Room Modes

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 distance from the wall behind them, and there should be approximate symmetry of the Left and Right speakers with respect to their respective side walls.

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. Simi- larly, 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 instance) 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.