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10.1 Standing Waves
The parallel surfaces of most listening rooms can lead to a potential problem
at any frequency. A sound wave can be repeatedly reflected from opposing
surfaces, back and forth. If the distance between the surfaces is an integral
multiple of one-half the sound wavelength, a standing wave will be set up.
This means that the incident and reflected waves combine with each other so
that a stationary pattern of high and low sound pressures is established in the
room. This irregular distribution of sound level is caused by cancellation and
reinforcement between the reflected and direct sound waves.
At high frequencies, this pattern of high and low sound pressure levels within
the room becomes too finely spaced to be discerned. However, when the
dimensions of the room are comparable to the wavelengths of the musical
notes, there will be obvious changes in the intensity of certain bass notes in
different locations within the room. Additionally, the existence of the standing
wave implies a resonant condition where acoustic energy is stored in the
room. This energy storage can result in "heavy", "muddy", or "slow" bass.
Since the presence of standing waves is caused by parallel reflective
surfaces, practically every listening room suffers from this problem to some
degree. However, several factors are working in our favor here. First, as the
room size increases, the affected frequencies become lower and thereby less
audibly apparent. Second, the presence of shelving or furniture against the
walls will break up the large surfaces, reducing the magnitude of the
problem. Third, upholstered furniture can absorb a significant amount of bass,
diminishing the build-up of resonant energy. Fourth, typical wall construction is
not completely reflective at low frequencies.
However, in some cases audibly objectionable standing waves will still be
present in the listening room. This can be noted by large variations of the
intensity of certain bass notes in different areas of the room. Another indicator
is an unevenness of loudness of different bass notes. (This is sometimes what is
actually on the recording, so be sure that this is consistently a problem on a
variety of recordings, or use a variable warble-tone generator.)
If you wish to reduce or eliminate standing waves that may exist in your room,
it will be necessary to reduce the low-frequency reflectiveness of at least one
of the parallel surfaces of opposing surfaces. The most effective method is to
use Tube-Traps, available from Acoustic Sciences Corporation. This is the only
commercially available sound treatment that absorbs significant amounts of
energy below 400 Hz. Experimentation will be needed to determine the
optimal locations.