Acoustics and sound reinforcement issues in a modern-style basilica

Transcription

1 Acoustics and sound reinforcement issues in a modern-style basilica Marko Horvat, Kristian Jambrošić, Hrvoje Domitrović University of Zagreb, Faculty of Electrical Engineering and Computing, Zagreb, Croatia Summary This paper describes a case study aimed at improving the acoustical situation in the basilica in the town of Solin, Croatia, still under construction at this time. The architectural design of the basilica puts the emphasis on visual appeal, but no measures related to obtaining adequate acoustical conditions have been envisioned in the project. Given the importance of this particular place of worship, it is an imperative to achieve favourable and appropriate acoustical conditions in the basilica, and, consequently, good intelligibility of speech as the basis of any event to be held in this space. The measures required to achieve this goal were divided into two stages. In the first stage, acoustic treatment was proposed on the basis of calculations made in a dedicated room acoustics simulation software, based on the blueprints and expected finishing of the surfaces inside the basilica. Given the size of the basilica, it became clear that spoken word cannot reach all the listeners with adequate volume and clarity. Therefore, in the second stage a design solution was proposed for the sound reinforcement system to be installed in the main liturgical space of the basilica, taking into account both acoustical and architectural requirements that needed to be met. The required calculations were based on the room acoustics model obtained in the previous stage, and were made in the same software. The paper shows the results of selected calculations made for possible solutions for acoustic treatment and the sound reinforcement system. The criteria and limitations that played a role in choosing the optimal solutions for both are discussed. PACS no Tj, Fw, Jz 1. Introduction 1 Many churches have been built in Croatia in the last 25 years, either as replacements for the ones destroyed during the war, or as newly built houses of worship. However, in all these years very little advancements have been made towards achieving a timely cooperation between architects and acousticians in the planning stages of construction of such buildings. The emphasis is still given on visual purity, with very little or no regard to acoustic conditions inside these buildings. This is achieved by choosing acoustically unsuitable building and finishing materials, and/or the techniques to implement them. In great majority of cases, the resulting building, although visually impressive, cannot be used for its intended purpose due to very poor acoustic conditions inside the building. The role of an acoustician begins when the building is already completed or near completion, with few options left to explore in order to improve the acoustics of the building. This paper presents the results of a case study made in the still unfinished Basilica of Our Lady s Island in Solin, Croatia. For the reasons described above, the building requires acoustic treatment, as well as a sound reinforcement system that will further improve the intelligibility of the spoken word and reach all the areas inside the building occupied by people. Following good practice, the decision was made to reduce the reverberation time to less than 3.5 seconds at middle frequencies, thus making it more appropriate for a modern house of worship. Leaving the reverberation time at a high value would be more consistent with acoustic conditions found in old churches and cathedrals, but this is neither necessary nor desired. As the spoken word is the dominant type of sound expected inside the basilica, care must be taken to ensure satisfactory speech intelligibility. Copyright 2018 EAA HELINA ISSN: All rights reserved

