Measures of acoustical spatiality in an Italian opera house

Proceedings of the 7th WSEAS International Conference on Acoustics & Music: Theory & Applications, Cavtat, Croatia, June 13-15, 2006 (pp60-64) Measures of acoustical spatiality in an Italian Opera house LAMBERTO TRONCHIN and VALERIO TARABUSI DIENCA – CIARM, University of Bologna, Italy http://www.ciarm.ing.unibo.it Abstract: The sound spatial characteristics of concert halls are extremely relevant for 3D auralisation of spaces for music, as well as for designing acoustical enhancements in the halls. Moreover, the theatre “Comunale” in Bologna is also an ancient Italian opera house (realized in late 18th Century by Galli Bibiena) with peculiar spatial characteristics. In the Theatre “Comunale” balconies are realized with different shape and materials respect to classical Italian opera house. This special feature of balconies provokes some effects in the listening conditions that are related with the position of sound sources in the stage and in the orchestra pit. These special characteristics have been experimentally investigated by means of an omni-directional, pre-equalized sound source located in different positions in the orchestra pit and stage, and a dummy head, accomplished with a soundfield microphone, located in many different listening positions in the balconies and the stalls. The special features of ACF and IACC, as well as other acoustical parameters, measured experimentally, have been reproduced in a second step in the listening room “Arlecchino” at the laboratory of DIENCA – CIARM, by means of the Stereo Dipole and Ambisonics technique. In this paper the results from these experiments are presented and analyzed Key words: - Teatro Comunale of Bologna, Acoustical measurements, Impulse Response, Acoustical Parameters, Spatiality . INTRODUCTION The acoustic properties of historical opera houses are considered one of the most important cultural heritages of Italian history. Since the paper of M. Gerzon, [1], recently revised [2], the acoustic properties of special opera house are considered at the same importance of ancient musical instruments. In this paper the acoustic properties of one of the most important Italian opera houses, the teatro Comunale in Bologna, are therefore measured and evaluated, by means of a dummy head and a B-format microphone in order to preserve their acoustics for posterity. These acoustic data will be utilized also for 3D auralization purposes. THE THEATRE IN THE HISTORY The Teatro Comunale in Bologna opened in 1763. The architect Antonio Galli Bibiena, one of the most active architects for spaces for music in 18th Century, designed the theatre. However since the early drawings, Galli Bibiena thought about a different shape of the teatro Comunale of Bologna: a bell shape, different from the horse-shoe shape. The Comunale di Bologna (which followed the Teatro Scientifico in Mantua and the Teatro of Four Horsemen in Pavia) was therefore the first example of a special “phonic” shape in the idea of Galli Bibiena. Nevertheless, other physicians and acousticians at that time did not agree with this idea. Moreover, the Comunale of Bologna has other different characteristics. One of the most significant innovations was the bricks structure of the main hall, instead of a wooden one. In this manner, it would have been easier to avoid the burning of the theatre. Proceedings of the 7th WSEAS International Conference on Acoustics & Music: Theory & Applications, Cavtat, Croatia, June 13-15, 2006 (pp60-64) Fig. 1 The cavity below the stalls But this theatre has also two relevant innovations in the balconies and in the stalls. The balconies were conceived giving the opportunity to the holders to personalize the walls, coloring the walls, modifying the interiors, etc. The pavement of the stalls was provided with a special device: a special mechanism was able to elevate the pavement until the stage. In this case a huge cavity would be created, and musicians and singers would have been at the same level. In the opinion of Bibiena, the movement of the floor could have also enhanced the intelligibility of singers. This device was active until 1820. MEASUREMENTS IN THE TEATRE Many acoustic measurements were performed during last 15 years in the Teatro Comunale of Bologna. However, all these measurements consisted of only binaural measurements by means of different dummy heads. Since the laboratory of DIENCA – CIARM has also created a listening room (called “Arlecchino”) able to reproduce virtual sound fields following Stereo Dipole and Ambisonics methodologies [3,4], a new throughout campaign of measurements was undertaken in order to properly describe spatial sound