ACOUSTIC DESIGN OF LARGE-SIZED SPORTS HALLS IN CROATIA

6th Congress of Alps-Adria Acoustics Assosiation 16.-17. October 2014 Graz, Austria ISBN: 978-3-200-03921-6 ACOUSTIC DESIGN OF LARGE-SIZED SPORTS HALLS IN CROATIA Kristian Jambrošić 1, Hrvoje Domitrović 2, Marko Horvat 3 University of Zagreb, Faculty of Electrical Engineering and Computing, Department of electroacoustics, Unska 3, HR-10000 Zagreb kristian.jambrosic@fer.hr 1, hrvoje.domitrovic@fer.hr 2, marko.horvat@fer.hr 3 Abstract: For the 2009 World Men s Handball Championship in Croatia, six medium to large sport halls were built from scratch: Arena Zagreb in Zagreb (capacity 15,200 visitors), Spaladium Arena in Split (capacity 12,500 visitors), Krešimir Ćosić hall in Zadar (capacity 8,600 visitors), Sport hall Varaždin in Varaždin (capacity 5,200 visitors), Sport hall Town garden in Osijek (capacity 3,600 visitors) and Sport hall Zatika in Poreč (capacity 3,500 visitors). The financing of these halls was not unified, depending on the decision of the local municipality for each case individually. Consequently, the architectural projects and in particular the interior acoustic designs of these sport halls were handled by each architect differently, or not at all. Similarly, the sound reinforcement systems were designed and installed with different design goals. This paper describes the acoustic design process for some of these halls, particularly for the halls in Zagreb, Split and Varaždin. It also shows measurement results for the typical acoustic parameters in the sport halls in Split and Varaždin and compares two sound reinforcement systems that were proposed for the sport hall in Split. Finally, the usability of medium and large sport halls for events other than pure sport considering their acoustic characteristics is considered. Key words: sport hall acoustics, large sized rooms, sound reinforcement systems 1. INTRODUCTION The 2009 World Men s Handball Championship in Croatia was a huge logistical, but also technical challenge for the organization committee. This championship provided an opportunity to build a number of sports halls from scratch to host all matches of the championship. In total, six medium to large sports halls were built: Arena Zagreb in Zagreb with a capacity of 15,200 visitors, Spaladium Arena in Split built for 12,500 visitors, Kresimir Ćosić hall in Zadar with the capacity of 8,600 visitors, Sport hall Varaždin in Varaždin for 5,200 visitors, Sport hall Town garden in Osijek for 3,600 visitors and finally Sport hall Zatika in Poreč with the capacity of 3,500 visitors. The local municipalities have financed each sports hall individually. Moreover, there were no unified design specifications concerning the acoustics of the halls that had to be met. Therefore, the question of interior acoustic design and the design of the sound reinforcement systems was handled individually for each of the mentioned halls, mostly depending on the architect s knowledge in this field, as well as those from the local investors of the halls that had to be built. Acoustics of sports halls is a specific part of architectural acoustics, with quite unique demands and constraints because of their dimensions and shapes. The main demand is the need of high speech intelligibility in a room of usually huge volume with many highly reflective surfaces [1, 2]. The reverberation time of sports halls is usually quite long, depending mainly on the size of the hall, which can vary considerably from hall to hall. At the same time, long reverberation times directly influence the speech intelligibility. Longer reverberation times cause the decrease of intelligibility, which is a big problem for sports teachers as well as for the users, but also for the audience, if any. In addition, when a sports hall is intended for other uses except sport events, e.g. for congress, musical events, exhibitions etc., "bad acoustics" of the hall considerably reduces its usability, thus reducing the value of the total financial investment, which can be enormous in this case, sometimes going over 20,000,000 EUR! Extensive financial problems are

