The purpose of a surround channel is to surround you with sound. The problem is that this is tough to do with just two speakers. Decorrelation takes care of Left/Right spaciousness, but what about wrap around envelopment? If a director chooses to place you in a wind storm, conventional speakers leave you with the impression that you're hearing the windstorm through two open windows.
The Home THX Diffuse Surround Speaker is designed to provide you with a "movie without walls" experience. It does this by painting the walls and the ceiling with sound. It sends very little sound towards the listener. The sound reflecting off the rooms walls and ceiling provides you with an enveloping soundfield, immersing you in the movie experience.
The total energy radiated by a speaker in all directions is called its "Power Response". The Home THX Diffuse Surround Speaker is required to have a flat power response. This means that the total energy radiated by the speaker (in front, above, below, behind, and to the sides) must average together into a flat frequency response. With the total speaker energy being flat, a listener anywhere within the surround speaker's null area will hear flat frequency response.
http://www.mksound.com/pdf/user_manuals/user_mains/ss500thx-man.pdf
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