Simulated acoustic field in mudejar-gothic churches

Simulated Acoustic Field in Mudejar-Gothic Churches M. Galindo’, T. Zamarrezo’, S. Gir&, J. J. Sendra’. and J. Navarro’ ‘Department of Applied Physics, ‘Department of Architectaral Constructions. Seville University. Avda Reina Mercedes. 2; 41012~&villa (Spain) Summary: In this work we have undertaken an acoustic simulation of the sound field in several churches in the city of Seville. The simulation has been carried out through the RAYNOISE program and has been based on the reverberation times measured in dmse space?.. We have focused on Sound Ressure Levels SPL, Clarity C80 and Definition D50 indices data in relation to their spectral variations and spatial distributions and their comparison with the experimental values. INTRODUCTION Mudejar style is a link between Islamic art and western Christian art, so it is a uniquely Spanish artistic movement. These churches were probably built between the thirteenth and fifteenth centuries and they were built of brick. All the churches analysed have similar architectural features: three naves covered with wooden frameworks following in the Moorish style and a vaulted presbytery. Sendra et al (l-3) have pointed out that the smaller size of these churches and mainly their wooden ceilings significantly improve their acoustical behaviour, as far as this can be indicated by reverberation times, over the Gothic contemporaries. The greater absorption in the ceilings tends to lead to a decrease of the great reverberation times expected in these large volumes, above all at low frequencies. Besides being used for worship which implies the use of speech and music, these buildings are sometimes renovated and used as lecture halls, theatres, or auditoria. According to the specialist Angulo (4), this type of church is called “the Seville parish type”. OBJECTIVES Our main aim in this work is to show the agreement between the experimental data and the simulated results performed by the ray tracing method (RAYNOISE version 2.la) in this type of architectural enclosures. We are specially interested in researching the ability of the so&ware to predict the spectral variations and spatial distribution of the measured acoustic parameters. We mainly focus on the SPL and on the acoustic indices C80 and D50. The measurement tests as well as the simulations have been carried out inside the unoccupied churches. GENERAL+ ASPECTS Using an iterative procedure we have adjusted the absorption coefftcients of the different wooden ceilings, which are the most singular ones, until we obtained the simulated reverberation times in a range of +lO% of the in situ experimental results (see table 1). Those absorption coefficient values are contrasted with other similar data in the bibliography. The geometrical and acoustical model is built in this way and the whole simulation process is executed with maximum order of reflection R equal to 10. The number of rays N is determined by the specifications of the pmgtam user’s manual by the formula 1256,6 1’ N’< S, with 1 the mean free path (m) and S, the characteristic surface (m’) of the geometric model. To decide when to abandon a my, in addition to the order of reflection, we use a dynamic rang of 60 dB and a time window, following the manufacturer’s suggestion, t&,0294 1 (s). Calculations have been carried out using the Conical Beam Method (CBM) and we have considered the air absorption and the sound velocity at the atmospheric conditions of 20°C and 50% relative humidity, similar to that at the measurement time. The background noise in each simulation process coincides with the measured noise spectra. Likewise we have made calculus modifying the diffusion conditions of the ceilings and audience zone. This does not have a marked effect on any evaluated parameters in the same way as Lam (5) has already pointed out. An omnidirectional source with the known sound power spectrum of the real one has been used to obtain simulated sound pressure levels. Both calculation options, image and mapping, have been set to obtain the acoustic