Originally Posted by ahender
Is there an obvious quality difference between using two vs one rear speakers?
Psychoacoustical difference. If you hear a sound louder in your left ear than right ear, you know that sound is towards your left. If you hear the sound louder in your right ear then left ear, you know that sound is towards your right. If you hear the sound equally in both ears, you know the sound is directly in front of you. But is it?
A single speaker place directly behind you can trigger a psychoacoustic phenomenon called imaging reversals, momentarily causing sounds coming from behind you to be reflexively heard in front of you. However, we humans tend to make small involuntary head movements to constantly recalibrate our surroundings. So the reversal effect would only last a few seconds and you'd eventually figure out that the sound is coming from behind you. Problem is, sound effects in movies are fleeting, so by the time your ear/brain mechanism got the direction correct, the sound effect could have come and gone, leaving an incorrect impression.
The solution to the reversal problem is as easy as it gets: just use 2 Rear speakers, spread at least 60 degrees apart. This is why Dolby and DTS and THX all
recommended using 2 Rear speakers to play back the mono surround-back channel of their 6.1 soundtracks. Note that it was the only
channel for which 2 speakers were recommended. Fortunately, 6.1 didn't last that long, either as mix format or speaker layout.