Auditorium/Theater

Goal: To properly balance absorption and reflection to provide a favorable acoustical environment. One must address both the need to hear and understand speech, and the desire to have a pleasant space for music.

Related Codes & Standards

• Noise Criteria (NC)
• Sound Transmission Class (STC)
• Reverberation Time (RT 60)

Tips/Considerations

• Recommended reverberation time is 1.0-1.5 seconds (might be higher for some auditoriums).
• Although the seating area will provide absorption, thereby reducing the reverberation time, you will most likely need to add absorptive materials to the other surfaces within the space.
• It is vital to control the reflections from the back wall. If you don’t control them, the presentation could reflect off the back wall and “slap back” to the presenter(s). This won’t necessarily impact the audience, but could be disastrous and distracting for the people on stage. Because of this, it’s usually necessary to treat the back wall with an absorptive material. A concave back wall could compound this problem. If you can’t avoid a concave back wall, it’s imperative that it be treated with absorptive material.
• Splay or use irregular surfaces on the walls to avoid flutter echoes. Parallel reflective surfaces can allow sound to “ricochet” back and forth between the surfaces. This potentially annoying condition is referred to as standing wave or flutter echo. It is avoided by constructing non-parallel surfaces or by adding absorptive materials to the surface(s).
• Consider faceting the ceiling to help with sound dispersion.
• Control the reverberation time on the stage. Ideally, the reverberation time in the stage area should be the same as in the house. Since the stage area might have a higher ceiling than the rest of the auditorium, more absorptive materials might be required in this area. Frequently, the back wall of the stage, and possibly one or two of the side walls, is treated with an acoustically absorptive material, typically black in color.
• Remember the space will be less absorptive when only half full, since the audience itself is absorptive. By using absorptive seating areas, the reverberation time will remain more consistent regardless of the audience size.
• Noise from the lobby area can be disruptive. Be sure openings such as doorways are properly sealed. Consider a vestibule door system.
• Persons seated deep under a balcony might experience auditory distortion. To avoid this, the balcony should be no deeper than twice its height. Ideally, the balcony should not be any deeper than its height.
• Even if everything else is controlled perfectly, the space might not be usable if the background noise (e.g. HVAC system) is too loud. To help protect your design, the NC level should not exceed 20 to 35. When specifying NC, specify an actual rating, such as NC 20, rather than a range, such as NC 20-30. Although specifying a lower number will ensure minimal background noise, it might be cost prohibitive to achieve. Be realistic about the amount of acceptable noise and the project’s budget when specifying an NC level.
• Beware of potential outdoor noise impacting your space. For example, is your location near a flight path, a railroad or freeway? If so, you might have to pay critical attention to blocking this noise. To do so effectively, you must address not only the STC or isolation quality of the exterior wall, but also for the possibly weaker building elements, such as the windows, doors and HVAC systems.