Variability of SCUBA diver s Acoustic Emission

Transcription

1 Variability of SCUBA diver s Acoustic Emission Dimitri M. Donskoy*, Nikolay A. Sedunov, Alexander N. Sedunov, Michael A. Tsionskiy Maritime Security Laboratory, Stevens Institute of Technology 711 Hudson Street, Hoboken, NJ 73 ABSTRACT Knowledge of the variability of the acoustic emission characteristics from SCUBA divers is critically important for designing and operating a passive acoustic SCUBA characterization system. Using modeling and experimental measurements in a controlled environment, we identified key source factors influencing the variability of the acoustic emission parameters including Source Band Level (SBL), Spectral Power Density (SPD), and breathing periodicity or emission modulation frequency. The key factors are: equipment, specifically, the design of the first stage (high pressure) regulator and its service life; diver s experience and training; and, finally, operating conditions of the equipment and diver, i.e. tank air pressure and the diver s motion activity. We found, for example, that the SBL could vary as much as 16 db depending on the equipment used and up to 15 db depending on the diver s intensity of motion. Keywords: SCUBA, divers, acoustic emission, variability, source level, breathing periodicity 1. INTRODUCTION Recently, [1], we have shown that the primary mechanism responsible for acoustic emission from SCUBA equipment is a turbulent air flow pressure fluctuation during an inhale phase of breathing. These turbulent fluctuations occur during the compressed air expansion mostly in the high-pressure (first stage) regulator, in its valve and channels. The fluctuations excite structural vibrations of the valve housing causing acoustic emission into surrounding medium. In subsequent studies, [2], we conducted a number of tests measuring acoustic emission from major SCUBA elements: first stage regulator, air tank, and the entire SCUBA. These tests clearly demonstrated that the main contributor to the underwater emission is not the tank (as previously believed because of its large surface area and numerous resonances), but the first stage regulator. This finding was somewhat surprising because the relatively small size of the regulator did not make it an effective acoustic source. However, despite its small size, regulator vibration is much stronger than vibration of other parts, making it the primary source of emission. The objective of the reported investigation is to identify the primary factors influencing the key characteristics of the underwater acoustic emission at the source only. Here we do not consider transmission losses, any effects of environmental conditions, etc. The key characteristics of the emission currently utilized for the passive acoustic detection and identification of SCUBA divers are Source Band Level, SBD, defined as total received acoustic power in defined broad frequency band; Spectral Power Density, SPD, or narrowband T spectral distribution of the received signal; Time and periodicity, T, defined by diver s breathing pattern. Fig. 1 shows a typical EXHALE INHALE spectrogram of the acoustic signal emitted Fig.1. Typical spectrogram of the SCUBA acoustic emission. *ddonskoy@stevens.edu; phone ; fax ; Frequency Optics and Photonics in Global Homeland Security IV, edited by Craig S. Halvorson, Daniel Lehrfeld, Theodore T. Saito Proc. of SPIE Vol. 6945, , (28) X/8/$18 doi: / SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol

2 by SCUBA, illustrating broadband nature of the emission, its periodic pattern, and dominating emission during the inhale phase of breathing. Numerous laboratory and field tests demonstrated that these acoustic characteristics vary widely depending on a number of factors. Both SCUBA equipment and the diver are equally important factors affecting the emission. We identified that the equipment related factors are: design of a high pressure regulator and its service life and, in a lesser degree, the tank air pressure. The diver s factors are: his experience and training as well as his motion activity. 2. ACOUSTIC EMISSION FROM HIGH PRESSURE REGULATORS All the tests were conducted with live divers and fully assembled SCUBA gear in Stevens Davidson Laboratory 1 m long tow tank. Fig.2 shows a diagram and pictures of the setup designed to identify the source of the acoustic emission and its spectral characteristics. In this setup, the diver was out of the water and only various parts of SCUBA were submerged. For example, if only the first stage (high pressure) regulator is submerged, as shown, only the regulator s acoustic emission is recorded. Thus, by submerging only the first stage regulator, only the air tank, or both, we were able to identify the primary acoustic source by comparing signals emitted from the regulator, tank, and whole assembly, respectively. Fig.3 shows an example of the recorded signals confirming that the emission directly from the high pressure regulator dominates over the emission from the tank. Reg. 2 Scuba tank Reg. 1 Hydrophone 1 Reson TC414 Hydrophone 2 Reson TC414 Hydrophone 3 Reson TC414 1ft 4ft 3ft Water Fig. 2. Diagram and pictures of the experimental setup #1 designed to identify the source of acoustic emission and its spectral characteristics. Proc. of SPIE Vol

