phase-induced irregularities at the crossover frequency of 80 Hz.) Of these two guidelines, the first is usually more significant sonically, as it tends to minimize the audibility of the room’s modes. Avoid corner placement, as this typically in- creases bass quantity at the expense of bass quality (by stimulating every room mode supported by the room).
The actual number of subwoofers used in an installation will depend on both the size of the room and on the manufacturer’s design. Of course, having multiple subwoofers enhances the maximum level at which they will play. Additionally, there is an advantage to having more than one subwoofer in the room. Smoother bass may be obtained by placing two subwoofers asymmetrically in the room, since different sets of room modes would then be excited by each. The peaks of one set of room modes could offset the valleys of the other one to some degree.
Ideally, the rear speakers will create a diffuse, non-localizable soundfield which envelopes the listener. Creating this effect depends in part on the loudspeakers chosen for the job.
If THX-certified dipolar surround speakers are used, they should be located high and to the sides of the listening area, firing to the front and the rear of the room. (See diagram, below.) This placement ensures that the audience will not be able to localize them as sources of sound, but rather will be enveloped in a diffuse soundfield. They should be at least two feet above ear level when seated. Ideally, they would be placed somewhat above ear level even when standing, so that they are unlikely to be localized as people walk around the room.
If more traditional speakers are used, they should be placed such that their out- put is scattered across the rear of the room. (See diagram, next page.) Often, the best results will be obtained by placing them relatively high and slightly behind the listener, cross-firing across the rear wall so as to create a large number of re- flections with relatively little direct sound arriving at the listening position.