characteristics of the hall, and especially the stage and orchestra pit, and their relations with the perception of sound in the stalls and balconies [5,6,7] Fig. 2 Measuring positions in the stalls During the measurements, the following instrumentations have been employed. ·- An omni-directional, frequency-equalized sound source (namely LookLine) was utilized, and positioned in the stage and in the orchestra pit [8,9]. ·- A dummy head was positioned at the receiver’s positions (Neumann KU-100). It allowed the measurements of binaural impulse responses, and consequently binaural parameters. ·- A Soundfield microphone (MK V) probe accomplished the dummy head in the theatre. The 4 channels output were utilized during the calculation of monoaural and 3-dimensional parameters. A log sine sweep (chirp), 30 seconds long, was utilized in the measurements. The signals coming from the microphones were directly stored in the PC by means of a 20 bit 96 kHz 8 channels soundboard. The measurements were conducted in 25 different positions, ranging from stalls to balconies, as depicted in figure 2, where also the numerical model, which will be used for simulation, is shown. The sound source was positioned in the stage and then the measurements were repeated moving the sound source into the orchestra pit. A reference position at 1 meter was added, in order to compute spatial maps of strength. Proceedings of the 7th WSEAS International Conference on Acoustics & Music: Theory & Applications, Cavtat, Croatia, June 13-15, 2006 (pp60-64) 125 250 500 1k 2k 4k 8k 16k STAGE C50 [dB] C80 [dB] -1.1 -5.2 -5.3 -2.6 -2.4 -2.8 -3.1 -2.4 1.1 6.0 0.8 -1.7 -1.5 0.6 0.6 12 10 8 6 4 2 0 -2 -4 31.5 63 125 250 500 1000 2000 4000 8000 16000 -6 -8 [Hz] Stage Pit 0.2 0.2 1.2 4.6 10.8 D50 [%] 44.4 26.2 25.0 36.5 37.6 35.3 34.1 37.4 55.8 78.2 Ts [ms] 161.3 179.9 154.2 117.8 111.3 114.7 113.1 94.9 58.5 28.0 EDT [s] 2.6 2.1 1.8 1.6 1.6 1.6 1.6 1.3 0.9 0.6 T20 [s] 2.4 2.3 2.0 1.8 1.7 1.6 1.6 1.3 1.0 0.5 These differences are particularly remarkable in the stalls, rather than in the balconies. EDT 2.5 2.0 1.5 [s] In the following paragraphs the results are briefly reported. From the measurements many different acoustical parameters were calculated. In tab. 1 the mean values obtained in the theatre are represented. The results are reported differentiating the position of sound source in the stage and in the orchestra pit, respectively. As mentioned previously, a sound source with directivity pattern was also used during the measurements. In order to better analyze the influence of both the position of sound source and both its directivity patterns, in the following pictures the results for one specific position in stalls are presented. 63 C 50 14 Fig. 3 Clarity C50 measured with sound source in the stage (blue) and in the pit (red) RESULTS Frequency 31.5 As an overall impression, the Teatro Comunale presents the typical sound characteristics of Italian-styled opera houses. The reverberation time at mid frequencies resulted about 1.4 s. However, a remarkable difference between the acoustics of orchestra pit and stage has been found. [dB] Some further measurements were conducted with a directive sound source, in order to appreciate differences in impulse responses (both binaural and B-format) during the 3D auralization process. The Soundfield microphone allowed both the measurements of mono-aural parameters, like reverberation time and clarity, both spatial parameters, like LE and LF, and B-format (3D) Impulse Responses. The dummy head allowed the measurements of binaural parameters, like IACC. Moreover, the B-format impulse responses obtained with the Soundfield, and binaural impulse responses obtained with the dummy head, allowed creating the virtual acoustics of the theatre [10] 1.0 0.5 0.0 31.5 63 125 250 500 1000 2000 4000 8000 16000 [Hz] Stage Pit Fig. 4 EDT measured with sound source in the stage (blue) and in the pit (red) Considering the graphics representing the values of acoustical parameters measured in one position not far from the orchestra pit, it could be appreciated the variation of the acoustical parameters especially in the initial part of the sound field (less than 100 ms). IACC Early 0.90 0.80 LF 0.68 0.94 0.81 0.81 0.78 0.78 0.68 0.58 0.48 0.38 0.70 0.60 PIT 0.50 0.40 C50 [dB] 3.9 -0.2 -8.7 -4.3 -4.2 -3.4 -3.0 -1.7 2.1 5.5 C80 [dB] 5.5 0.30 0.20 3.5 -0.8 2.1 1.3 0.8 1.0 2.2 5.9 11.5 D50 [%] 70.0 49.1 17.7 31.2 32.0 34.5 35.3 41.1 59.9 72.3 Ts [ms] 93.0 112.4 148.5 110.8 108.2 110.9 104.1 86.7 54.0 34.1 0.10 0.00 250 500 1000 2000 [Hz] EDT [s] 1.2 1.0 1.7 1.4 1.4 1.6 1.5 1.2 0.7 0.4 T20 [s] 2.0 2.1 2.0 1.7 1.6 1.6 1.5 1.3 0.9 0.5 0.6 0.8 0.7 1.0 1.1 1.0 0.8 0.6 0.5 0.5 LF