Just to amplify a bit and provide some background on the rationale for the above advice…particularly with respect to the benefits of 7 speakers and in particular of placing side speakers at 90 degrees…
The reason for using two pairs of rear surrounds is grounded in a solid body of psychoacoustical research that clearly establishes why a 5.1 speaker configuration is inadequate when attempting to create a seamlessly immersive soundfield. This has been well-known in audio engineering since the definitive work by Theile and Plenge, as reported in their paper “Lateralization of Phantom Sound Sources,” published in the Journal of the Audio Engineering Society, April 1977. Their tests into side panning and phantom imaging effects showed quite conclusively that due to differences in the way we hear sounds originating from the side vs. the front, if we attempt to create phantom images at the sides using only front and rear speakers, the images are highly unstable. Their conclusion is that: “in the search for a loudspeaker arrangement that allows for ‘all round effect,’ the directions right and left on the lateral (90 degrees) must be represented through real sources.” In other words, stable side imaging *requires* a *real* pair of speakers placed directly to each side of the listener.
It is easy enough to demonstrate this fact for yourself in any system or room. Sit or stand in the sweet spot, and play a mono source through your two front L&R speakers--you’ll hear a typical stable phantom center image. Now turn 90 degrees, so that one ear is pointed towards that phantom center--rather than remaining locked to a fixed location, the “image” will break apart. Hence the reason why a pair of physical speaker is needed directly to the sides of the listening position.
Of course, placing the side speakers at 90 degrees inevitably leaves a huge gap in the rear soundfield. That is why many 5.1 speaker setups have the surrounds positioned farther behind the listening position. The popular speaker placement at 110-degrees is a compromise that narrows (but does NOT eliminate) the rear hole, but this partial improvement comes at the expense of seriously weakened side imaging, because of the problem described above. Thus, there is no way to achieve a seamless soundfield using only one set of surrounds. Dr. Robert E. Greene wrote a very cogent article explaining the inherent limitations in attempting to create surround with only 5 speakers in “The Perfect Vision,” Issue 30. May/June 2000.
Adding a second pair of surrounds for a 7-speaker setup solves this problem handily. The sides can be placed at 90 degrees for stable side imaging, the rears can be placed at 150 degrees to close the rear gap, and voila!--seamless, smooth envelopment.
This is why home surround processors from manufacturers with expertise in psychoacoustics (Lexicon, Meridian, and PL II developer Jim Fosgate) have been building their technologies around 7-speaker configurations for decades. This is also why in developing even the Surround EX format, THX and its Pro-Logic II partner Dolby Labs adopted a 7-speaker playback configuration for an immersive soundfield.
Once you know what to listen for, the ambient envelopment which makes up 99% of a film soundtrack (rain, wind, traffic, crowd noises, etc.) will never sound realistic with only 5 speakers because of the soundfield gaps that cannot be solved with only 5 speakers. And of course you obtain the intended directionality of EX and ES rear channel effects that are specifically encoded to be extracted and reproduced from the rears as distinct from the sides.
With respect to an earlier question in another thread about the use of a single rear speaker, this too is not a good idea for reasons that are grounded in a substantial body of psychoacoustic research dating back to the 1970s showing that 6.1 configuration is inherently flawed.
In the first place, our hearing behind us does not mirror the acuity of our hearing in front. Quite the contrary--sounds that originate along the median (center axis) plane between our ears, whether from above or directly behind are extremely difficult to place accurately, and not just because of those fleshy appendages called ears that sit between us and the sound source.. In large part, this reduced localization ability is due to the lack of certain fundamental localization cues, such as slight timing offset in the arrival at each ear, that are present when sounds originate from one side or the other. The result is the localization problem known as "back-to-front reversal," in which we actually hear sounds from directly behind as coming from in front of us. In an ambiguous situation, we "hear" the sound from the front center because that's where the focus of our visual field is, and basic survival instinct has taught us to give it more of our attention. There are certain sounds, such as voices, that we expect to hear coming from in front of us, so we do. And when we're watching a film with an EX soundtrack, our attention is firmly fixed ahead, hence we're even more susceptible to the auditory confusion.
The research documenting this phenomenon is abundant, but you don't have to take my word for it; here are some references to get you started: Butler, R.A. and K. Belendiuk, "Spectral Cues Utilized in the Localization of Sound in the Median Sagittal Plane," Journal of the Acoustical Society of America, 61 (1977); Morimoto, M. and H. Aokata, "Localization Cues of Sound Sources in the Upper Hemisphere," Journal of the Acoustical Society of Japan, 5 (1984); Musicant, A.D. and R.A. Butler, "The Influence of Pinnae-Based Spectral Cues on Sound Localization," Journal of the Acoustical Society of America, 75, (1984); Kuhn, G.F. "Physical Acoustics and Measurements Pertaining to Directional Hearing," in Directional Hearing, edited by W.A. Yost and G. Gourevitch, 3-25. New York: Springer-Verlag, 1987; and Wightman, F.L. and D.J. Kistler, "The Dominant Role of Low-Frequency Interaural Time Differences in Sound Localization," Journal of the Acoustical Society of America, 91 (1992).
The reversal problem tends to be contextual--what we expect to hear has a lot to do with where we place sounds. Not all sounds will get incorrectly localized, but some will, and that is even more problematic because only hearing some of what are meant to hear from behind lulls us into thinking we are hearing the soundtrack correctly, wheresas we can never be sure whether we are or not.
So, in light of this problem, we can now see that far from being MORE accurate, even with discrete rear channel content, the use of a center rear speaker in a 6.1 speaker configuration is actually FAR LESS accurate --unless your definition of accuracy is hearing a sound coming from the opposite side of the room from where it's meant to be heard.
The back-front reversal problem disappears, however, when using two rear speakers placed off the center axis. This is precisely the reason why THX, Dolby and DTS ES all mandate the use of TWO rear speakers, even when they're only reproducing the same mono rear content.
Lexicon's chief scientist David Griesinger recommends rear speaker placement at 130-150 degrees for the best balance of excitement and envelopment. Check his presentation "How Many Loudspeaker Channels are Enough?" at the bottom of his website page athttp://world.std.com/~griesngr/