Great comments as usual Alan, however this comment has me concerned.

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But my proposal of using two centers wasn't to deliver sonic improvement; rather, it was to remove the extremely off-putting sensation that may occur with some large-screen front projection systems where the dialog gets localized at the single center channel speaker because it's too far away from the visual screen center. That is what using two centers (above and below) is meant to correct, and it does that quite well in the installations I've heard with no audible downsides.





No one can argue with trying to centre the channel in the middle of the display. I'm not so sure two identical speakers above and below the screen achieves that objective and it has the unfortunate effect of creating less linear frequency response throughout the room. Two identical signals in phase running through two identical speakers symetrically positioned would produce a lobe running down the vertical axis. And given the height of most screens the lobe would be fairly wide over several seats. Not my idea of good sound reproduction across the room.

On the other hand, if you have no other choice, and you sit pretty much on center, it may acceptable despite the acoustical problems dual centres create from comb-filtering, lobing, cancellations, and different arrival times.

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I have not noted any of these effects in several installations nor in my own experiments with single and dual centers, and I would point out that "cancellations and reinforcements" are always occuring in any multiple speaker installation in any given room. Despite these effects, multi-channel music reproduction and home theater can be enormously satisfying and highly realistic.



I can see why you would make these observations if you sit in one sweet spot and don't move. However, as you stated earlier, the whole point of a center is to anchor dialogue to the screen. By using dual centers the voices will move to the left or right depending on where you're sitting. In light of the acoustical damage done by duals I would lean toward not wasting money on dual centers, and instead either running without a center and use the left and right to derive the (phantom) center, or go with a carefully placed horizontal centre. I guess I've been in enough HTs using duals where the audibilty of voices is too heavily compromised by the resulting artifacts to blame on room acoustics.

John