Wilson Audio WATCH Center Series 3 - Manual (Page 17)

Structural Resonance
Structural resonances are familiar to most people as buzzes and rattles, but this type
of resonance usually only occurs at extremely high volume levels and is usually masked by
the music. In many wood frame rooms the most common type of structural resonance prob-
lem is “booming” of walls and floors. You can test for these very easily by tapping the wall
with the palm of your hand or stomping on the floor. Most rooms exhibit mid-bass “boom”
when struck. The loudspeaker playing in the room also excites these resonances. To give
you an idea of what the perfect wall would sound like, imagine rapping your hand against
the side of a mountain. Structural wall resonances generally occur in the low to mid-bass
frequencies and add a false fullness to the tonal balance. They, too, are more prominent
at louder levels, but their contribution to the sound of the speaker is more progressive.
Rattling windows, picture frames, lamp shades, etc., can generally be silenced with small
pieces of caulk or with blocks of felt. However, short of actually adding additional layers
of sheet rock to flimsy walls, there is little that can be done to eliminate wall resonances.
Volume Resonance
The physical dimensions and volume of air in a room will also support standing wave
modes and resonances at frequencies determined by the size of that room. Larger rooms
will resonate at a lower frequency and have more complex (better) modal distributions
than will smaller rooms. Volume resonances, wall panel resonances, and low frequency
standing waves combine to form a low frequency coloration in the sound. At its worst, it is
a grossly exaggerated fullness, which tends to obscure detail and distort the natural tonal
balance of the speaker system.
Occasionally, however, there is just enough resonance to give a little added warmth
to the sound – an addition some listeners prefer. Careful placement of loudspeakers in the
room can dramatically reduce the speakers’ destructive interaction with low frequency
modes. ASC Tube Traps™ are effective in reducing some of this low frequency room col-
oration. Custom designed bass traps, such as perforated Helmholtz resonators, provide the
greatest degree of low frequency control.
Section 3.4—Your Room
Room Shapes
Standing waves are pressure waves propagated by the interaction of sound and op-
posing parallel walls. This interaction creates patterns of low and high acoustical pressure
zones that accentuate and attenuate particular frequencies. Those frequencies are depen-
dent on room size and dimension.
There are three basic shapes for most rooms: square, rectangular, and L-shaped (see
Figure 2).
A perfectly square room is the most difficult room in which to set up speakers. By vir-
tue of its shape, a square room is the perfect medium for building and sustaining standing
waves. These rooms heavily influence the music played by loudspeakers, greatly diminish-
ing the listening experience.
Long, narrow, rectangular rooms also pose their own special acoustical problems for
speaker setup. They have the ability to create several standing wave nodes, which will have
different standing wave frequency exaggerations depending on where you are sitting. Ad-
ditionally, these long rooms are often quite lean in the bass near the center of the room.
Rectangular rooms are still preferred to square rooms because, by having two sets of dis-
similar length walls, standing waves are not as strongly reinforced and will dissipate more
quickly than in a square room. In these rooms, the preferred speaker position for spatial
placement and midrange resolution would be on the longer walls. Bass response would be
reinforced by speaker placement on the short walls.
In many cases, L-shaped rooms (see Figure 2) offer the best environment for speaker
setup. Ideally, speakers should be set up along the primary (longest) leg of the room. They
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