In a multi-speaker surround system there is also the possibility of significant reflections from the surround speaker cabinets, diaphragms or frames so room treatment is even more desirable in such systems. Since so many of my readers are devotees of video home theater and its ad-hoc arrangement of two surround speakers placed at the rear sides, I think it would be best to first discuss the shortcomings of this arrangement before proceeding to describe something more realistic and scientifically based. The home theater movie people recommend two dipole speakers placed on edge so that the acoustic null such speakers produce is facing the listening position. Dipole speakers are speakers that radiate sound equally from opposite sides. Additionally, these sounds are of opposite polarity and so cancel where they collide in a room.
Some dissenters argue that monopole, that is direct, single polarity, radiators, are better.
Either type of rear surround speakers may be reasonable some of the time for movie and video sound reproduction. The use of dipole speakers assumes that the listening room is quite live, because otherwise, the dipoles would be relatively inaudible. But a live home theater room means that the direct sound from the front speakers will be reflected, in spades, from all these nearby surfaces, especially since there are three of them up front emitting direct sound. To add insult to degradation, their reflections cannot be thoroughly eliminated by room treatment with absorbers or diffusers, if the dipoles are to function properly. In movies, these spurious early reflections only slightly impede our ability to locate dialog and sound effects because of the precedence effect and because the brain has no preconceived notions of the acoustic spaces the rapidly changing scenes are supposed to be set in.