Acoustical Society of America
ICA/ASA '98 Lay Language Papers

The Teatro Comunale in Bologna, Italy.

THE BOXES: SOUND ABSORPTION AND EFFECT OF CAVITIES
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.

DESIGN OF A NEW ORCHESTRA SHELL
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.