figure 4.3.-Test course configuration for A-applications.- Three runs per each side and vessel conditions shall be performed at distance to CPA according to the Figure 4.4 with a margin of +/-10m. The nominal distance to CPA (dn cpa ) shall be 1 x ship length or 200m, whichever is greater. For some very silent vessels, this requirement may cause problems in terms of the SNR. In these cases, a shorter dn cpa can be considered taking into account that the closer the distance, the less realistic the assumption that the vessel is a point source is. The direction of the course of each run shall not vary more than 5º among them to minimize the differences in the vessel hydrodynamic performance due to the effects of the currents. 89

90 Figure 4.4.-Test course configuration for B-applications Repeatability checking This paragraph is aimed at describing the procedure to check that the repeatability of the actual measurement is according to what is required for the grade considered. This test is only applicable for grade A. As seen in Figure 4.3, three runs at 1 x dn cpa should be performed. The aim of this is to check that there are no significant variations in the measured source levels obtained from these runs under the same conditions. In particular, the average of the differences in each one-third octave band among the resulting source levels obtained from the runs performed at the closest distance shall not be higher than 1.5dB Uncertainty checking As can be seen in the equation below the uncertainty of a given measurement procedure will be mainly determined by the uncertainty of the distance measurement system, the noise measurement system, the transmission loss and the variability of the vessel as a source. This paragraph will detail a way of quantifying the actual uncertainty of the measurement using the data obtained from the very same measurement. This test consists on comparing the source level obtained from the runs performed at different distances. In particular, the average of the differences in each one-third octave band among the resulting source levels obtained from these runs shall not be higher than 3dB for grade A and 6dB for grade B. 90

91 Vessel conditions Vessel conditions shall be fixed before the test. The following parameters shall be defined and kept constant during the test: Vessel speed through water Power provided by the main engines Rotational speed, pitch and load of any propeller or thrusters. Load conditions. Fore and aft draught. Machinery that are working during the test. The vessel conditions for the test shall be defined according to contract specifications. Commonly it will be as close as possible to the expected operating profile of the vessel. The vessel shall carry a load within her normal load range. For vessels with large variations in the load conditions during her normal operation, measurements at two loading conditions (full load and ballast) are recommended since the underwater noise radiated by the vessel will strongly depend on her operational conditions (different depth of the propeller, power requirements, etc). Therefore, these two measurements will represent the characteristic range for the underwater radiated noise level of the vessel. 91

92 Test sequence The following table tries to summarize the sequence of actions to perform during this measurement procedure. Action Check the weather conditions Deployment of the hydrophones Background noise measurement Measurement of sound celerity profile and bottom characteristics. Verification of the level of background noise Comments The same location as the previous day. Record of the background noise When applicable. Checking that the level of background noise is 10dB lower than the expected noise of the vessel at the hydrophone position for a reasonable amount of one-third octave bands. Configure the ship according to run Check the distance measurement system Runs Notify that the measurement system is operational. Notify that the ship configuration is consistent with the test procedure, start the run. Start the recording at start data record point position. Inform the vessel crew that the recording has started. Stop the recording at end data record point position and notify the end of the run. Inform the vessel crew that the recording has stopped. Inform the measured ship if the run is OK or need to be repeated. Background noise measurement Measurement of sound celerity profile Check the weather conditions Record of the background noise When applicable. Note variations in the weather conditions during the measurement Table 4.1.-Test sequence.- 92

93 4.8. Post-processing Acoustic centre This procedure will consider that the acoustic centre is collocated for all frequencies and it will be halfway between the propeller and the main engines on the centre plane of the vessel and 2/3 ship draught depth Data window Multiple data windows centred at different angles from -45º to 45º (see Figure 4.5) will be defined to obtain for each one of them an averaged spectrum to take into account somehow the directivity of the vessel. The data window length (DWL) for each data window will be equal to the ship length (SL) or 100m whichever is greater, and its middle instant occurs when the acoustic centre goes by the centre of the respective data window. Figure 4.5.-Definition of the data window.- For each data window,a linear average one-third octave band spectrum of the sound pressure level measured by each hydrophone will be obtained in accordance with the IEC 1260:1995 and ANSI S Background noise The fact that background noise will be always present during the measurement of the URN signature of the vessel introduces error whose magnitude will depend on the signal-plusnoise-to-noise ratio (SNR). When this value is smaller than a certain value, corrections can be applied, however the magnitude of the error of this correction will depend on how much the background noise varies. This procedure fixes the limit of the SNR from which the background noise can be neglected and how much the background noise can vary so the corresponding correction can be applicable to obtain a maximum uncertainty because of this phenomenon of 2dB for A- applications and B-applications. For instance, if the signal plus background noise measured is 100dB and the background noise in the area is 90dB, the actual noise coming from the vessel has a level of 99.5dB, so 93

