John, my reference to Dr. Toole's expertise is based on his book "Sound Reproduction-Loudspeakers and Rooms". Modes and reflections are discussed in detail at several points, but the specific question of side-wall reflections was discussed in summary at pp. 503-504 "...because of the widespread belief that these reflections should be eliminated as a matter of ritual"(p.503). He explains how these reflections in fact are somewhat beneficial in aiding spaciousness for home listening in relatively small rooms and states that "...my recommendation is to leave some blank wall at the location of the first lateral reflections from the front loudspeakers"(p.504), and that "There appears to be no evidence in the now substantial literature that these first-order lateral reflections are problems in normally furnished or the equivalent moderately treated rooms"(p.504).
Various absorption and diffusion possibilities are suggested for other areas of the room, but it's emphasized that the absorption should be down to about 200Hz, which requires treatment at least 3-4" thick . "If only part is absorbed, then the performance of the loudspeakers is compromised, and some of the reflection remains; there is just no point"(p.504). Coverage in the middle portion of both the front and back walls is specifically suggested.
