Room Acoustics

Room Reverberation

The Boundary Effect

The following information on room acoustics does not need to be considered in every installation. Rather, it is provided for those who plan a dedicated listening room, or for those who feel they have a problematic room and therefore need ideas about how to improve their system’s performance further.

Once again, the value of your dealer’s experience should never be underesti- mated. Many installers have been involved in dozens or hundreds of home the- ater installations similar to yours, and have proven solutions to whatever problem you might be experiencing. The information provided here is best used as a starting point for your discussions with your dealer.

In a perfect world, your room would have no characteristic sound of its own, no acoustical “fingerprint.” The ideal room would be perfectly neutral and would not superimpose itself on the sound within it in any way. After all, any reverberation or ambient sound which the director wished people to hear will be recorded in the soundtrack. And much of the inherent ambience in music recordings will normally be reproduced by the rear speakers (using the stereo surround mode), where such ambience belongs. Anything beyond this added by the room would be redundant and would actually detract from the realism. In general, then, the ideal listening room will be somewhat more “dead” acoustically than the average living room. This goal can be accomplished through the use of drapes, plush car- peting, or various acoustical treatments.

Note, however, that the surround speakers depend on reflecting sound to develop the proper enveloping characteristic, and that they therefore need some reflective surfaces. Ideally, these would be diffusive in nature, providing randomized reflec- tions in many directions. Bookcases and other irregular surfaces provide diffu- sion, as do some commercially-available wall treatments. If there is some degree of choice in the matter, it is generally better to have the rear 1⁄3 of the room be reflective and diffusive, while the front 2⁄3 of the room is relatively absorptive. Re- sist the temptation to “go overboard,” however, lining the room with absorptive material from floor to ceiling. It is possible to have too much absorption, resulting in reduced subjective dynamic impact and therefore less excitement.

A well-known effect of room acoustics is the change in bass and mid-bass re- sponse which results from moving a speaker near a wall. This so-called “Bound- ary Effect” is the result of the reflection of the extremely long bass wave off the wall being substantially in-phase with the direct sound radiated toward the lis- tener. This in-phase reinforcement effectively “doubles-up” on the amplitude of the bass relative to what would have been heard without the wall reflection. If a speaker was originally designed to produce flat response when situated in the middle of the room (not near any room surfaces), placing it on the floor or against a single wall often makes it sound somewhat bass-heavy. Placing it where the floor and the wall meet will produce even more bass, and placing it in the corner (at the intersection of three room surfaces) is enough to make almost any speaker sound congested and muddy, unless it was specifically designed for that type of placement in the first place. (In practice, the actual difference you hear may vary slightly from room to room, depending on how solidly the walls are built. A light, flexible wall may “leak” bass into the next room, reducing the mag- nitude of the effect.)