3 Fig. 3. Signals emitted by a high pressure regulator (left) and an air tank (right). These tests were conducted for a variety of regulators and tanks. A total of seven different types (balanced and unbalanced piston type, balanced and over balanced diaphragm type) of SCUBA regulators were tested. The emission signals from each regulator were recorded for different tank pressures: 3psi, 2psi, 1psi, and 5 psi. Fig. 4 shows measured total emitted power, SBL, from all seven regulators as a function of the air pressure in the tank. 15 SPL, db re 1 1m.. SBL db U.S. R1-1 Divers/ Conshelf 21 Mares R1-2 / MR12 III Apex R1-3 / TX5 Beuchat R1-4 / V5 Scubapro R1-5 / MK25T G25 Scubapro R1-6 / MK25 Scubapro R1-7 / MK Air Tank Pressure, psi Fig. 4 Total emitted power from seven different type high pressure regulators at different air pressure in the tank. Proc. of SPIE Vol

4 This data shows significant variability of SBL: up to 16dB between quietest regulator R1-7 and one of the loudest R1-6. Tank air pressure has a smaller effect on SBL variability amounting for approximately 3 db. In narrow frequency bands, the variability could be even higher: as high as 37 db. During its useful life, the regulators are subjected to wear and tear, repairs, adjustments, etc. All of that may contribute to variability of the acoustic emission from the same type of regulator. In order to get a sense of this variability we tested three regulators of the same model. These measurements took place in the same laboratory tank with a diver and all SCUBA gear underwater. The acoustic emission from all three regulators was measured consecutively using the same diver motionlessly lying near the bottom of the tank, Fig.5. The data for each regulator was recorded for two minutes of normal breathing. The measurements yielded app. 5 db SBL variability and 1 db SPD variability. 3. EFFECT OF REGULATOR SERVICE LIFE a 4. VARIABILITY OF ACOUSTIC EMISSION ASSOCIATED WITH DIVER S EXPERIENCE/TRAINING AND MOTION ACTIVITY Four divers participated in this test: Diver #1 was very well trained with over 2 years of commercial diving experience, Divers #2, 3, and 4 had 1-5 years of recreational diving experience and minimum training. Each diver used the same SCUBA equipment, under the same air pressure and was in the same position near the bottom of the tank, at the same distance from measuring hydrophones, Fig.5. For each diver, the data was recorded at three levels of activity: no motion, light flapping, and heavy flapping. Significant variability of acoustic emission depending on the diver s experience/training and motion activity was observed in these tests. Figures 6-8 show waveforms of the acoustic emission signals recorded from each diver at each level of activity. These recordings demonstrate signals during exhale and inhale phases of breathing. There is a number of interesting observations that could be made from these recordings. First, there is a noticeable difference in breathing patterns and emission amplitudes between the experienced/trained and the inexperienced divers. Second, the motion activity has a pronounced effect of the periodicity and intensity of the acoustic emission. Third, only the signal associated with the inhale phase of breathing is increased with the increased motion activity. The signal emitted during exhaling is not changed regardless of motion activity. This is yet another confirmation that inhaling and associated sound generating mechanism is a primary source of SCUBA acoustic emission in the studied frequency band. Fig. 9 demonstrates SBL variability due to divers experience/training and motion activity. The estimated SBL variability determined from this test is the following: Experience/Training: (SBL) = 6 db, Motion Activity: Fig.5. Diver testing position (SBL) = 15 db Proc. of SPIE Vol

5 1. pi to.6. _.4: - Expert diver jsø'4 *isijs Pi-Msj Exhale Inhale 9 Breath/min - - '. :. :5.:lO.:l5.:2. :25. :3. :35. :4. :45. :5. :55.1:. Recreational divers 1- Lfl 2j4f4444f$444$ Exhale Inhale 15 Breath/min 1!!!!!!!!!!!! '.. : : l. o o 11 Breath/min -n f4$p4i i4rn$4ii4$ø$t$ 4 o o o o 1! 11 Breath/min Fig. 6. Acoustic emission from four divers: no motion. Proc. of SPIE Vol

6 ' _.4: -.5 Expert diver ii III MS ' -PIiH -. -'. :. :5.:lO.:l5.:2. :25. :3. :35. :4. :45. :5. :55.1:. Recreational divers ' Breath/min Breath/min '. ;' il ii.ll. IL,.1 A]!E, i.- 11 Breath/min LD '. 14 Breath/min :. :5.:l.:15.:2. :25. :3. :35. :4. :45. :5. :55.1:: Fig. 7. Acoustic emission from four divers: light flapping. Proc. of SPIE Vol

7 '.- '.- Sn Expert diver Breaths/min -. :. :5.:l.:15.:2. :25. :3. :35. :4. :45. :5. :55.1:: Recreational divers 25 Breath/min -1. :. :5.:lO.:l5.:2. :25. :3. :35. :4. :45. :5. :55.1:. 1. : r 4sf p Breath/min - '. :. :5.:lO.:l5.:2. :25. :3. :35. :4. :45. :5. :55.1:. o:4ot I. pi to I.. -1.! :. OO:5.:lO.:l5..2.O :25. :3. :35. :4. :45. :5. :55.1:. 42 Breath/min Fig. 8. Acoustic emission from four divers: heavy flapping Proc. of SPIE Vol