94 an error of about 0.5dB would be made if the background noise was neglected. In general, the error caused by not considering the background noise is: So, the signal-plus-noise-to-noise-ratio of each hydrophone shall be at least 10dB for each one-third octave band in order not to take it into account. See equation below to obtain the SNR for each hydrophone. Where, is the signal-plus-noise-to-noise-ratio. is the sound pressure level, in db, recorded by the hydrophone vessel is performing one run. when the is the sound pressure level recorded by the hydrophone the background noise. during the record of If it is between 3dB and the 10dB and sufficiently stationary, corrections according to the following equation shall be made in the affected one-third octave bands and properly indicated in the report. Where, is the sound pressure level by the hydrophone after applying the background noise correction. As it was said before, the background noise correction can be performed when background noise is sufficiently stationary. That relies on the fact that when the correction is performed an error is made because the background noise recorded before the trials will not exactly be equal to the background noise existing during the run. That error will depend on how much the background noise has varied according to the following equation: Where, is the estimated variation of the background noise. It can be computed as the difference between the background noise at the beginning and at the end of the trial. 94

95 Considering the equation above and taking into account that the maximum error permitted because of the background noise correction is fixed to 2dB, the graph of the Figure 4.6 shows the maximum variation of the background noise per each one-third octave band allowed so the correction can be performed. Otherwise, it would be a deviation from this standard. For example, if we have a SNR equal to 4 db for a particular one-third octave band; then, we cannot have a variation between the level of background noise measured at the beginning and at the end of the trial for that particular band higher than 2dB, so we can perform the background noise correction. Figure 4.6.-Applicability of background correction Specific post-processing For each hydrophone and data window a distance correction to refer the measurements at 1m shall be made using the numerical range-source depth average transmission loss according to this equation. Where is the one-third octave band spectrum of the sound level received by each hydrophone is the one-third octave band spectrum of the source level computed from the hydrophone is the horizontal distance from the hydrophones to the midpoint of the corresponding data window for grade A or the total distance for grade B. is the centre frequency of the corresponding one-third octave band. is the horizontal angle of the data window. 95

96 is the transmission loss at distance.for the one-third octave band whose centre frequency is obtained for the hydrophone Then, the power average of the source level obtained from each hydrophone is obtained according to the following equation.. Where is the power average one-third octave band spectra of the source level obtained by each hydrophone. is the number of hydrophones deployed. Then, the envelope of the resulting power average of the source levels obtained for each data window shall be computed according to the following equation: Where is the envelope, corresponding to the run, of the family of curves made up by the resulting power average the source levels obtained for each data window whose angle is. Finally, all the runs of data are then arithmetically averaged to determine the final sound source value for each side. Where is the resulting signature source level for the side portside or starboard., which can be either is the number of runs per each side. 96

97 Figure 4.7.-How to obtain the directivity enveloping spectrum per each side of the vessel.- Finally the power spectral density of both directivity enveloping spectra will be computed to obtain the resulting signature source level per each side: Where, is the bandwidth of the one-third octave band considered. each side. is the power spectral density of the resulting signature source level per The Figure 4.7 summarizes this way to proceed Narrow band analysis For A applications narrow band analysis shall be performed for noise signals recorded and the level of the main spectral narrow band components shall be showed in the corresponding report. The inherent bandwidth of lines in radiated noise spectra will be function mainly of the stability of the source and the Doppler Effect. Each data window will be divided into 2s samples and for each one a narrow band spectrum shall be obtained windowing the signal by means of a Hanning window. Then before averaging the spectra, the frequency resolution of each one shall be adapted as follow to avoid that the frequency lines are hidden because of the Doppler Effect: 0.5Hz for frequencies below 50Hz. 1% for frequencies above 50Hz. 97