unfortunately a part of the everyday existence of most of the newly built sports halls mentioned in this paper. 2. ACOUSTICS OF SPORTS HALLS Although the reverberation time of a sports hall is not the only acoustic parameter important for describing its acoustic quality, it is still the most used and most indicative one [1, 3]. Many case studies show that the simulated reverberation times in sports halls using analytical expressions such as the Sabine or Eyring formula, or even using more complex ray-tracing and image source calculation method programs often do not coincide with the reverberation times measured in the finalized halls. One of the reasons for this is that there is often very little sound scattering of the hall's boundary surfaces, especially if standard shoe-box type gyms are considered, [4]. There are many different recommendations concerning the reverberation time of sports halls. For example, the recommendation of the Building Bulletin 93 [5] recommends the reverberation time in indoor sports halls to be <1.5 s, not taking into consideration the volume of the hall. There is another reference, a DIN norm [6] that provides a recommendation for the reverberation times for empty sports halls and swimming pools as: 3. SPORTS HALLS In this paper, some aspects of acoustic design and sound reinforcement systems of three sports halls will be shown. It is important to mention that the halls were all built for the handball championship, but it was clear from the beginning that the investment would pay off only if the halls would be used for other types of events due to their capability of hosting a large number of people. These events mostly include concerts, but also all other types of non-sportive events, such as exhibitions, congresses, special installations, etc. The problem is that specific demands of such events were rarely considered during the process of designing the acoustic interior. 3.1. Sport hall Arena Zagreb The sport hall "Arena Zagreb", Figure 1, is the largest sports hall in Croatia with a net volume of the main hall of 207,000 m 3, and a total surface of 30,100 m 2. It is clear that the reverberation time of such a hall should be much smaller than 4.6 s, as calculated with the expression (3). In particular, a foreign consultancy company recommended the reverberation time to be between 1.8 s - 2.0 s for the empty hall! RT 60 = 1.27 lg(v) - 2.49 (1) RT 60 = 0.95 lg(v) - 1.74 (2) where V is the hall volume. Expression (1) is used for classes and sport groups with only a single communication going on at a time, and expression (2) is used for multiple, parallel classes or sport groups. However, they are valid only for halls with volumes from 2,000 to 8,500 m 3. The Netherlands has also issued a guideline for the reverberation times in sport halls, but in this case depending on the hall net volume [2, 3]. As this paper deals with very large indoor sports halls, only the recommended reverberation time for the largest sports halls, with volumes >29,000 m 3, will be mentioned here. The reverberation time should be calculated as: Fig.1. Arena Zagreb sports hall. The hall itself was planned to be used firstly for sport events (Figures 2 and 3), and secondly for concert and similar other events (Figures 4 and 5). The main difference between those two setups is the possibility to retract the lower part of the bleachers in order to provide more space on the parquet. RT 60 = 2V/3S (3) where V is the total volume in m 3, and S the total inner area of the hall in m 2. It is worth mentioning that the same guideline defines and limits the recommended value of reverberation time for halls smaller than 29,000 m 3 to not longer than 2.3 s. Besides the reverberation times, limit values of other acoustic parameters are usually defined, with the noise level being the next most important one. Typically, the noise level in halls coming from the installed technical equipment should be less than 40 dba. Fig.2. Sport events held in Arena Zagreb sports hall.

The optimum places for applying absorptive materials were found by simulation, using the density of rays from a source placed at the typical loudspeaker location, both for regular sport events and concerts, Figure 6. Fig.3. 3D model of Arena Zagreb prepared for sport events. Fig.6. Model of Arena Zagreb with an example of rays coming from the sound source position to all hall surfaces. 3.2. Sport hall "Spaladium Arena" The second largest sports hall in Croatia was built in Split, also for the aforementioned handball championship, Figure 7. The hall has also retractable bleachers for changing the hall setup for usual sport events to a setup for musical events, or even athletics, Figures 8-10. Fig.4. A music event held in Arena Zagreb sports hall. Fig.7. Spaladium Arena Split sports hall. Fig.5. 3D model of Arena Zagreb prepared for musical events with retracted lower part of the bleachers. The expected reverberation time after installing absorptive materials on the ceiling and the walls of the hall was 1.6 s for mid frequencies. The hall filled with audience should lower the reverberation times at mid frequencies to 1.0 s, which is a very low value for a hall of this size. Fig.8. Music event held in Spaladium Arena sports hall.

The sound reinforcement system of the Spaladium Arena hall had to be chosen between the following offered possibilities: 1. combination of a centralized + distributed system 2. centralized sound system (4 spaced clusters) 3. distributed speaker systems (14 clusters + 20 additional loudspeakers, in total 160 loudspeakers!) 4. distributed systems (8 clusters) The position of loudspeakers for systems 1 and 2 are shown in a 3D wireframe graphics in Figures 12 and 13. It is interesting to evaluate which system is overall the best one for this hall. For various reasons, only the first two mentioned systems were considered as the possible solutions for the Spaladium Arena hall. Fig.9. 3D model of Spaladium Arena prepared for sports events. Fig.12. Sound reinforcement system 1 (centralized + distributed) for the Spaladium Arena sports hall. Fig.10. 3D model of Spaladium Arena prepared for indoor athletic meetings. The required reverberation time of the hall was 2.0 s at mid frequencies for the empty hall. Measurements in the hall showed that the actual reverberation time is somewhat higher, with an average value of 2.83 s at 500 Hz, Figure 11. Measures for reducing the reverberation time to the required values were proposed to the investor. Fig.13. Sound reinforcement system 2 (strictly centralized) for the Spaladium Arena sports hall. Fig.11. Measured reverberation time in Spaladium Arena. There were some requirements that had to be met, such as providing sound pressure level (SPL) at audience ears of at least 110 db. Both systems were capable of fulfilling this demand, as seen in Figures 14 and 15. It would be an easy job to choose the better one in terms of the total SPL, but there are also other criteria that have to be met.

