LEGAULT & DAVIDSON
Comments on architectural acoustics — Preliminary design phase
14 June 2006
Dear Paul
As per your request, here are my comments regarding the acoustics of the architecture for the preliminary design phase of the Dolbeau-Mistassini theatre project.
1. Theatre use
The theatre's main function is to host multiple activities, from speech (plays and conferences) to musical activities (rock concerts). The majority of events will require a sound amplification system to insure adequate acoustic volume for the whole audience.
Classical concerts with natural sound will be the exception. They will require the addition of a movable orchestra shell. The acoustical parameters of the house are not designed this sound source.
2. Acoustical criteria
The acoustical criteria applicable to the base building deals with two major aspects:
—sound intimacy
—auditorium acoustics
2.1. Sound intimacy
Sound intimacy is about providing an environment sufficiently calm to hold the programmed activities, amidst the building's systems. Background noise is measured when there is no activity in the hall. It is composed of noises generated by the building's systems and residual noise inside and outside the hall. The criteria for ambient noise allow one to plan the different sound barriers that control airborne noise coming from inside and outside the building. These insulating sound barriers are the walls, the ceilings, the floors, the doors and the windows.
The recommended background noise level is:
—NC-25 for the hall (a.k.a. auditorium / front of house)
—NC-35/40 for the foyer
2.2 Auditorium acoustics
The acoustics of the auditorium has to provide good hearing conditions for the totality of listeners. Many parameters contribute in obtaining those conditions. They mainly consist of the acoustical volume, sound diffusion, reverberation time and reduction of acoustical defects to an acceptable level.
The recommended RT (reverberation time) for the auditorium is about 1 second at 500 Hz.
This compromise is suitable for most of the scheduled activities, namely speech and music.
The adequate volume level for every listener in the audience is insured by a sound system.
3. Acoustical integration with the architecture
The acoustical parameters have been integrated in the project during the preliminary design phase.
Sound intimacy requires the use of vertical and horizontal barriers made of assemblies that allow the ambient noise level of the auditorium to be respected. Sound insulating vestibules have been provided between the auditorium and adjacent spaces allowing proper sound attenuation, specifically for the access door to the hall.
Good acoustical conditions have been developed within the hall by reducing the distance between the source and the seat furthest to the stage. The orchestra's gentle slope allows better dispersion of direct sound. Wall surfaces closest to the stage and sound source have hard reflecting surfaces projecting the sound while the back walls have highly absorptive surfaces made of fabric covered porous materials. Those porous materials are essential to control natural reverberation and to reduce acoustical defects found in confined spaces such as delayed sound reflections and sound concentrations.
When final details will be designed, the profile of the balconies and the ceiling above the orchestra will be adjusted to reduce the risk of acoustical nuisance at an acceptable level.
4. Conclusion
It appears to me that the concept of the auditorium will be able to successfully fulfill the expected acoustical parameters.
The required adjustments will be integrated during the construction documents phase and within the architectural specifications.
Jean-Pierre Legault, acoustical consultant