98 If the user wants to perform narrow band analysis for B-applications wider frequency resolution is valid. Spectra obtained for a given data window will be RMS averaged with linear weighting. The spectral narrow band component measured by each hydrophone deployed shall be corrected to 1m using the transmission loss applicable according to the following equation: Where is the narrow band component obtained from the signal measured by the hydrophone at distance and data window whose angle is. is the transmission loss at distance.for the one-third octave band the frequency of the component belongs to obtained for the hydrophone is the narrow band component obtained from the signal measured by the hydrophone and data window whose angle is referenced at 1m from the source. If more than one hydrophone is used then the spectra of the different hydrophones shall be power average as follow: Where is the power averaged narrow band spectrum taken and data window whose angle is referenced at 1m from the ship hull. Then, the maximum of the amplitudes obtained for this component among all the data windows shall be computed: Where is the maximum amplitude obtained among all the data windows for the narrow band component of interest. Finally the final amplitude for the narrow band component of interest will be computed averaging arithmetically the amplitudes obtained for each run according to the previous equation: 98

99 Where is the final amplitude for the narrow band component of interest at either portside or starboard (which is defined by Directivity Due to the minimum required distance between the hydrophones and the CPA of the run and the sea-depth allowed by this procedure, just the slice of the hemisphere representing the directions the vessel emits at can be computed (see Figure 4.8). Fortunately, at larger distances of the vessel (commonly tenths of sea depths) the beam aspect of the vessel is much more important than the keel aspect. For this reason the horizontal directivity will be obtained averaging the values along the vertical. The directivity diagrams shall be obtained for all the one-third octave bands of the frequency range applicable unless the user requires something different. In order to obtain the directivity diagrams of the vessel the following post-processing shall be applied. 1. Each run will be divided into two second samples whose horizontal angle is α i and horizontal distance to the hydrophones is d i (see Figure 4.9). 2. For each sample and hydrophone deployed, the one-third octave band spectrum of the sound pressure received by the corresponding hydrophone shall be obtained in accordance with the IEC 1260:1995 and ANSI S The spectra obtained from the previous step shall be referenced to 1m using the transmission loss applicable and the distance d i specified in the step 1. Then the power average of the results obtained from each hydrophone shall be computed. 4. Then, the amplitude of the one-third octave bands will be extracted obtaining the value. 5. Finally, the amplitude of each frequency of interest of all the samples with certain, and whose signal to noise ratio is above the limits specified will lead to obtain the directivity diagram as shown in the Figure

100 Figure 4.8.-Directivity hemisphere covered by the procedure.- Since runs at four different horizontal distances will be performed, four directivity diagrams will be obtained, each one corresponding to each horizontal distance. These four diagrams can be compared with each other to check the validity of the assumption of the vessel acting as a point source. Figure 4.9.-Post-processing for directivity diagrams.- Figure Directivity diagram.- 100

101 If values of the vessel directivity close to bow and stern aspects are required, it is necessary that the CPA of the run will be closer than the nominal distance to CPA given by the procedure. The worse the signal to noise ratio is the closer the hydrophones need to be. Anyway, the distance d i shall always be higher than the nominal distance to CPA so the spectrum obtained for the corresponding sample could be valid UNCERTAINTY AND REPEATABILITY STUDY Theoretical study This paragraph will summarize the study of the uncertainties associated to the procedure proposed in this paragraph: Firstly an evaluation of the individual sources of error will be performed. To this end, different studies, both theoretical and experimental have been done. In summary, the following items have been considered as a source of error: o Uncertainty regarding the distance measurement: It was divided into two items, one associated to the accuracy of the GPS system used, and other associated to the uncertainty caused by the hydrophone line tilt. o Uncertainty associated to the instrumentation; both, the one associated to the frequency response of the hydrophones and the one associated to their directivity. In spite of indicating these values of uncertainty (according to what is specified in the procedure) they are not considering in the computation of the final uncertainty as they commonly affects only the very high frequency. o Uncertainty associated to the transmission loss. With the aim to estimate this value a sensitivity analysis of the model was performed considering the uncertainty of its input parameters. o Uncertainty associated to the post-processing. Only, the uncertainty associated to the background noise correction (when it has been applied) has been considered as the other factors (acoustic centre location, effect of vessel direcitivity, ) have shown to have negligible importance with regard to other factors. Finally, the final uncertainty associated to the measurement is obtained combining the individual uncertainties obtained previously. Before detailing the development of this work, it is important to take into account that in this study the uncertainty is computed so 95% of a hypothetical large list of URN measurements of the same vessel are within the limits made up of, where is the measured URN source level of the vessel and is the uncertainty associated to this measurement. The Table 4.2 shows the final values obtained from this theoretical study for the different source of uncertainties as well as the final value for the measurement procedure. The Table 4.3 shows similar information for the analysis of the repeatability of the measurement. 101

