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s
e C T i o n
2 . 3 — r
e s o n a n C e s
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W i l s o n A u d i o S p e c i a l t i e s
speaker systems. Some benefit is achieved by placing the stereo pair of loudspeakers
slightly asymmetrically in the listening room. This is so 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
The “comb filter” effect is a special type of standing wave noticeable primarily at
higher frequencies and shorter wavelengths.
Acoustical comb filtering occurs when sound from a single source, such as a loud-
speaker, is directed toward a microphone or listener from a distance. The first sound to
reach the microphone is the direct sound, followed by a 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 frequencies are
180 degrees out of phase. Further, there is augmentation at other frequencies where the
direct and the reflected sounds arrive in phase. Because it is a function of wavelength,
the comb filter effect will notch out portions of the audio spectrum at linearly spaced
intervals. Subjectively, comb filter effect evidences itself as follows:
• Added roughness to the sound
• Reduction of harmonic richness
• Smearing of lateral soundstage image focus and placement
Comb filter effects are often caused by side wall reflections. They are best controlled
by very careful speaker placement and by the judicious placement of Illbruck Sonex® or
air duct panels applied to that part of the wall where the reflection occurs.
Section 2.3—Resonances
Resonance in listening rooms is generally caused by two sources: