THORBURN ASSOCIATES

Acoustic and Technology Consultants

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To RT60, or Not To RT60

It's surprising how often we're asked to design a space for a specific reverberation time (RT60), when in actuality it is a completely inappropriate criteria for the project. We understand that "reverberation" is just a really fun word to say, but it is very important to utilize the proper tools when analyzing the acoustical design of a space. For the case of RT60, large rooms are the key. Strictly going by size, a room can be considered "large" when it starts to reach 6,000 cubic feet.

Acoustically speaking, a room qualifies as "large" when the reverberant field dominates the properties of the room, such as loudness, clarity, balance, and intimacy. When the amount of sound that reaches your ear due to reflected sounds is greater than the amount of sound coming from direct sound waves, you are in the reverberant field. Within this zone, the loudness of sound is relatively constant from one position to the next, and the overall level depends primarily on the total amount of the room's absorption.

This is the major difference between large and small rooms, where small rooms are not able to develop this diffuse, uniform sound field, and are instead chiefly characterized by acoustic abnormalities due to the room's physical dimensions.

Large rooms demand a great deal of respect when designing for proper acoustics. With the clarity of sound being of utmost importance, three elements should be addressed: reverberance, background noise, and reflections (echo).

While reverberation and reflections play an important role in distributing sound throughout a room, if left unchecked reverberation can seriously deteriorate the understanding of speech and the perception of individual lines of music as sounds blur together. Excessive background noise can additionally raise the threshold of audibility in the room, masking quieter passages of music and spoken consonants that give speech its intelligibility.

Rooms where the perception of music or speech is key to its functionality - gymnasiums, rehearsal rooms, worship spaces, ballrooms, cafeterias, and corporate boardrooms - will greatly benefit from good acoustics. Good acoustic design creates a comfortable, safe, and usable environment within any large space.

Storrs Hall, a part of the College of Architecture at the University of North Carolina Charlotte, is a good example. The building has a large gallery for exhibitions, social events, educational gatherings, convocations, and meetings. The problem was that nobody within the room could hear or understand each other, so TA was brought in to review the space and provide recommendations.

Initial measurements showed a reverberation time three times longer than the space should have been designed for. Focusing on the excessive reverberation and reflections, we provided recommendations on the placement and form of additional interior finishes. With the Hall's renovations complete, the Dean of Architecture, Ken Lambla, happily informed us that at a recent meeting held in the gallery, "it was the first time we could all hear each other speak!" We're glad to have helped!

The capacity for communication and understanding is a fundamental aspect of a large room. To enjoy an excellent sounding room from day one, designing for reverberance, background noise, and reflections will provide the quality of acoustics you need for enhancing speech and music intelligibility.


Thorburn Associates, December 2007

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