Fig.14. Total SPL in Spaladium Arena of system 1. Fig.17. Direct SPL in Spaladium Arena of system 2. It becomes clear that system 2 is performing better than system 1 according to this criterion. Another criterion is the speech transmission index STI which should be minimally 0.5. The parameter is shown in Figures 18 and 19. The average value for system 1 is 0.52, and for system 2 is 0.64. Again, system 2 clearly performs better. Fig.15. Total SPL in Spaladium Arena of system 2. A very important parameter is also the direct SPL (without the diffuse part of the SPL). If the graphical representations are similar to the ones for the total SPL, the loudspeakers are emitting sound with good control, otherwise a lot of sound energy is scattered. Figures 16 and 17 will show these facts. Fig.18. STI in Spaladium Arena of system 1. Fig.16. Direct SPL in Spaladium Arena of system 1. Fig.19. STI in Spaladium Arena of system 2.

Another interesting parameter to be considered when choosing the sound reinforcement systems is the ratio of direct to reflected sound energy in the hall, which is the main requirement for good speech intelligibility, Figures 20 and 21. Again, system 2 performs better than system 1. It is always interesting to see the number of overlapping loudspeakers, as calculated by the simulation software, Figures 22 and 23. The white colour indicates the area where only one loudspeaker can be heard, and dark areas where more than one loudspeaker is audible. Once again, system 1 behaves worse with 3.8 audible loudspeakers on average, while system 2 has 1.7 audible loudspeakers on average. At last, the speech clarity parameter C 50 can also be used for comparing the overall quality of sound reinforcement systems, Figures 24 and 25. It can be clearly seen that system 2 has C 50 values of up to 10 db, whether system 1 has lower C 50 values of up to 4 db. Fig.22. Number of simultaneous loudspeakers heard in Spaladium Arena for system 1. Fig.20. Radio of direct to reflected energy, system 1. Fig.23. Number of simultaneous loudspeakers heard in Spaladium Arena for system 2. Fig.21. Radio of direct to reflected energy, system 2. Fig.24. Clarity C 50 in Spaladium Arena of system 1.

Fig.25. Clarity C 50 in Spaladium Arena of system 2. Fig.28. Interior of the sports hall Varaždin. The Spaladium Arena sports hall accommodates also a smaller, shoebox-type sport hall, but there is no space for the audience, Figure 26. Fig.26. Smaller sports hall in the Spaladium complex. 3.3. Sports hall Varaždin The Sports hall Varaždin is a smaller sports hall, Figures 27 and 28. Nevertheless, the acoustics of the hall was considered while building the hall. The acoustics in the hall was designed in order to achieve a mid-frequency reverberation time of 1.6 s for the large and 1.5 s for the small hall. The large sports hall had installed foam baffles on the ceiling, Figure 29, which provided overall a nice and balanced interior of the hall. Fig.29. Foam baffles covering the ceiling of the hall. Fig.27. Sports hall Varaždin. Figures 30 and 31 show the 3D model of the hall and the ray-tracing pattern for determining the interior surfaces that are most often hit by the sound energy from the typical source position. The smaller sport hall in the same complex is shown in Figures 32 and 33.

Fig.30. 3D model of the sports hall Varaždin. halls. This is even truer if a hall is used not only for sport events, but also for music events. People coming to a concert, paying for a ticket and not hearing good sound due to high reverberation time will most likely avoid coming again to this venue. Moreover, if the speech intelligibility is low as well, the audience can hardly follow sport events in a satisfactory manner. For all these reasons, it is very important to follow all existing recommendations for designing good acoustics in sports halls. If it is possible to influence the architect's decisions of the interior surface shapes of a hall, surfaces with high scattering coefficients should be proposed wherever possible. If interior surfaces, mostly walls, are scattering sound energy well, the overall absorption of the surfaces in the room will be quite higher and the reverberation time consequently lower. Fig.31. Model of the sports hall Varaždin with an example of rays coming from the sound source position to all hall surfaces. Fig.33. 3D model of the small sports hall Varaždin. REFERENCES Fig.32. Small sports hall Varaždin. 4. CONCLUSION The design of the acoustics of sports halls is usually a very complicated process. It often involves convincing the architects of the importance of quality acoustics in the [1] Y.C.M. Wattez: Acoustics in sports halls absorbing behaviour of perforated steel panels, Technical University Delft, Faculty of Architecture, 2012 [2] M. Luykx, M. Vercammen: Acoustic solutions for the middle part of a sport hall: Reverberation time and sound strength, Proceedings of the 7 th Forum Acusticum, Krakow, 2014, 1-6 [3] M. Luykx, M. Vercammen: Evaluation and measuring procedure for strength in sport halls, Acoustics in Practice, 1(1), 2013, 5-10 [4] C.O. Hosoien: Effect of scattering on predicted reverberation time, Proceedings of the 7 th Forum Acusticum, Krakow, 2014, 1-7 [5] Building Bulletin 93: Acoustic Design of Schools - a Design Guide, Architects and Building Branch, London [6] DIN 18041:2004-05: Hörsamkeit in kleinen bis mittelgroßen Räumen, 2004