Placing Surround Speakers
No matter what the setup of your speakers for surround sound — whether you have two, three, four, or more side and back speakers — your surround speakers play a fundamentally different role than your front speakers. You want things highly localized in the front part of your soundfield, but the surround field is more diffuse.
The main duty of surround speakers is to create the sense of environment, the background, the unobvious. So if there is a howling wind, the pitter-patter of a rainstorm, or the background of a busy city, it comes from these speakers.
Reviewers tend to talk about the diffuse sound of the surround sound speakers because they often are not really making any specific sound per se, but a cacophony of sounds that together create an atmosphere. And where the front speakers (the center, left, and right) are arrayed at or around your ear level, the surround sounds tend to work higher, to the sides, and behind you, enveloping you in the surround soundfield.
Surround sounds tend to rely less on direct radiated sound and more on reflected and indirect sound. Just like in a city where sounds bounce off buildings, or in a stadium where it's sometimes hard to localize a specific noise.
There are several different modes of surround sound, often dependent on the encoding of your source input. There is a fair amount of debate about the appropriate speaker to use to create surround sound. For instance, Dolby Pro Logic encoders have a monophonic surround channel with a limited range of frequencies supported; by comparison, Dolby Digital encoders plan for full-range speakers. So do you use full-range speakers that are more directional, in order to take advantage of the newly enabled signals of Dolby Digital? Or, do you use more diffuse speakers to add to the shared atmosphere of the "surround"?
The choice isn't clear-cut. In general, you should focus on creating a broad, diffuse soundfield to the sides, back, and top of the room. A good installation uses dipole speakers to help provide localization where needed. For the higher-end encoding, such as THX Surround or DTS-ES, where there are at least four surround sound speakers, you should consider four dipole speakers — two on the sides and two in the back. You set up your two back surround speakers so that their drivers are in phase with one another. This maximizes the ability to create a specific sound image right behind you and serves as a good solution for home theater surround sound issues.
Some folks (typically on the high end of the home theater spectrum) recommend using direct radiating speakers for your surrounds. (In fact, DTS recommends this for DVDs encoded with their surround sound system.) For most people, dipoles are a better bet, mainly because, for most films, audio engineers mix the sound specifically for the diffuse speaker arrays used in theaters. So the majority of DVDs have been designed specifically for dipole-style speakers for your surrounds.