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found at certain discreet locations in the room. These locations dif fer according to room
dimension and size. A room generating severe standing waves creates dif ficulty in setup. In
these rooms, the speaker will sound radically dif ferent as it is moved around. The ef fects
of standing waves on a loudspeaker's performance are primarily in the areas listed.
• Tonal balance
• Resolution of low-level detail
• Soundstaging
Standing waves are more dif ficult to correct than slap echo because they tend to oc-
cur at a lower frequency. Absorbent materials, such as Illbruck Sonex®, are inef fective at
controlling reflections in the bass region. Moving speakers about slightly in the room is, for
most people, their only control over standing waves. Sometimes a change of placement of
as lit tle as two or three inches can dramatically alter the tonal balance of a small system.
Fortunately, minor low frequency standing waves are 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 constructed
of concrete or brick. These materials trap the bass in the room unless it is allowed to leak
out of the room through windows and doors.
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 loud-
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.
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Comb Filter Effect
The "comb filter" ef fect 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 cer-
tain frequencies, cancellation occurs because the reflected sound lags in phase relative
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 ef fect will notch out portions of the audio spectrum at linearly spaced intervals. Sub-
jectively, comb filter ef fect evidences itself as follows:
• Added roughness to the sound
• Reduction of harmonic richness
• Smearing of lateral soundstage image focus and placement
Comb filter ef fects are of ten 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 3.3—Resonances
Resonance in listening rooms is generally caused by two sources:
• Structures within the listening room.
• The volume of air itself within the listening room.
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