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Room Acoustics
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.
room reverberation
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.
the boundary effect
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.)