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“Early reflections” will tend to diminish the sound stage in the direction of the reflections – “early reflections”
from the side walls tend to reduce sound stage width, while “early reflections” from the back wall will reduce
image depth. Some listeners are finding that a strong sense of depth enhances the feeling of involvement when
listening, due to three-dimensional solidity of images. Therefore, it’s more important to have a greater distance
from the speakers to the rear wall than to the side walls. Typically, this is easier to achieve if the speakers are
placed along the short wall of the listening room.
Getting fast, tight, realistic bass
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
a multiple of one-half the sound wavelength, a standing wave will be set up. This means that the incident and
reflected waves combine so that a stationary pattern of high a 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 wavelength 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. Finally, typical wall construction isn’t 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).
In order to reduce or eliminate standing waves that may exist in your room causing “heavy”,
“muddy” or “slow” bass, it will be necessary to reduce the low-frequency reflectiveness of at least one
of the parallel surfaces opposite of each other. The most effective method is to use Tube-Traps,
available from different manufacturers such as Acoustic Sciences Corp. 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.
Improve musical detail
Parallel, reflective surfaces can also produce a different audible problem. If there is little absorption at higher
frequencies, musical transients containing high frequencies, such as a hand clap or strike of a percussion
instrument, can be heard bouncing repeatedly between the surfaces. This effect is called flutter echo (or slap
echo), and such multiple reflections can obscure music detail. The situation is analogous to standing between
two parallel mirrors, when the outline of your reflections becomes more difficult to discern, due to the
additional reflected images present.