Evaluation of a Modern Catholic Church by Acoustical Software Based on the Ray-Tracing Method

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1 rychtarikova.qxd :52 StrÆnka 21 RECEIVED ACCEPTED M. RYCHTÁRIKOVÁ, M. MUSABUDA, F. OHRABLO Evaluation of a Modern Catholic Church by Acoustical Software Based on the Ray-Tracing Method Monika Rychtáriková (PhD student) Slovak University of Technology, Faculty of Civil Engineering, Department of Building Structures, Radlinského 11, , Bratislava, Slovak Republic Mons. Maxwell Musabuda (PhD student) Catholic University of Leuven, Faculty of Theology. St.-Michielsstraat 6, B-3000, Leuven, Belgium Prof.Ing.František Ohrablo, PhD. Slovak University of Technology, Faculty of Civil Engineering, Department of Building Structures, Radlinského 11, , Bratislava, Slovak Republic ABSTRACT KEY WORDS This article is based on a acoustical analysis of a modern Catholic church in the region of Antwerp (Belgium). The main issue is the bad intelligibility of speech. This problem intended to be resolved using Acoustical Software based on the ray-tracing method. In the space at issue, elementary room acoustic measurements according to ISO 3382 (Acoustic-Measurement of the reverberation time of a room with reference to other acoustical parameters) using Dirac software were made. Three microphone positions (marked in Fig.1) in the middle of each audience area were defined. A computer model and simulation in the CATTacoustic ver 7.2 was done. In the analysis, results of in situ measurements and computer simulation were considered and compared as well as a new purpose for the ceiling was given. This text includes the results of the computer simulation of the church model with the new purpose for the ceiling. room acoustics, ray-tracing method, reverberation time (RT), Clarity C80, Centre of gravity TS. Fig.1. 3D Model and Ground Floor Scheme of St. Paulus Church. (M1-M3 - microphone positions, B - position of sound source) 2003 SLOVAK UNIVERSITY OF TECHNOLOGY 21

2 rychtarikova.qxd :52 StrÆnka 22 INTRODUCTION The fundamental requirements of a church are a good listening environment for both speech and music and all the spaces within the church require quiet surroundings which are conducive to undisturbed meditation and prayer. Yet, people still have problems with hearing and understanding what is being said and music that is being played. Good sound in the church is still asking for many congregations. To evaluate a church as an acoustical space, classic room acoustic criteria of the ISO standard are used. Usually, problems arise with determining the recommended values for the space. The modern day worship service involves the active participation of the pastor, the performing musical group, and the congregation. Each participant experiences a unique set of problems. This case raises philosophical questions concerning people s attendance at worship. It also leads to one of the greatest problems for churches these days, estimating the percentage of people in church during ceremonies in orden actual to obtain values of the total absorption in a space. General Problems, Requirements and Basic Recommendation for Church Acoustics The prime consideration in the acoustical design of a church is the noise control (NC) level of the church and the proper control of reverberation. The noise control level of the church describes the background noise in the sanctuary and is too high, especially in the large cathedrals placed in the centers of big cities. The typical conversation levels of people are around 60dB on the average. To hear speech at a comfortable level and to be able to understand the words being spoken, the level of speech usually has to be 25dB above any noise. The other concern is that NC is not a constant throughout the sanctuary. The hum of florescent lights or a ceiling fan above the platform or altar can raise the NC in the front of the church to NC40, while the rest of the church is NC30. [Tab.1] The average reverberation time (RT) in a church is one of the first clues as to whether there are problems or not. The traditional or Liturgical style of worship can make good use of longer Tab.1 Noise Control level values [db] 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz NC-40 [db] NC-35 [db] NC-30 [db] reverberation times. However, any reverberation time that is longer that 2.5 seconds is not needed, and it is no longer musical unless it is well diffused. If the reverberation time in the church is longer than 2.5 seconds, then it is most likely to have a problem with both music and speech, and this also degrades the performance of a church organ. But not all reverberation is equal. Long reverberation times in the low/mid range of speech is not good. Longer reverberation times below 500 Hertz will have a profound effect on speech degeneration. What is interesting is the effect of bass sound in the church. From experience, the ideal reverberation time at 200 Hertz is when it is within 0.2 of a second, of the measured average time, for an appropriate style of worship. Because of low frequencies, it is good to check the position of the sound source. The wave length of 200 Hz is 1.7m, and for 100 Hz, is 3.4m, so if the bass player is standing closer to the loudspeaker in order to hear the sound wave, he has to wait for the sound to bounce off a wall that is further than the wave length and perpendicular to the source. RT [s] 2,2 2,0 1,8 1,6 1,4 1,2 1,0 0, m 3 (St. Paulus Church) Catholic churches Protestant churches synagogues V [m 3 ] Fig. 2 Optimal reverberation time at 500 Hz, assuming two-thirds occupied, as a function of enclosure volume [2] This often means that the sound has traveled to the other side of the room and come back. Also, if the bass player is standing against a wall, he/she still will have problems hearing the bass sound. High frequencies (over 2 khz) give a sound its character and wholeness. There are high frequency sounds which give us our intelligibility clues and to music the expressions we love and enjoy. Today many newly - built Catholic churches carry considerable sound absorbent treatment for improved hearing conditions, even when organ music renditions are planned during services. Optimal reverberation time for churches is shown in Fig. 2, where both speech and music are considered. 22