102 Grades A1 A2 B1 B2 Distance Measurement Accuracy 0.5dB 1dB 1.5dB 1.5dB Noise recording accuracy 2.5dB (1) 2.5dB (1) 4.3dB (1) 4.3dB (1) Transmission loss 3dB 3dB 7dB 6dB Vessel 1dB 1dB 1.2dB 1.2dB Post processing 2dB 2dB 2dB 2dB Total 4dB 4dB 7dB 6dB Table Final uncertainty for each item in each one of the grades defined.- (1) Due to the fact that this uncertainty is only important for high frequencies, it is not accounted in the value of the final uncertainty of the measurement. (2) Grades A1 A2 B1 B2 Distance Measurement Accuracy 0.5dB 1dB 1.5dB 1.5dB Noise recording accuracy 0dB 0dB 0dB 0dB Transmission loss 0dB 0dB 0dB 0dB Vessel 1dB 1dB 1.2dB 1.2dB Post processing 2dB (1) 2dB (1) 2dB (1) 2dB (1) Total 1.2dB/2.2dB (1) 1.5dB/2.3dB(1) 2dB/3dB(1) 2dB/3dB(1) Table Final repeatability of the grades defined in the URN measurement procedure.- (1) Only when the background noise correction is applied Experimental study This paragraph will detail the tests performed in the FS4 aimed at the experimental verification of the real uncertainty of the measurement of the measurement procedure described here. The aim of this trial is, on one hand, to support experimentally the suitability of the procedure, and on the other side, to obtain an experimental value of the actual repeatability and uncertainty of the measurement which supports the theoretical values obtained in the paragraph Firstly, the experiment will be described and finally, the results will be shown and analysed. During the second full scale measurements the underwater radiated noise of the vessel FS4 was measured twice according to the measurement procedure in the first day to assess the repeatability and once more the following day under as different conditions as possible (sea state, heading, variable bathymetry, etc.), to assess the uncertainty of the measurement. The Figure 4.11 shows the different locations and the schematic directions of the runs performed in these three measurements. As can be seen in this figure, the runs performed during the first day were perpendicular to the ones performed during the second day. The first one corresponds to constant bathymetry and the second one to variable bathymetry. 102

103 Figure General overview of the trial to quantify uncertainty and repeatability.- The Figure 4.12 shows the two final URN signatures obtained, after applying the whole postprocessing, during the first day of the trial. Due to the rough sea state during the first day the frequency range below 100Hz had to be discarded as it was background noise affected. This is why a red shaded zone is present in this frequency range. The Figure 4.12 clearly shows the high repeatability of the measurement procedure. Figure Signatures taken during the same day to evaluate repeatability.- The Figure 4.13 shows the signature taken during the second day and one of the signatures taken during the first day so we can assess the uncertainty of the measurement. This figure shows worse accordance between these two signatures than what Figure 4.12 shows. This fact was expected and confirms that there were changes in the condition of the measurement (celerity profile, effect of bathymetry, sea state, etc.). Moreover, the average difference between these two signatures is about 2dB, which shows that the uncertainty of the measurement procedure looks reasonable and does not contradict the uncertainty obtained theoretically in the paragraph Therefore these results seem to be in accordance with the theoretical values obtained for the repeatability (1.5dB) and uncertainty (4dB) and support, by means of experimental data, the final definition for the underwater radiated noise measurement procedure. 103

