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Q & A: Slapback Sound

Reflections are almost always unique for a given source and receiver position.

Q. I've got a large church sanctuary that is 80 feet from the altar to the back wall, and is hard, untreated Sheetrock and curved. There is an audible slap-back echo from sounds produced at the altar area. What's the best way to determine where those reflections are coming from, and how should we treat the offending surfaces?

A. Reflections are almost always unique for a given source and receiver position. If your source is the church sound system, measure the impulse response at several listener locations around the altar. Then, see if you can find a reflection that matches the approximate travel time for a sound wave from the loudspeaker, to the curved wall, to the listening position. Knowing the approximate speed of sound, you can look at the impulse response and determine the location of the reflecting surface from the geometric relationship of the loudspeaker, the reflecting surface, and the listener location. Once you have identified the reflecting (or focusing) surface, you can temporarily hang some sound-absorbing materials, such as packing blankets, and retest to confirm your findings.

To remedy the situation, first try to re-aim the sound system to reduce the amount of sound energy hitting the offending wall. This often is the most cost effective way to reduce this type of problem. After you have done this, then treat the surfaces with either absorption, diffusion, or some combination of both.

–Sam Berkow, principal

SIA Acoustics, New York

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