Acoustics in theatres: CSTB sets the tone
Clients and contractors share the same ambition both for renovation and construction of theatres, to obtain the best possible acoustics. "Every demand requires a specific response, emphasizes Isabelle Schmich, Manager of the Room and Complex Volume Acoustics cluster at CSTB in Grenoble. Depending on the case, we make recommendations, for example about the configuration of the rooms or the choice of materials. We are working on drawings and we are also carrying out acoustic simulations and model studies." We sometimes recommend installing the Carmen® system. With this patented system, an innovative acoustic technique designed by CSTB researchers, it is possible to overcome bad sound propagation affecting some concert halls. The principle is the use of active virtual walls composed of quasi-local reaction cells distributed on the walls and ceiling around the room. Each cell picks up and restores sound in real time as if it was naturally reflected, so that the distribution of sound, clarity or intelligibility of sound can be managed.
Before installing Carmen®, the number of seats installed, the volume of the room, the empty room reverberation time and the reverberation time required by the client have to be known. Drawings of the room are also essential to determine the distribution of volumes, the acoustic distribution and any defects or sharp edges in the architecture that could influence sound propagation. Finally, a "visual diagnostic" has to be made to confirm the initial conclusions.
Silence, we are installing Carmen®…
The next step is interaction between CSTB's acoustic team, the architect and the client to determine the best locations for integration of active cells forming the Carmen® system. "24 cells were necessary for the Miskolc Palace des Arts that contains 580 seats. The orchestra conductor wanted ten configurations to be programmed including theatre, chamber music, recital, concerto, classical symphony, romantic symphony and oratorio, with sound reverberation time varying from 1.3 seconds to 2.2 seconds."
Note that the required reverberation times vary from one type of show to another, from 0.8 to 1.2 for a jazz concert to 1.8 to 2.5 seconds for a romantic symphony. Carmen® can easily control the variations necessary for quality and acoustic comfort. Once installed, it has been used immediately in the freshly renovated Norwich Theatre Royal in the United Kingdom, where CSTB's Acoustic team worked at night for fifteen days before the public opening on November 13 2007. 27 cells were installed in this room and concealed in the decoration, to obtain six configurations and host 1 280 persons.
"Room acoustics is precision work," says Isabelle Schmich. Silence is essential when making the adjustments, to test sound reverberation times and system response. Once the system has been installed, CSTB provides remote maintenance by taking control of the Carmen® system from its Grenoble site, where it can make any corrections to settings if necessary."
High-performance acoustics are not beneficial only to large theatres, Carmen's little sister, Carmencita is currently being developed for more modest concert rooms of approximately about 300 seats.