2 2. Room acoustics design The calculations made for the main space of the basilica in its designed state show that acoustic treatment is absolutely necessary. Reverberation time was calculated for the initial state, and for all considered solutions for acoustic treatment as the spatially averaged value obtained at 12 receiving points inside the listening zone in the basilica. The values are given in octave bands with centre frequencies from 63 Hz to 8 khz. The calculations were made for the unoccupied state as the worst case scenario. As the basilica is still unfinished, its initial acoustic situation was simulated, rather than measured. The simulation was made based on available information on the materials planned to be used as the finish in the interior. The resulting reverberation time in the initial state is shown in Figure 1. Acoustic treatment in the basilica can be applied to three different elements: the ceiling, the walls, and the benches people will sit on. Several possible solutions were devised by combining these three elements, taking into account the requirements made on visual appeal of the treated space. The proposed acoustic treatment of the ceiling is to be implemented by installing a 50 mm thick layer of mineral wool with a density of 70 kg/m 3 under the existing wooden ceiling, with the air gap of 200 mm. The layer of mineral wool is to be supported by wooden slats, and the surface of the mineral wool covered by the slats is not to exceed 50 % of its total surface. A layer of sound-transparent canvas or felt is to be set over the slats, in order to keep the particles of mineral wool from falling through the openings. A sketch of the proposed treatment of the ceiling is shown in Figure 2. Figure 2 The proposed acoustic treatment of the ceiling. All measures are expressed in milimetres. Figure 1 The reverberation time in the unoccupied basilica with no acoustic treatment With the reverberation time of 8 seconds at middle frequencies, rising to more than 10 seconds in the low-frequency range, the initial acoustic situation is unacceptable for any kind of liturgical purposes with the spoken word as the basis for such events. Therefore, the primary task is to reduce the reverberation, thus improving speech intelligibility. The set goal is to achieve the best possible speech intelligibility within the constraints set by the architect and the investor, with STI greater than 0.45 in the listening area. The sound reinforcement system will bring further improvement; however, the less the room is dampened with absorptive materials, the less pronounced this improvement will be. The proposed acoustic treatment is fully functional in the acoustic sense, and maintains the foreseen visual identity of the basilica. To increase the amount of overall absorption in the room, especially in the listening zone, i.e. the space occupied by people, the seats and the backrests on all the benches need to be upholstered. Small seating cushions can be added instead of upholstering. Additional low-frequency absorption is to be added by using the closed air volume below the seats as a Helmholtz resonator lightly filled with sound absorbing material. Even more absorption can be introduced into the space by acoustically treating the wall surfaces. The intention is to decorate the walls with mosaic made of small pieces of ceramic tiles. To examine different possible solutions for acoustic treatment, a comparison of acoustical conditions in the room was made, with mosaic, roughly laid plaster and

3 Odeon Licensed to: Unversity of Zagreb, Croatia Euronoise Conference Proceedings acoustic plaster as the three candidates for the finishing material for the walls. The solutions taken into consideration are the following: 1. the ceiling treated as described, and the walls finished with rough plaster 2. the ceiling treated as described, and the walls finished with mosaic 3. the ceiling treated as described, the walls finished with rough plaster, and absorptive benches 4. the ceiling treated as described, the walls finished with mosaic, and absorptive benches 5. the ceiling treated as described, the walls finished with acoustic plaster, and absorptive benches To introduce the maximum amount of absorption while meeting the criteria on visual design of the basilica, solution 4 was chosen as the only viable one, although it is not the optimal solution from the acoustical point of view. The reverberation times obtained for all five solutions are shown in Figure 3. Solution 5 would yield the lowest reverberation time of only 2.3 seconds. The chosen solution 4, in which absorptive benches are added to the already treated ceiling and mosaic on the walls, provides the final value of reverberation time of 3.4 seconds. The total surface area of the ceiling treated as described above is 458 m 2. The total floor area occupied by absorptive benches is 365 m 2. All acoustically treated surfaces are shown in dark colour in Figure 4. Figure 4 The visualization of acoustically treated surfaces (dark colour); the ceiling surfaces between the load-bearing beams, and the absorptive benches To illustrate the difference between the occupied and unoccupied state, the reverberation time obtained for these two cases with solution 4 implemented is shown in Figure 5. For the occupied state it is assumed that all the seats are occupied. However, it is quite possible that additional people will want to attend the liturgy, and they would be standing in the entrance area or next to the walls. Their presence would introduce additional absorption into the room, thus further reducing the reverberation time. Figure 3 Reverberation time for all five investigated solutions Solution 1 yields the single-number value of reverberation time at middle frequencies (500 Hz and 1 khz) of 3.9 seconds, and shortens the reverberation to half of its initial value obtained without acoustic treatment. Solution 2 replaces the rough plaster on the walls with ceramic tile mosaic, resulting in a 20 % increase of reverberation time to 4.6 seconds. With absorptive benches added to solution 1, solution 3 yields the reverberation time of 3.1 seconds. Figure 5 Reverberation time in the basilica in unoccupied and occupied state Figure 5 shows that the reverberation time in the occupied state is reduced to 3.2 seconds, compared