parameters implemented and to compare with the experimental data Receiver surfaces and microphones have been placed at 1,20 m above floor level. TABLE 1: Measured reverberation tima Tms and S~IIIU~&~ TSim Santa Marina San vicente Santa Catalina Freq. (W Tmas TSti Tmcas TSh Tmas TSi#ll 250 3,62 3,71 1,71 1,70 I ,50 1.48 500 3396 4.11 1,81 1.79 1,72 I,64 IO00 4,00 4.01 1,82 1,78 1.75 1.68 2GQo 3.57 3,70 1.68 I,70 I,67 1.57 4000 2,62 2,68 I,40 I,45 I ,45 I,36 RESULTS AND CONCLUSIONS Figure 1 shows a view of the 3D model created for San Vicente church. Figure 2 shows the measured and computer calculated values of the sound pressure levels (relative to that produced by the sound source in a free field at 1Om) versus source-receiver distance at 500, and 2000 Hz octave FIGURE 1.3D model used for simulations in San Vicente Church - - Adjustment 1 Cm) r (ml FIGURE 2. LpLpref (dB) versus distance r (m) at 500 and 2ooO Hz octave bands for San Vic&e church. r(m) 1 (ml FIGURE 3. C80 (dB) versus distance r (m) at 500 and 2000 Hz O&W bands for San Vic&e church. bands in this church. The same graphics show the theoretical curves obtained through classical diffise field model, Barron’s modified model (6) and an adjusted curve of the experimental results. As we can see the simulated levels continue to decrease further we go beyond the critical distance according to Barron’s model, but the measured values differ appreciably from expected theoretical levels although the same tendency suggested by Barron’s modified model for attenuation by distance is observed. Similarly figures 3 and 4 compare the measured and simulated values of C80 and D50 respectively as a function of distance at the same octave bands. In both cases we superimposed (continuous line) the values from Ban-on’s modeL Both sets of values show similar spectral and spatial patterns, and they show these two acoustic quality descriptor are rather sensitive to local variations, there being no agreement with the predicted theoretical model. Quite a satisfactory agreement between measured and simulated results has been achieved by RAYNOISE computer simulation, mainly for the early to late energy indices in these complex rooms. In this respect it is useful to remember the uncertainties that arise from the reproducibility of measurements and absolute positions of microphones in a real church and its model description (7). a 5 10 15 20 25 30 35 0 5 10 I5 2” 25 3” 35 r (ml r Cm) FIGURE 4. D50 (%) as a timctioa ofdistance r (m) at 500 and 2000 Hz octave bands for San Viceate church We have optimised the simulation procedure taking into account the measured reverberation times, so the simulation process can be used in this way to improve any remodeling process from an acoustic point of view. In the future we are interested in contrasting these facts with other acoustic indices such as STIiRASTI and LEF, also using other software tools. REFERENCES 1. Sendra, J. J., Zamarrerio, T., “El camp0 sonoro en las iglesias Gotico-Mudejares con cubierta de madera: Aplicaci6n de1 modelo de Barron,” in Proceedings of Tecni-Anisticu ‘95, XWI Jomadas Nacionales de Anistica, La CON@ Spain pp. 87-90, 1995. 2. Se&a, J. J, Zamarrtio, T., Navarro, J., “An analytical model for evaluating the sound field in Gothic-Mudejar churches,” in Proceedings of Second International conference on Computational Acoustics and ifs Environmental Applications Compac 97, Temi, Italy, pp.l39-148, 1997. 3. Zamarrefio, T., Galindo, M., Gir611, S., “Prediction de Indices actisticos en iglesias Gotico- Mudejares,” in Proceeding of XV7 Reunidn Bienal de la Real Sociedad Espariola de Fisica y Quimica, Las Palmas de Gran Canaria, Spain, pp.25-26, 1997. 4. Angulo, D., “‘Arquitectura mud&jar sevillana de 10s siglos XIII, XVI y XV,” Ayuntamiento de Sevilla, Sevilla , pp. 32, 1983. 5. Lam, Y. W., Journal of the Acoustical Society ofAmerica 100, pp. 2181-2203 (1996). 6. Barron, M., Lee, L. J., Journal of the Acousticul Sociev ofAmerica 84, pp. 618-628 (1988). 7. Pelorson, X., Vian, J. P., Polack, J. D., AppliedAcoustics 37, pp. 175-198 (1992).