Wilson Audio WATCH Dog Subwoofer Series-2 - Manual (Page 19)

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• Sound staging - low frequency component of image shifted
Standing waves are more difficult to correct than slap echo because they tend
to occur at lower frequencies, whose wave lengths are long enough to be ineffectively
controlled by absorbent materials such as Sonex. Moving speakers about slightly in
the room is, for most people, their only control over standing waves. Sometimes a
change of placement as little as one inch can dramatically alter the tonal balance of a
system affected by standing wave problems. Fortunately, minor low frequency stand-
ing waves are sometimes well controlled by positioning tube traps in the corners of
the room. Very serious low frequency accentuation usually requires a custom-
designed bass trap system.
Low frequency standing waves can be particularly troublesome in rooms con-
structed of concrete or brick. These materials trap the bass in the room, unless it is
allowed to leak out of the room through large window and door areas.
In general, placement of the speaker in a corner will excite the maximal number
of standing waves in a room and is to be avoided for most direct radiator, full range
loudspeaker systems. Some benefit is achieved by placing the stereo pair of loud-
speakers slightly asymmetrically in the listening room so that the standing waves
caused by the distance between one speaker and its adjacent walls and floors are not
the same as the standing wave frequencies excited by the dimensions in the other
channel.
Comb Filter Effect
A special type of standing wave, noticeable primarily in the midrange and lower
high frequencies, is the so-called “comb filter effect.”
Acoustical comb filtering occurs when sound from a single source, such as a
loudspeaker, is directed toward a microphone or listener at a distance. The first sound
to reach the microphone will be the direct sound, followed by delayed reflected sound.
At certain frequencies cancellation occurs because the reflected sound lags in phase
relative to the direct sound. This cancellation is most apparent where the two are 180
degrees out of phase. There is augmentation at other frequencies where the direct and
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