Advanced techniques for the determination of sound spatialization in Italian Opera Theatres ENRICO REATTI, LAMBERTO TRONCHIN & VALERIO TARABUSI DIENCA University of Bologna Viale Risorgimento, 2, Bologna ITALY Abstract: - As Italy possesses a rich heritage of Opera houses and since, owing to the period in which they were built, they were not planned for electroacoustical amplification systems and the quality of the listening conditions in the theatres was still wholly due to their spatial characteristics, the materials and the techniques used to build them, the Teatro Comunale of Bologna was chosen for this research. An experimental measuring campaign was conducted in this late 18th century Italian-style theatre so as to define a map of the impulse responses (IR) able to significantly represent the acoustics perceived by the audience seated in the stalls or balconies. Using the geometric and constructional data, a three-dimensional model of the theatre was then made and used to numerically simulate the acoustic properties in the theatre and, after appropriate calibration, the respective impulse responses were calculated in the same points as where the measurements had been taken in the experimental campaign. Lastly, the method for auralising an anechoic signal was used for qualitatively comparing the simulation of the theatre s acoustic response. This was done by calculating an IR from the experimental data and extrapolating an IR from the numeric model. Key-Words: - Teatro Comunale of Bologna, Acoustical measurements, Impulse Response, Acoustical Parameters, Spatiality, Acoustical simulation, Auralisation 1 Introduction Opera houses are an important world heritage and Italy possesses several, dating back to different periods. Many of them were built in the 18th century with the typical Italian-style structure. Although they were all created for the same purposes, being designed as important social venues and places where a certain type of music was performed, they all featured different shapes, furnishings and building techniques. This is why experiments and research into their acoustic properties have proliferated for a great number of years. In this particular case, the research began with a new measuring campaign conducted in 2005 using the most recent measuring techniques (B-format and dummy head). A three-dimensional model of the entire auditorium and stage complex was made and used to calibrate a numeric model able to provide an acoustic estimate of the theatre. Lastly, an anechoic excerpt was auralised and the result of the experimental investigation was then set in parallel with that of the model, on a numeric basis. 2 Description of the theatre The research was conducted in the Teatro Comunale of Bologna, which holds an audience of 900 seated in the stalls and in the four tiers of balconies. It was built in the second half of the 18th century by the architect Antonio Galli Bibiena, who had previously designed other Italian theatres like the Teatro Scientifico in Mantua and the Teatro of the Four Horsemen in Pavia. Fig.1 longitudinal section ISBN: 978-960-6766-74-9 133 ISSN 1790-5095
This theatre differs in various ways from others built at the same time. It is bell shaped (different from the more typical horse-shoe shape), while plastered bricks were used to build the theatre s auditorium, very different from the more traditional auditorium made of wood. Teatro had already been analysed from the acoustic aspect using the dummy head only. Measurements were taken in a representative number of listening points in the theatre (25, of which 9 in the stalls and 16 in the balconies) using two microphonic techniques at the same time: B-format measurements (with the Soundfield system) and binaural measurements (with the Neumann dummy head). The omnidirectional source was positioned both on the stage and in the orchestra pit. Fig.2 theatre overview plan Once the prerogative of wealthy folk, the balconies in this auditorium still reveal traces (in the plasterwork brought to light during the restoration work of 1980) of the personal touches that each family was allowed to add to its private balcony. On the other hand, no trace now remains, even of an historical nature, of the presumably varied furnishings with which the balconies were once adorned. Theatre specifications: Capacity: 420 spectators seated in the stalls and about 480 in the balconies. Volume of the auditorium: 6000 m 3 Stage area: 460 m 2 Height and width of the stage complex: 16 m height, 15 m width. Fig.3 - measuring positions in the theatre 3 Experimental analysis A new analysis, using present day investigation techniques (sine sweep) and the more recent instruments, was conducted despite the fact that the Fig.4 - perspective view of the measuring positions The recordings made were used to process the impulse responses, while these latter were used to ISBN: 978-960-6766-74-9 134 ISSN 1790-5095
