--------------------- Acoustical Society of America -Boxes and sound quality in an Italian opera house

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Acoustical Society of America
135th Meeting Lay Language Papers

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Boxes and sound quality in an Italian opera house

Alessandro Cocchi - alessandro.cocchi@mail.ing.unibo.it
Massimo Garai - massimo.garai@mail.ing.unibo.it
and Carla Tavernelli - carla.tavernelli@mail.ing.unibo.it
DIENCA, University of Bologna
viale Risorgimento 2
40136 Bologna, ITALY

Popular version of paper 2aAAb1
Presented Tuesday morning, June 23, 1998
16th ICA and 135th ASA Meeting, Seattle, WA
Embargoed until June 23, 1998

The object of the present study is the "Teatro Comunale" (City theatre) in Bologna, an Italian opera house of the 18th century, designed by Antonio Galli Bibiena. The theatre has a "bell" shape in plan, four order of boxes plus a gallery on the walls and a vaulted ceiling. There are 540 upholstered seats on the wooden floor of the stall and 466 in the boxes and in the gallery.

Theatre picture

The Teatro Comunale in Bologna, Italy.

In a former study the listening quality in the theatre was evaluated by measurements in the stall and in the boxes, while an old orchestra shell was placed on the stage; the raw data were further processed following the Ando's theory. The measurements revealed clear differences between the listening quality in the boxes and in the stall, especially regarding intimacy, clarity and enveloping of sound. A late arrival of sound energy on the central boxes was detected and related to a second order reflection on the flat, reflecting, rear wall of the orchestra shell and the ceiling.

There are two different mechanisms by which the boxes influence the sound field in the hall: the sound absorption of the box internal walls and the whole effect of the cavities on the box-covered walls. As it is impossible to alter an ancient theatre for studying these effects in an experimental way, computer simulation is the only way to investigate them. A computer model of the theatre was built for use with the simulation software Ramsete. The model was validated using a procedure, elsewhere detailed (e.g. in a paper presented at the MCHA 95 international congress), to adjust the model geometry, the characteristics of the sound sources and the sound absorption of the surfaces until the simulated results match the measured ones. Each orchestra section was simulated by a model source with the appropriate sound power and directivity. During the several restorations occurred in the theatre, the velvet lining and curtains of the boxes were almost completely removed and now it is difficult even to guess the acoustic characteristics of the original materials. At present, the theatre is claimed to be more "live" than similar Italian opera houses. Thus, the first aim of the present study was to assess how and how much the introduction of some modern fabric lining or curtains in the boxes could change the theatre acoustics. In other words, the work shows how the sound in the historical theatre could be if the absorption of the box internal walls were changed, e.g. adding some velvet lining or curtains, which are supposed to exist in ancient times. In the computer model, velvet curtains were added into the boxes and the simulation results were compared to those obtained for the actual all-plaster boxes. For several acoustic quantities, the change from the actual situation was computed and averaged over the stall. For example, the reverberation (sound tail) of the room is reduced, due to the added absorption. The predicted variations are non-uniform over the audible frequency range, because the sound absorption coefficient of the velvet curtains takes into account an air gap between the curtains and the wall. Computer simulation can also help in understanding the role of the cavities which constitute the boxes: the effect of these cavities was "cancelled" inserting in the computer model a smooth wall closing the opening of each box. simulations show that without the box cavities the reverberation would be reduced at low frequencies and enhanced at high frequencies. The hypothesis can be made that at low frequencies the boxes act as resonators to sustain the reverberant sound, while at high frequencies they are "sound traps" which recall some energy from the hall. The transition is smooth and located in between the low- and mid-frequency regions. This hypothesis would confirm that the "warmth" of Italian opera houses is also due to the box-covered walls. The same simulation shows that the effect on loudness of sound would be very small, as the temporal redistribution of sound energy doesn't affect its overall value.

As measurements revealed that the actual orchestra shell is not completely suited to the theatre, a completely new orchestra shell has been designed by means of computer simulation. The new shell has a variable configuration to be made suitable for different orchestra sizes, with and without chorus; it has diffusing surfaces and a shape exactly fitted to uniformly redirect the sound on the stall and to prevent the problem of late arrival of sound into the central boxes.

New orchestra shell

Rendering of the newly designed orchestra shell.

Computer simulation predicts noticeable improvements of the acoustic quality. As an example, the following maps show the improvements in the C80 values, one of the most used acoustic criteria for evaluating the acoustic quality of theaters and concert halls.

Clarity map - boxes Clarity map - stall

Improvement of the listening "clarity" (technically speaking: in the C80 values in decibels) in the 2nd order boxes (left) and in the stall (right) with the newly designed orchestra shell (in the configuration without chorus) in place of the actual orchestra shell.

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