8 Diver1, ActivityChange Diver2, ActivityChange -5-5 Amplitude, db Amplitude, db Test12 Test13 Test14-25 Test15 Test16 Test17 No motion Light Heavy Flapping No motion Light Heavy Flapping Diver3, ActivityChange Diver4, ActivityChange -5-5 Amplitude, db Amplitude, db Test18 Test19 Test2-25 Test21 Test22 Test23 Fig.9. SBL variability for four divers at three levels of motion activity As can be seen from Figs. 6-9, the breathing period is not constant even for the same diver and the same motion activity. Therefore, we have to analyze the periodicity pattern statistically. Fig. 11 shows the probability distribution functions, presented as box plots, for breathing periodicity for all four divers for three motion activities. Proc. of SPIE Vol

9 1 No Motion 9 8 Period, T, sec D1 D2 D3 D4 1 Test12 Test15 Test18 Test21 1 Light Kicking 9 8 Period, T. Sec D1 2 1 D2 D3 D4 Test13 Test16 Test19 Test22 7 Heavy Kicking 6 Period, T, sec D1 1 D2 D3 D4 Test14 Test17 Test2 Test23 Fig. 1. Box plots of breathing periodicity of signals shown in Figs.6-8 (D1, D4 denote respective divers #1, #4, D1 is an expert diver). Proc. of SPIE Vol

10 Table 4 shows mean values of the periodicity, <T>, for the experienced diver, D1, and averaged mean values for the three recreational divers at each level of activity. From these data we can evaluate the variability of breathing periodicity, T, due to the motion activity (last row of the table) and due to the experience (last column). Interestingly, the difference between the experienced and inexperienced divers for each motion activity is very close (last column). Averaging data in the last column yields the averaged periodicity variability due to the divers experience/training: T (experience) =1.58 sec. Difference between values shown in the bottom row yields an estimate of the periodicity variability due to the divers motion activity: T (motion) =2.95 sec. Table. 4. Variability of the breathing periodicity for experience and inexperience divers (D1.D4) at three levels of motion activity <T(D1)> sec <T(D1, D2, D3)> sec No Motion Light Flapping Heavy Flapping <T(motion)> <T(D1)> - <T(D1, D2, D3)> = <T(experience)> sec 5. SUMMARY Following our initial investigation of the acoustic emission mechanism from the SCUBA equipment, we conducted a number of tests designed to better understand the acoustic emission characteristics and the key factors influencing them. Thus, we learned that there is a significant variability of the emission characteristics. Using measurements in the controlled environment, we identified key source factors influencing the variability of the acoustic emission parameters including Source Band Level (SBL), Spectral Power Density (SPD), and breathing periodicity, T, or emission modulation frequency. The key factors are: equipment, specifically, the design of the first stage (high pressure) regulator and its service life; the diver s experience and training; and, finally, operating conditions of the equipment and the diver, i.e. tank air pressure and the diver s motion activity. Table 5 summarizes measured variability estimates of the key parameters for the defined factors. For example, the SBL could vary as much as 16 db depending on the equipment used, and up to 15 db depending on the diver s motion activity. The total range of variability of the SBL could be as high as 45 db (!). Understandably, this data is just an initial evaluation of the variability based on a limited number of test subjects. More experiments are needed to obtain more reliable data. But even these initial evaluations demonstrate a significant variability of the key acoustic emission parameters used for diver characterization. Understanding statistical significance of this variability becomes a critical step in evaluating, designing, and operating a SCUBA passive characterization system. This initial study shows that there is a need to build a statistical (probabilistic) model of the source. Proc. of SPIE Vol

11 Table 5. Variability of the acoustic parameters vs. key factors Acoustic Parameters Key Factors Equipment R1 design Service life Tank air pressure Diver Experience/Training Motion intensity Source Band Level, (SBL) 16 db 5 db 3 db 6 db 15 db Modulation (periodicity) (T) 1.58 sec 2.95 sec Spectral power density (SPD) 37 db 1 db ACKNOWLEDGMENTS This work was supported by ONR project #N : Navy Force Protection Technology Assessment Project. REFERENCES [1] [2] Donskoy, D., "Acoustic Emission Mechanism from Scuba Diving Equipment," J. Acoust. Soc. Am., 121(5), Pt. 2, 386. (27). Donskoy, D., Imas, L., Timothy Yen, T., Nikolay Sedunov, N., Michael Tsionskiy, M. "Air Turbulence-Induced Vibration of SCUBA Breathing Apparatus," J. Acoust. Soc. Am., 122(5), Pt. 2, (27). Proc. of SPIE Vol