calculate certain significant acoustical parameters, such as: Strength (G), Definition index (D50), Clarity index (C50 and C80), Early decay Time (EDT), Reverberation time (T30), Lateral Fraction (LF), Interaural cross-correlation index (IACC). 4 Numeric model A 3D cad model was then made of the theatre s auditorium and stage complex behind, using present day plan and elevation views. After this, research was conducted into the materials and furnishings with which the theatre is made and the absorption coefficients at the various different frequencies were estimated for each type of material used. Once this three-dimensional geometric model had been made, it was converted into a 3DFaces model (flat triangular elements) in which the previous continuous solid was approximated with around 22000 flat faces. Fig.5 - acoustical parameters The sampling frequency used for acquiring the data was 96 khz and 20 bits by means of a Layla echo audio board, while post-processing was conducted with signals at 96 khz and 32 bits. Fig.7-3D cad model Fig.6 - diagram of the measurement acquisition chain The Ramsete calculation software, based on the pyramid tracing method (a variant of the conetracing algorithm) was used for processing. The associations between the properties of the materials and the flat elements were entered and the same grid of 25 receiver points used for the experimental investigation was reconstructed. ISBN: 978-960-6766-74-9 135 ISSN 1790-5095
Fig.8 - file of absorption coefficients of the materials Lastly, the simulation program was run and the simulation repeated several times. The settings of both the geometric model and those of the characteristics of the materials were adjusted on each run until the required degree of precision had been obtained. Fig.11 acoustical parameters extrapolated from numeric model The virtual estimation of as many impulse responses as were provided by the experimental investigation was obtained at the end of the process, after which the acoustic properties of the auditorium could be estimated again. A very good correspondence was observed between the experimentally measured data and those extrapolated from the geometric-physicalmathematical model. Fig.9 - Ramsete model 5 Auralisation Auralisation was carried out in order to virtually reproduce the acoustic properties of the theatre as extrapolated by means of the two different methods described above. A recording made in an anechoic chamber of an excerpt from an opera (Torna a Surriento) was processed by means of the experimentally and numerically obtained impulse response of a seat in the stalls (20d). Fig.12 anechoic waveform Fig.10 - graph of the impulse response at 1000 Hz Fig. 13 impulse responses (from measurements and from the numeric model) ISBN: 978-960-6766-74-9 136 ISSN 1790-5095
Fig.14 - wave shape of the anechoic and auralised excerpt with the two different impulse responses Two, very similar auralised excerpts were obtained, thus confirming that modelling had been successful. 6 Conclusions In conclusion, the acoustic properties of an Opera house were studied in two different ways, i.e. experimental investigation and numeric simulation using the pyramid tracing method. These two methods testified to the reliability of numeric modelling, of great use in many different fields: from brand-new projects to restoration and the ability to forecast the effects on acoustical efficiency caused by "simple" modifications to furnishing or embellishments. 7 References 1. L. Tronchin, A. Farina (1997) "The acoustics of the former Teatro "La Fenice". Venice". J. Audio.Eng Soc. 45(12), pp. 1051-62 2. Farina, L. Tronchin (1998) 3D Impulse Response measurements on S. Maria del Fiore Church. Florence. Italy Proc. of 16th International Congress on Acoustics. Seattle. USA. 3. Farina, R. Ayalon (2003) "Recording Concert Hall Acoustics for Posterity" - 24th AES Conference on Surround Sound. Techniques. Technology and Perception Banff. Canada 26-28 June 2003 4. Farina (2000) Simultaneous measurement of impulse response and distortion with a swept-sine technique. 110th AES Convention. Paris 18-22 February 2000 5. Farina, L. Tronchin (2004) Advanced techniques for measuring and reproducing spatial sound properties of auditoria. invited paper. Proc. of RADS 2004. Hyogo. Japan. 6. L. Tronchin, A. Farina, M. Pontillo, V. Tarabusi (2000) The calculation of the impulse response in the binaural technique. Proc. 7 th International Congress on Sound and Vibration (ICSV). Garmisch. Germany. 7. Cocchi, D. Massobrio, L. Tronchin (1999) Room Acoustics and Cultural Heritage: how to preserve? Proc. 2 nd International Congress on "Science and Technology for the Safeguard of Cultural Heritage in the Mediterranean Basin. Paris. France. 8. L. Tronchin, V. Tarabusi, A. Giusto (2002) The realization of Ambisonics and Ambiophonics listening room Arlecchino for car sound systems evaluation, 21 st AES Conference on architectural acoustics and sound reinforcements - St. Petersburg, Russia 9. J.Ahrens and S. Spors (2008), Reproduction of a plane-wave sound field using planar and linear arrays of loudspeakers, IEEE Int. Symposium on Communications Control and Signal Processing (ISCCSP). ISBN: 978-960-6766-74-9 137 ISSN 1790-5095