Phase cancellation is the reason Bose does not recommend using a pair of PAS with a stereo keyboard. Apart from the problem described by Esh, the half-stereo signals emanating from a stereo PA will interfere causing some relative dead spots in the audience, depending on room acoustics, e.g., the bouncing described by Al. Perhaps, the stereo PA system that Esh dealt with was set to mono to avoid these dead spots with the unfortunate side-effect of causing phase cancellation as stereo signals were mixed to mono.

Pick your poison:
Mono--No spatiality and no stereo effects, but no phase cancellation ("everyone hears the same thing.
Stereo--Spatiality and stereo effects, but phase cancellation (people hear different things, dead spots).
Multi-mono--Spatiality, no phase cancellation, but limmited stereo effects.

Hey Gary,

FYI, sound travels at around 1130 feet per second, 342 meters per second (for our European friends) , and/or 760 miles an hour.

And while I agree with your equilateral triangle analogy (can be isosceles too) to describe the 'sweet spot', I think it's fair to say that this area is not as small or precise as the apex of a triangle...it's much broader (unless the speakers we're talking about are incredibly directional). The wave launch from a conical shaped speaker/driver broadens exponentially as it leaves the source. When two well-designed speakers are properly placed in a room, the steering information (left and right) provided by a stereo signal can be experienced by many. Sure the optimum position is in the middle, but the leeway to one side or another can be quite far before we lose the ability to sense position accurately. (this doesn't apply if you've only got one working ear)

I also agree with Esh, phase cancellations (and don't forget additions), both electronic and acoustic can be a huge can of worms. And you're right too; they can play havoc on any system.

And I agree with the folks that use the Bose system and say it's wonderful. The best acoustical guitar / vocal amp system I've heard yet is the Bose PAS.

-mike

Al,

You are right about the speed of sound in air. I was thinking back to the old days when I worked with sonar systems, which was about one mile per second in salt water. I stand corrected.

Cheers,

Gary

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Travlin' Easy