Structural Resonance - Lexicon CP-3 Plus Supplementary Manual

PLUS
CP-3 Theory
and Design
Room echoes
Irregular bass response

Structural resonance

Maximizing the effect of the
surround soundfield
28
Room echoes cause interference which results in an unnaturally "bright"
sound. These echoes can also be absorbed or diffused. Use heavy insulating
drapes over large expanses of glass.
Bass frequencies have long wave lengths — these may be as long as one of
the dimensions of your room. This can cause over-emphasis of some
frequencies and cancelling out of others. This effect is most noticeable in
rooms which have two or more dimensions which are equal (for example,
length and width). This type of room will exhibit irregular bass response in
different parts of the room. Keep in mind that this effect is worst in nearly
empty rooms. Large pieces of furniture will break up the reflections
between parallel surfaces quite nicely. Moving speakers further in from the
walls can also make a noticeable improvement.
Note that speakers placed against a solid wall can alter the bass and mid-
bass response of your system — making it sound bass-heavy. Placing a
speaker in a corner (where 3 surfaces meet) will make any speaker not
designed for such placement sound muddy. Very thin walls will allow some
of this amplified bass energy to escape, reducing the effect. Moving the
speakers in from the walls also works nicely.
Structural resonance (pictures and windows rattling. etc.) can be caused
even by desireable bass frequencies. If the bass response of your system is
at the desired levels, these rattles can be eliminated by putting felt on picture
frames, or around window cracks.
The listener should not be able to pinpoint the source of surround effects,
therefore output from the surround speakers should not be aimed directly
at the listening area. This is easily accomplished by raising the surround
speakers. Although surround speakers require some reflected sound to be
effective, these reflections should be diffusive — providing randomized
reflections in many directions. Bookcases and other irregular surfaces
provide this sort of diffusion, as do some commercially-available acoustic
panels.
Lexicon