Wilson Audio WATT Series 7 - Manual (Page 39)

• Sound staging- LF 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 of as little as one inche can dramatically alter the tonal balance of a system
because of standing wave problems. Fortunately, minor low frequency standing waves
are sometimes well controlled by positioning ASC 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 al-
lowed 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 loudspeak-
ers 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 mid-range and lower
higher 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 rela-
tive to the direct sound. This cancellation is most apparent where the two are 180 de-
grees out of phase. There is augmentation at other frequencies where the direct and the
reflected sounds arrive in phase. Because it is a function of wave length, the comb filter
effect will notch out portions of the audio spectrum at regular octave-spaced intervals.
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