3 rychtarikova.qxd :52 StrÆnka 23 St. Paulus Church St. Paulus Church is a modern Catholic church constructed in the second half of the 20th century (1964) in the region of Antwerp (Belgium), by the architect P. van Aerschot. The main shape of the church is simple and logical. The application of traditional materials such as brick, wood and glass create an intimate atmosphere in its interior [Fig. 3]. The Church is located in the quiet of the forest in Westmalle, close to the Abdij, where one of the famous traditional Belgian Trappist beers is brewed and bottled. The placement of the church in the forest already avoids any problem of background noise. The main construction of the room, designed by keeping to fundamental room acoustic requirements, makes the space diffuse enough and the application of reflective materials on the inner surfaces makes it reverberant. The floor is made from stone tiles without the application of carpet on it. The walls are created from unplastered bricks, and the whole ceiling is made of gypsum board with very low sound absorption. The church use wooden chairs without upholstered materials for seats. The total volume V= 2870 m 3 with a total floor surface area (audience included) is S floor = 439 m 2. The area of the audience is S audience = 200 m 2. The total surface area of the ceiling is S ceiling = 630 m 2 (the ceiling over the audience is 304 m 2 ). The ground clearance of the ceiling is relatively low: h 1 = 2.8 m over the altar, and the audience is doubled to h 2 = 5.5 m. reverberation time, as a first indicator of the room s acoustical problems, is more than 4 sec. in the middle frequencies, which is too long for having good hearing conditions for speech in the room [Fig.4]. On the other hand, the decay curve [Fig. 5] in all the measured positions is straight without showing any special acoustical problems of echoes, bad diffusion or other effects in the room. Fig.4 shows a comparison of the measurements and computer simulation. In fact, the longer the RT is, the more difficult it is to estimate its accurate duration. Also, with the increase in RT, the human ear gets less sensitive to recognize the difference in the Measurements and Computer Simulation Measurements according to ISO 3382 (acoustical measurement of the reverberation time of a room with reference to other acoustical parameters) were done, and the results showed, that the Fig.4 Reverberation time RT, Early decay time EDT, according to measurements and computer simulation. Difference in results obtained from measurement and computer simulation according to position of microphone Fig.3 Fotodocumentation of the Interiour of the Church Fig.5 Decay curve at the position M1 (Dirac software measurements) 23

4 rychtarikova.qxd :52 StrÆnka 24 length of the reverberation. In our case, the incorrect results are only in the low frequencies in T 30, which is the typical problem of raytracing programs. The differences in the results of the high and middle frequencies are negligible. In order to test the range of the validity of the ray-tracing method, other room acoustic criteria were regarded. Fig. 6 shows the differences in Central Time Ts, Clarity C80 and Deutlichkeit D50. Ts the values of the actual ceiling are highest in the low and the middle frequencies (Ts= 350ms). In the high frequencies is Ts = ms. In this case, the simulation results are good. The values of C80 and D50 are more dependent on the position of the receiver, but because of the limited extension of this article, the average values are given. According to the statistics, the best results obtained from simulation are in the middle frequencies, and the curves are flatter. The differences in Rasti values (3-6) are negligible. Purpose of the New Ceiling Because of speech domination (in comparison with the played music), our task was to solve this problem without using or placing any electroacoustical instruments in the church. We decided to change the total absorption of the room to a value that will still keep some reverberance appropriate for musics performances. As seen in Tab.1, we decided to take RT = 1.6s as a reference value of the reverberation time. Thanks to the diffuse sound field [Fig.5], we can use the Sabin formula, based on statistical calculations, using the total room volume and the total absorption present, as the first step in our purpose. To obtain the value of the total absorption area, the basic formula of RT was used: V 0, 163* V RT = 0, 163 = A RT [] s A [] s actual total abs.: A ac = (0.163 * 2870)/4.5 = 104 m 2 necessary absorp.: A n = (0.163 * 2870)/1.6 = 290 m 2 For total absorption we need to add: A = A n A ac = = 186 m 2. If we have a look into the interior of the church, the only possible surface we can change without violation of its inner architecture is the ceiling. The surface area of the ceiling over the audience is S = 304 m 2. We need A = 186 m 2. To obtain the necessary absorption coefficient, we used the simple formula: a = A/S = 186 m 2 /304 m 2 = Regarding the requirement of new absorption properties of the ceiling over audiences (a = 0.61), a new computer simulation using Fig.6 Results and differences in other room acoustic criteria: Ts, C80 and D50 24

5 rychtarikova.qxd :52 StrÆnka 25 Fig.7 Computer simulation results of T30 and EDT in alternative of new ceiling with (a = 0.61) and their comparison with actual values in, the church. the new properties of the ceiling was made. Fig.7 shows the reverberation time and the early decay time as obtained from the computer simulation. The exchange of reflective gypsum boards to new absorptive boards, with properties a = 0.61, will not change the visual impression of the church s interior, but it will considerably change its acoustical conditions. It will surely cause a new subjective sensation for the audience. The question is wheter they will like the new sound of the church, but the hearing conditions will certainly be appropriate to the purpose of this church. CONCLUSION Our example confirms that the range of the validity of Ray tracing software is mostly limited to low frequencies. Room acoustics, digitally modeled with impulse responses calculated by ray-tracing or image-source methods, are based on geometrical acoustics. This is applicable to high frequencies or in simple rooms, without a large amount of columns and special interior elements in the space, where diffraction can be ignoreded and so the results obtained from the simulation are satisfactory. The article is published with in the grant No. 1/8331/01 VEGA REFERENCES ISO 3382 Acoustic measurement of the reverberation time of a room with reference to other acoustical parameters RETTINGER, M. (1968) Acoustics, room design and noise control, New York KNUDSEN, V. O. - Harris, C. M. (1978) Acoustical Designing in Architecture, Acoustical Society of America FURRER, W. (1961) Raum und Bauakustik Laermabwehr, Basel 25