Originally Posted by Jetmeck
Else we would have two center channel speakers also ???
No, because our hearing behind us is very different than our hearing in front of us. This is why imaging reversals happen typically happen from back to front (sounds directly behind us can momentarily image in front of us), rarely the other way around.
When our ear/brain mechanism hears something equally in both ears, the immediate reflex is to think the source is directly in front of us. This is why a you hear a phantom centre image when listening to music on a stereo set-up, because the vocalist is heard equally in both ears. Same thing can happen when with a single speaker directly behind you, because we hear the same sound equally in both ears. These are called imaging reversals.
Luckily, we humans are always making tiny head movements to recalibrate direction. So despite hearing a momentary reversal, we'll eventually realize that the sound is indeed coming from directly behind us. Problems is, movie sound effects are usually too brief to give us enough time to recalibrate. What to do?
Turns out the solution couldn't be simpler. Use 2 rear speakers, spread well away (at least 30 degrees) from the centre line. This is why Dolby, DTS and THX all
recommend playing the mono surround-back channel of their EX and ES soundtracks through 2 rear speakers. It is the only
channel for which 2 speakers are recommended for playback.
But if you are going to spread the rear speakers apart anyway, why not take advantage of the separation by feeding them a stereo signal? Hence 7.1 soundtracks and 7.1 surround processing (PLIIx, Logic7, Neo:X).