Tab. 1 Values of acoustic parameters measured 4000 8000 Stage 16000 Pit Fig. 5 IACC measured with sound source in the stage (blue) and in the pit (red) Both the energetic parameters (i.e. clarity) showed a relevant difference between the stage Proceedings of the 7th WSEAS International Conference on Acoustics & Music: Theory & Applications, Cavtat, Croatia, June 13-15, 2006 (pp60-64) (which its direct sound that is clearly perceived by the receiver) and the orchestra pit, where the position of the sound source and the effect of the fence provoked a diffuse sound field without direct sound from source to receivers. C 80 30.00 Stage L 25.00 Stage Lem L 20.00 Stage R 15.00 Stage Lem R 10.00 Pit L Pit R 5.00 Pit Lem L 0.00 -5.00 31.5 63 125 250 500 1000 2000 4000 8000 16000 Pit Lem R -10.00 Fig. 8 RT30 measured in stalls with different sound sources and positions Only Reverberation Time remained fairly stable with sound sources. But analyzing the values of IACC, it resulted quite depending from the sound source, with variations ranging from 0.15 to .075 at the frequency of 2 kHz. This was not a surprising result, since IACC is calculated from the initial 80 ms of impulse responses, i.e. the component of IRs that strongly depended on different sound sources and positions. Fig. 6 Clarity C80 measured in stalls with different sound sources and positions Even reverberation time resulted considerably different, moving sound source from stage to pit. The Early Decay Time, which is calculated in the early decay time, resulted more influenced than the RT30, which take into account a longer decay time. EDT at mid frequencies varied of also 0.5 s in some cases, whereas RT30 varied of 0.2 s. T 30 ACKNOWLEDGEMENTS The authors wish to thank Enrico Reatti, for his help during the measurement, and for his help in getting the historical background of the theatre, and Gerando Pece, for his collaboration during the experimental measurements. REFERENCES [1] M. Gerzon - "Recording Concert Hall Acoustics for Posterity", Journal of Audio Engineering Society, 23(7) p. 569, 1975. [2] A. Farina, R. Ayalon - "Recording Concert Hall Acoustics for Posterity" 24th AES Conference on Surround Sound, Techniques, Technology and Perception – Banff, Canada 26-28 June 2003. [3] L. Tronchin, V. Tarabusi, A. Giusto: The realization of Ambisonics and Ambiophonics listening room “Arlecchino” for car sound systems evaluation, Proc. 21st AES Conference, St. Petersburg, Russia, 2002. [4] L. Tronchin, G. Curà, V. Tarabusi: The enhancement of the Arlecchino listening room: adding stereo dipole to ambisonics, Proc of. Forum2005, Budapest, Hungary, 2005. 3.00 Stage L 2.50 Stage Lem L 2.00 Stage R Stage Lem R 1.50 Pit L Pit R 1.00 Pit Lem L Pit Lem R 0.50 0.00 31.5 63 125 250 500 1000 2000 4000 8000 16000 Fig. 7 RT30 measured in stalls with different sound sources and positions One more analysis involved different positions of sound source and directivity patterns. In the figs 6-8 some acoustic parameters measured in the hall are represented. The variations of these parameters were really remarkable. Even clarity changed considerably with sound source. IACC Early 0.90 Stage L 0.80 Stage Lem L 0.70 Stage R Stage Lem R 0.60 0.50 Pit L 0.40 Pit R 0.30 Pit Lem L Pit Lem R 0.20 0.10 0.00 250 500 1000 2000 4000 8000 16000 Proceedings of the 7th WSEAS International Conference on Acoustics & Music: Theory & Applications, Cavtat, Croatia, June 13-15, 2006 (pp60-64) [5] A. Farina, L. Tronchin: “3D Impulse Response measurements on S. Maria del Fiore Church, Florence, Italy” Proc. 16th International Congress on Acoustics, Seattle, USA, 1998. [6] A. Cocchi, D. Massobrio, L., Tronchin: “Room Acoustics and Cultural Heritage: how to preserve?” Proc. “2nd International Congress on "Science and Technology for the Safeguard of Cultural Heritage in the Mediterranean Basin”, Paris, France, 1999. [7] Tronchin, L., Farina, A., Pontillo, M., Tarabusi, V.: “The calculation of the impulse response in the binaural technique”, Proc. 7th International Congress on Sound and Vibration (ICSV), Garmisch, Germany, 2000. [8] A. Farina: “Simultaneous measurement of impulse response and distortion with a swept-sine technique”, Proc. 110th AES Convention, Paris 18-22 February 2000. [9] A. Cocchi, L. Tronchin, A. Farina, M. Cesare-Consumi: “A comparison between some measurement techniques in the Foligno auditorium” Proc. Auditorium Acoustics: Historical and contemporary design and performance, IOA, Imperial College, London, UK, 2002. [10] A. Farina, L. Tronchin: “Advanced techniques for measuring and reproducing spatial sound properties of auditoria”, Invited Paper. Proc. of RADS, Room Acoustics: Design and Science, Awaji, Japan, 2004.