4 to 3.4 seconds in the unoccupied state, which suggests that the number of people in the basilica will have a marginal influence on the acoustical situation inside, regarding the resulting reverberation time. The total surface area of the acoustically treated surfaces is small in comparison with the volume of the room, and the majority of absorption is on the ceiling, far away from the listening zone. Nevertheless, the devised acoustic treatment results in reasonably controlled reverberation and provides a solid basis for the implementation of the sound reinforcement system. of sight in the basilica. The results of the simulations and the subjective assessment of auralised sound samples support the decision to dismiss all three configurations. The chosen solution for the sound reinforcement system is the fourth configuration that consists of pendant loudspeakers hung above all surfaces of interest, thus not being limited only to the audience area. The proposed positions of the loudspeakers are displayed in Figure Sound reinforcement system The basis for the project that deals with sound reinforcement system is the chosen solution for room acoustics described above. Several possible configurations of the sound reinforcement system were considered, as follows: 1. a central cluster hanging above the altar at a height of 7 metres, oriented towards the audience area 2. left and right line array hanging at a height of 7 metres, oriented towards the audience area 3. a distributed system consisting of column loudspeakers mounted on the walls at a height of 2 metres, oriented towards the audience area 4. a distributed system of pendant loudspeakers hanging at a height of 5 metres above the audience area, the presbytery, the bishop s seat, the choir area on the left, and the organist chair on the right All investigated configurations were evaluated through calculations, but also through auralised samples of a male voice. Given the purpose of this particular space, male voice was chosen as the most appropriate sound sample for such an evaluation. The inherent disadvantage of the first three configurations is that they all cover only the audience area, and additions to the system would have to be made in order to extend the coverage to all areas of interest. These additions would most likely be pendant loudspeakers hanging from the ceiling. Furthermore, none of these three configurations provides uniform coverage, as certain parts of the audience area are too far away from the loudspeakers. The first configuration was dismissed as aesthetically displeasing, as it contains a large cluster of loudspeakers hanging directly over the altar, thus disrupting the longitudinal line Figure 6 The proposed positions of the loudspeakers in the basilica (red circles) The altar is chosen as the reference position for the sound reinforcement system. A ring is defined around the altar, containing the four loudspeakers that cover the presbytery and the bishop s seat, as well as two loudspeakers closest to the altar that cover the front of the audience area. The rest of the loudspeakers that cover the audience area, the choir area and the organist chair are set in a quadratic grid defined by these two loudspeakers, with the distance between the adjacent loudspeakers of 4.5 metres. The hanging height for all the loudspeakers is 5 metres. Quadratic arrangement yields a consistent overlap between the coverage zones of adjacent loudspeakers, thus providing a uniform total coverage. As quadratic arrangement cannot be maintained near the walls, the coverage in the side aisles is not as good as in the rest of the audience area, but it is still satisfactory for the people occupying these aisles. The loudspeakers were grouped into six zones, depending on their distance from the altar. Appropriate delay was calculated for each individual zone and then applied to each loudspeaker

