Proceed PAV - Manual (Page 69)

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The Boundary Effect is particularly important when it comes time to position
the front speakers. If they are not placed consistently with regard to adjacent
room boundaries, the tonal balance or timbre of sounds panned across the
front may change, weakening the illusion of a real object moving across the
soundstage. The front Left, Center and Right speakers should be the same dis-
tance from the wall behind them, and there should be approximate symmetry
of the Left and Right speakers with respect to their respective side walls.
Room Modes Another acoustic property of concern is the concept of “room modes.” (These
are also sometimes called “standing waves.”) Bass frequencies have long
wavelengths, some of which are the same size as some dimension of the room
itself. If the sizes work out such that the reflection of the wave between two
walls is in phase with the original wave, that particular frequency will be over-
emphasized. Similarly, some frequencies will very nearly cancel out in certain
parts of the room, being out of phase with each other. This effect is much
worse in rooms in which two or more dimensions (width and length, for in-
stance) are even multiples of one another, since the same frequencies are then
being affected no matter which way they turn. The result is wildly irregular
bass response in various parts of the room, resulting in either an unnatural,
boomy quality or a thin, lifeless character to the sound depending on where
you sit.
All rooms have room modes, but you can ameliorate their effect greatly. In the
case of new construction, where you may have some latitude specifying the final
room dimensions, make sure that no two dimensions of the room are even mul-
tiples of each other. (Consult with your dealer for more information.)
Most of the time, you will not have the luxury of specifying where walls should
go. The next best thing (and something to be tried even when you can move
walls) is mid-bass diffusion. Break up the reflections between parallel surfaces
with large pieces of furniture or almost anything else. The worst sounding rooms
are the ones which are almost empty, since the standing waves can bounce back
and forth unimpeded.
Another strategy to be tried in minimizing the audibility of standing waves is
speaker placement. The placement of the speakers in any home theater system is
somewhat restricted by the need to have the sound closely associated with the
screen, which in turn must be in a location convenient to the seating area. But
sometimes moving a speaker a bit can make a noticeable difference in the
smoothness of the mid-bass response, due to changes in the room’s modes. For-
tunately, the subwoofers can be moved with relative freedom, since they normally
will not be localized as sources of sound. Experiment with an ear toward having
the smoothest bass response and the best “splice” or transition to the LCRs.
Of course, there are various acoustical treatments which can be utilized—ask
your dealer. There is also electronic equalization, but this is best done after every-
thing else has been optimized. Electronic EQ is best for providing the finishing
touches to a room, rather than for doing major surgery. And in all cases, elec-
tronic EQ should be done by a trained professional with equipment having at
least one-third octave resolution. Anything less than that is likely to introduce as
many problems as it solves.