104 Figure Signatures taken during different days to evaluate the uncertainty.- 104

105 5. CONCLUSIONS A preliminary study to identify the needs concludes that the measurement procedure shall address three topics: Sources identification, assessment of the URN of the vessel to compare with future limits and measurements of rough estimates of the URN of the vessel for traffic management and development of URN patterns. The measurement procedure developed here covers the first two topics. Other important need identified comes from the fact that large areas of the European continent are surrounded by shallow waters, so the procedure shall address measurements in shallow waters. On the other hand, in other areas, performing measurements in shallow waters is rather difficult so the measurement procedure shall also address measurements in deep water. After carrying out the study of the key factors/parameters affecting the measurement of the underwater noise radiated by the vessels, reviewing how the current standards deal with them and developing of a new measurement procedure suitable for both, shallow and deep water, the following conclusions can be drawn: One of the most important deficiencies/setbacks of the current standards is the lack of a paragraph dealing with the uncertainty and repeatability associated to the procedure or the lack of experimental data or facts supporting the values claimed by some of them, which in some cases are rather optimistic. Consistent theoretical, numerical and experimental studies have been performed to support the uncertainty and repeatability claimed by this procedure. Moreover, individual estimations for each one of the main topics of the measurements has been obtained. This enables us to see which topics require more effort and which do not. In particular, the computation of the transmission loss is the most important source of uncertainty as well as the measurement of the distance. Finally, the procedure provides means of checking experimentally that the value of uncertainty and repeatability of the measurement is under control, and therefore the uncertainty specified in the procedure is valid for the final measurement. Most of the modifications introduced by the procedure developed in this deliverable as well as certain key requirements coming from existing standards have been experimentally or numerically verified. Moreover, the levels of uncertainty and repeatability of the measurement procedure have been specifically checked during a real measurement being in accordance with the values obtained by the analysis performed to study these topics. After the experimental results, we can conclude that the final measurement procedure developed in this deliverable complies with the initial aims. It is also true, that some topics can be improved (see below) but this procedure definitively is a real progress beyond the current state of the art. In particular, the exhaustive study of its uncertainty and repeatability and the experimental verification of some important topics like the minimum distance to the CPA for one run, the improvement regarding with the requirement of performing runs at different distances, the study of the transmission loss and the low frequency masking. 105

106 On the other hand, there are also topics that can be addressed in the future to support and improve the procedure for measuring the underwater radiated noise of the ships, in particular: This deliverable has not dealt with the need of measuring rough estimates of the URN signature of the vessel for traffic management without disturbing its route. It was a need identified initially in this deliverable that, in spite of not being specifically studied in this task, should be covered with a standard procedure to perform this kind of activities. On the other hand, more experimental data of uncertainties and repeatability, as the one described in the paragraph 4.9.2, should be collected to support more strongly the uncertainty and repeatability claimed by the procedure developed here. Indeed, due to the budget and time constraints, the uncertainty and repeatability of the measurement procedure was experimentally verified with the measurement of four vessels in two places. In order to have a more statistically significant support, more data of this type are required. Transmission loss has been identified as the biggest source of uncertainty. The measurement procedure developed here introduces important novelties with regard to the computation of this topic and similarly, what is probably more important, estimates the uncertainty associated to it. Anyway, deeper studies of how to model the sea bottom, how to deal with a moving source and possible different models of the source may be beneficial to develop a way of computing the transmission loss more accurately. One of the studies carried out in this deliverable suggests that the current measurement layouts (bottom supported and floating configuration) are not able to measure below 10Hz, which makes impossible to measure the blade pass frequency for certain kind of vessels. Moreover, rough sea states (3 or higher) makes difficult to measure in the low frequency range (below 40Hz) in the case of silent vessels in compliance with the ICES Nº 209. The design of new measurement devices insensitive to the sea state as well as developing a procedure to measure more closely the vessel (near field) should be studied to overcome this problem. The study of the last topic will imply to develop source models accounting for the characteristics of the vessel as a source in the near field. Other possible solution to deal with the problem previously described may be the introduction of the concept of distance of acoustic invisibility, which is the maximum distance at which the vessel can be listened for a certain frequency range. That is, the maximum distance where the sound pressure level measured in this frequency range when the vessel goes by at that distance is sufficiently above the background noise measured by the same recording system. This concept, among other things, would permit to assess the goodness of the signature of the vessel in the low frequency range, which is really difficult to measure at the large distances required by the current and new procedure as seen in previous paragraphs of this deliverable. 106

