"Stewart Pinkerton" wrote in message
...
*snip*
OK Stewart, I'll bite. How does one know if sound is from the back?

One doesn't, unless one turns one's head. We often have difficulty in
locating sound directly behind us - until we turn our heads slightly.

This comment actually helps makes the case for multi-channel. Unless you
sit at a live event wearing a head clamp, your ears' positions relative to
both direct and indirect sound is constantly changing. Moving around,
changing your position, looking around the hall, etc. The reality of the
source of ambient sound is thus the reality. You hear the ambience from all
directions, because it really is coming from all directions, and while your
head position changes, the source of the sound is constant and thus you have
audible confirmation that you are moving your head. (This is one of the
reasons that headphone listening, even with an AirHead processor, is
somewhat unrealistic. While your head moves around, you have no audible
sense that your are.)

The only way of coming close to this in a listening room is with multiple
channels. The only question, is how many channels are required and how
should they be arrayed. Thinking out loud, here's one possibility. It's
been well established for nearly 50 years, that two channels is sufficient
to provide the illusion of a continuous lateral soundstage. This is the
basis of stereo. If two channels can accomplish this laterally at the
front, it seems reasonable that two channels should be able to create the
same illusion for both the rear and sides of an enveloping sound field. In
other words, two rear channels should be able to do no worse of a job
reproducing the rear ambience of a hall, than the front two channels do of
producing the front. Likewise with the side reflections. In such a
scenario (ie 4 discreetly recorded & played back channels), the front
speakers work together for the front image, the rear speakers provide the
rear, the left speakers (front & rear) provide the left side, and the right
speakers (front & rear) provide the right side. The problem with such a
setup would be a relatively limited sweet spot, as you would be limited
longitudinally (for the front-rear speaker combinations) in much the same
way as stereo is limited laterally. One way to minimize the sweet spot
problem might be the addition of one extra side channel on each side. This
would have a similar effect as the addition of a third channel to stereo.
(Never having experienced discreet 3 channel playback, I don't know how the
sweet spot is compared with 2 channel, but I suspect it is larger.)

So, while 4 channels would seem to provide adequate coverage of the entire
horizontal sound field albeit with a rather limited sweet spot, perhaps the
addition of 4 center channels (front, sides & rear) minimizes this
limitation.

Any comments? Just thinking out loud.