5 The proposed sound reinforcement system consists of 30 loudspeakers altogether. The hanging height of 5 metres places them close enough to the listeners to yield a favourable ratio of direct to reverberant energy, but still high enough to maintain a favourable visual impression of the entire space. The distance of 4.5 metres between the loudspeakers provides a good coverage of all the areas of interest, while keeping the loudspeakers far enough apart for them not to obstruct the view to the aesthetically crucial parts of the basilica, such as the walls decorated with mosaic. The results shown in this section refer to grid calculations made for all floor surfaces in the basilica, thus showing the spatial distribution of the values of key parameters. The calculations were made for both the occupied and the unoccupied state. However, the results are shown only for the unoccupied state as the worse of the two cases. Consistent with room acoustics calculations presented in section 2, the difference between the occupied and the unoccupied state is quite small, suggesting that the number of people in the basilica will have very little influence on acoustical conditions inside. All calculations were made for the listening height of 1.5 metres. Figure 8 A-weighted sound pressure level Figure 9 shows the spatial distribution of clarity C 50 calculated in the octave band around 1 khz. In the areas covered by the sound system, its value is stable and remains between 0 and 2 db, suggesting a balance between the direct and reverberant sound. Lower values were observed only at the altar and the ambo, suggesting the dominance of reverberant sound in these positions. This is in accordance with the design of the sound system, which does not strive to cover these positions due to the risk of acoustic feedback. The values obtained for the occupied state are less than 0.5 db higher, making this change imperceptible. Figure 7 Unweighted sound pressure level Figure 7 shows the unweighted values of total sound pressure, while Figure 8 presents the A- weighted ones. In both cases, the variation of total sound pressure is smaller than 2 db, suggesting that an even coverage of all areas of interest has been achieved. In the occupied state, the values of sound pressure level drop by less than 0.5 db, meaning that the sound pressure level will be virtually independent of the number of people in the basilica. Figure 9 Clarity C 50 around 1 khz Figures 10 and 11 show the spatial distribution of the speech transmission index. Figure 10 shows the general speech transmission index, whereas Figure 11 presents the same parameter with the focus on male voice as the most probable sound that will be transmitted through the sound system. The calculations were made with noise present in the basilica. The defined overall noise level was set to

6 48 dba, and the spectrum was adjusted to represent typical noise spectrum in spaces of this kind. Given the fact that the acoustical conditions are virtually unaffected by the number of people occupying the basilica, the improvement of speech intelligibility is negligible when the number of people increases. If anything, the level of noise made by people is likely to rise in this case, thus nullifying any positive effects gained by introducing additional absorption. Figure 10 General speech transmission index The values of the speech transmission index calculated for male speaker exceed 0.5 in the entire area covered by the sound system. Lower values are again found on the altar and ambo. Figure 11 Speech transmission index for male speaker Based on the calculations, a sample of male voice was auralised at 14 positions in the basilica, namely, the altar, the ambo, the bishop s seat, two positions in the presbytery, the choir area, the organist position, and 7 positions in various parts of the audience area. Subjective evaluation of these auralised samples confirms the results of the calculations. The proposed sound system configuration is superior to all considered alternatives. The quality of voice transmitted through the sound system is satisfactory and remains constant, regardless of the number of people in the basilica. The altar and the ambo are the only positions where the perceived speech intelligibility is lower than in the rest of the listening zone. 4. Conclusions This study was made as the response to the task of devising an appropriate acoustic treatment and a matching sound reinforcement system for the Basilica of Solin, with the ultimate goal of improving speech intelligibility. The basilica is in final stages of construction. The calculations have proved that acoustic treatment is absolutely necessary. As the walls are to be decorated with mosaic, the logical solution is to treat the ceiling, and to design and construct absorptive benches in order to introduce additional absorption in the listening zone. The reverberation time is to be reduced from the initial 8 seconds to 3.4 seconds at middle frequencies in the unoccupied state. The sound reinforcement system is designed as a distributed system with pendant loudspeakers hung from the ceiling. It covers the audience area, as well as the presbytery and the bishop s seat, the choir area, and the organist seat. The design of the system is based on the acoustic conditions in the basilica after acoustic treatment. The calculations and the auralised samples of male voice reveal that adequate speech intelligibility will be achieved by combining the proposed acoustic treatment with the devised sound reinforcement system. An even coverage of all the areas of interest will be achieved. Acknowledgments To the memory of Professor Hrvoje Domitrović, our dear colleague, friend, and mentor, who passed away on 21 January References [1] ISO :2009 Acoustics -- Measurement of room acoustic parameters -- Part 1: Performance spaces [2] F. Martellotta, E. Cirillo, A. Carbonari, P. Ricciardi: Guidelines for acoustical measurements in churches. Applied Acoustics 70 (2009), [3] M. Horvat, H. Domitrović, K. Jambrošić: The improvement of acoustic situation in two modern churches. Proc. Forum Acusticum 2011,