107 References [1] NATO Standardization Agreement N 1136 (STANAG), Standards for use when measuring and reporting radiated noise characteristics of surface ships, submarines, helicopters, etc. in relation to sonar detection and torpedo risk, May 29, [2] AMERICAN NATIONAL STANDARD - Quantities and Procedures for Description and Measurement of Underwater Sound from Ships Part 1: General Requirements. ANSI-ASA S /Part1 [3] ISO/DPAS "Acoustics Quantities and procedures for description and measurement of underwater sound from ships Part 1: General requirements" [4] Ainslie, M.A., TNO-DV 2011 C235 - Standard for measurement and monitoring of underwater noise, Part I: physical quantities and their units [5] Madsen 2005 (JASA) [6] ANSI 1994 (Acoustical Terminology) [7] Hazelwood, R.A. and Connelly, J. Estimation of underwater noise a simplified method. Int. J. Soc. Underwater Tech., Vol. 26, No. 3, 2005, pages [8] Michael Bathiarian, M., A standard for the measurement of underwater noise. NOAA Vessel Quieting Symposium. [9] DNV. Part 6, Chapter 24 - Silent Class Notation. Rules for Classification of Ships. s.l. : DNV, 2010 [10] Gloza I., Transmission of low frequency sounds from ships into water environment. Acoustics 08. [11] Alzir, C., Marin-Curtoud P., Correction factor for radiated noise measurement in shallow waters. Undersea Defense Technology Conference, Hamburg, [12] Bong-Chae Kim et al. Measurement of low-frequency underwater noise by a selfrecording hydrophone. [13] T. Gaggero, M. Van Der Schaar, E. Rizzuto and M. Andre. Ship underwater noise emissions: Uncertainties in the measurements and in the effects on the marine environment. [14] J.R. Vidal Bosch. Teoría de olas y comportamiento del buque del mar. [15] R.J. Urick. Principles of Underwater Sound. Peninsula Publishing, New York, [16] F. Jensen, W. Kuperman, M.Porter. Computational Ocean Acoustics. American Institute of Physics, New York

108 [17] Measurement of underwater noise arising from marine aggregate dredging operations. Final Report. [18] Jason I. Gobat Reducing Mechanical and flow-induced noise in the surface suspended acoustic receiver. [19] Deliverable On site Measurements (SILENV Project). [20] Xavier Lurton. An introduction to Underwater Acoustics. Principles and applications. [21] International Association of Oil and Gas producers. Fundamentals of underwater sound. Report nº 406. May [22] L. Brekhovkikh, Y. Lisanov, Fundamentals of Ocean Acoustics. Springer-Verlag, [23] W.S. Burdic, Underwater acoustic system analysis. Prentice-Hall Signal Processing Series, [24] Mark V. Trevorrow, Boris Vasiliev & Svein Vagle. Directionality and maneuvering effects on a surface ship underwater acoustic signature. [25] Ainslie M. A., Springer-Praxis, Chichester. Principles of Sonar Performance Modelling (2010). [26] ICES Cooperative Research Report Nº 209. Underwater noise of research vessels, review and recommendations. ISSN , May [27] Deliverable Noise & Vibration label proposal (SILENV Project). [28] MARINE STRATEGY FRAMEWORK DIRECTIVE, Task Group 11 Report, Underwater noise and other forms of energy, APRIL 2010, M.L. Tasker, M. Amundin, M. André, A. Hawkins, W. Lang, T. Merck, A. Scholik-Schlomer, J.Teilmann, F. Thomsen, S. Werner & M. Zakharia. Prepared under the Administrative Arrangement between JRC and DG ENV (no /2010), the Memorandum of Understanding between the European Commission and ICES managed by DG MARE, and JRC s own institutional funding Walree. 108

109 A. ANNEX A - EUROPEAN WATERS ENVIRONMENTAL DATA The purpose of this annex is to present some general environmental data in relationship with underwater acoustics, and a few examples in order to support the analysis of the present study. Data is split into general data (sea bottom profile and characteristics, meteorological data, etc ), and data directly related to acoustic propagation and measurement issues (profile of speed of sound along depth, ambient noise). All information shown here must be considered as statistically averaged values, as data can vary significantly depending on local weather conditions and sea state. A.1 GENERAL DATA Figure A-1 shows a contour plot of sea bottom level. It allows distinguishing shallow waters and deep waters, and the geographical limit of continental shelf. Isovalues for the contour plots are: 50 m, 100 m, 500 m, 1000 m, 2000 m, 2500 m, 3000 m, 3500 m, 4000 m and 4500 m. If we take 100 m as the limit for shallow waters, we can see that they are mainly located in the English Channel, the North Sea and the Baltic Sea. Figure A-1 shows also two sample locations (red dots) where acoustical data is available, one for deep waters, one for shallow waters. Figure A-2 shows the characteristics of seabed. Rock is in red colour, Gravel or small stones generally in green or brown colour. Sand is shown generally in orange or yellow colour. Other cases refer to different kinds of mud. In most cases, deep waters seabed is muddy, and shallow waters seabed composed of sand or gravel. Figures A-3 shows an example of map of water temperature at sea surface. This data is of importance regarding heterogeneity of water masses and sound propagation in the ocean. Figure A-4 gives an average density of ships. This data can be related to sea ambient noise regarding traffic. Other data, not shown here, could be displayed, such as average wind speed, sea currents (permanent or tidal), etc. 109

