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8.1 Standing Waves
The parallel surfaces of most listening rooms can lead to a potential problem in the low
frequencies. 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.)
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.