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Acoustics
Acoustics
Magazine reviewers and audio system
owners spend much time critically apprais-
ing speakers and other audio components.
Unfortunately, a phenomenon that has a
very large effect upon sound is not easily
judged or changed. That effect is the
ACOUSTICS of the environment in which
you are listening.
Room acoustics is a complicated subject
about which hefty textbooks have been
written, and entire galaxies have gone to
war over. We simply want you to be aware
of a few basics that have a direct effect on
real time audio analysis.
As you probably learned in high school,
sound travels in waves. In an audio
system, these waves are created by the
speakers. Like waves in a pond created by
a splash, sound waves emanate from the
transducers (speakers) and spread out into
the room. If your room were infinitely big,
that’s all there would be to it. But just as
waves in a pond reach the bank and reflect
back, sound waves bounce off walls, ceil
-
ings, and floors, reflecting, reinforcing and
canceling each other as shown here:
Since sound is energy, the way it reflects
depends upon the angle of the surface,
the type of material and the frequency of
the sound wave. Because your listening
position is likely to be towards the back
of the Free Field (waves shown in the
diagram), you also get part of the reflect
-
ed Reverberant Field as well.
Now we add the next set of complications:
Different frequencies of sound have differ
-
ent wave lengths (a function of frequency
and the speed of sound). Each frequency’s
wavelength contributes differently to the
Free and Reverberant Fields because they
are different sizes. For example, a 32 Hz
bass note has a wavelength of 35 feet,
while a 16,000 Hz note has a wavelength
just under a tenth of an inch. Tiny treble
waves can be caught and neutralized by
draperies, carpeting, upholstered furniture
and gangs of indolent Persian cats…while
gigantic bass waves simply slosh back and
forth in the room.
Another set of variables is the shape and
volume of your listening room. Large
rooms require more bass energy to excite
waves within them. Small rooms need less
energy, but reflect it differently. And then
there’s the fact that most rooms don’t
have four walls anymore, but open into
dining rooms, lofts, cathedral ceilings, etc.
All of this means that predicting sound
interaction patterns is very difficult due to
the irregularities of the room shape.
As you can see, room acoustics is an
important but complicated subject. To
learn more about room acoustics, get a
copy of AudioControl’s Technical Paper
107, “Small Room Acoustics De-Mythol-
ogized”. You can download this paper
from www.audiocontrol.com (search
“De-mythologized”) or if you’re still into
the printed page, call us and we’ll mail
you a copy. The overall point that we’re
trying to make is that the various rooms
in a home function as gigantic mechanical
equalizers, boosting or cutting certain
frequen-cies depending on size, shape,
volume, acoustic treatment and the posi-
tion of the speakers.