110 Figure A-1.-Sea bottom level.- 110

111 Figure A-2.-Geological seabed map, colour corresponding to different types of seabed (source : 111

112 Figure A-3.-Sea surface water temperature acquired on 24 May 2006 (source : 112

113 Figure A-4.- Density map of ship traffic based on AIS data from mid-december 2012 to mid-january The colour scale is logarithmic and represents the average number of ships present in a 1km 2 cell at any time. Source Quiet-Oceans based on Marine Traffic website.- 113

114 A.2 ACOUSTICAL DATA Data will be shown here for the two sample locations identified on Figure A-1, one for deep waters, and one for shallow waters. Acoustic propagation at sea depends greatly on sound speed profile along depth. Speed of sound depends mainly on salinity, depth, and temperature. As temperature varies along time, the sound speed profile varies also. Then, it is important to get this information for different periods of the year. Figure A-5 gives the information (average, minimum, maximum) for the two sample locations and three months in the year (March, August and November). Note the variability for the shallow waters. Ambient noise level along frequency is key information when performing acoustic measurements at sea, the quality of the result depending on signal to noise ratio. Figure A-6 gives the isotropic equivalent ambient noise spectral level (db ref. µpa 2 /Hz) as a function of frequency, for the two sample locations. Isotropic equivalent noise means the noise sensed at a given location by an omnidirectional hydrophone. Noise level for shallow waters is higher in the low frequency range because it is affected by ship traffic noise. 114

115 Figure A-5.-Sound speed profiles.- 115

116 Figure A-6.-Ambient noise level as a function of frequency.- 116

117 A.3 PROPAGATION LOSSES A computer model has been used to predict propagation losses (in db) as a function of depth and range at a given frequency (1000 Hz), for different environments presented previously. Note that the scale for depth and range are not the same. The model uses ray theory and takes into account acoustic reflection on sea surface and seabed. In each case, two figures are shown, one for large propagation distances (commonly used for sonar system performance studies) and one for smaller distances (more relevant for URN measurement purposes). The depth of the sound source is 5 meters. For the deep waters case in summer (Figure A-7), the acoustic rays tend to plunge towards sea bottom, and a convergence zone appears. For low distances, sound propagates regularly with a spherical spreading behaviour. In winter (Figure A-8), the propagation loss pattern is more complicated due to the shape of the sound speed profile. On the other hand, at short distances, propagation remains similar. For the shallow waters case (Figures A-9 and A-10), multiple reflections on sea surface and bottom change strongly the propagation losses by comparison to the deep waters case. Propagation loss is smaller close to the surface. We recall that here the frequency is 1000 Hz. At lower frequencies, another model should be used, for example based on normal modes. At shorter distances, the acoustic pressure tends to be constant on a vertical line, which is characteristic of cylindrical propagation instead of spherical propagation. This effect is more visible on Figure A-10). In some cases, a layer of water close to the surface has a speed of sound slightly smaller than at greater depths. Then, a surface propagation duct appears, with a concentration of acoustic waves and higher sound levels. This effect is shown on Figure A-11 with the limit of the duct at 30 m depth. 117

118 Figure A-7.-Propagation losses for location N 1 in summer at Hz.- 118

119 Figure A-8.-Propagation losses for location N 1 in winter at Hz.- 119

120 Figure A-9.-Propagation losses for location N 2 in summer at Hz.- 120

121 Figure A-10.-Propagation losses for location N 2 in winter at Hz.- 121

122 Figure A-11.-Propagation losses in shallow waters at 1000 Hz with a surface propagation duct 122