Nickbuol and Ratpack,
Not true re using two centers and "the sound waves can cancel each other out."
When you run two center-channel speakers in parallel, connected in phase, there is no more cancellation takes place than what occurs all the time when you listen to two speakers in stereo, or 5.1 speakers in DD or dts 5.1, when any two speakers share common audio signals--and they do, all the time.
Without going into tedious acoustic detail, what cancellation occurs--it's called "comb filtering"--isn't enough to cause serious problems. If it were, exactly the same effect would occur with two stereo speakers, or any two speakers run in mono with the same signal.
Comb filtering results from interchannel crosstalk--the signal from the left front speaker reaches your left ear, then goes around your head and reaches your right ear slightly delayed by the time it takes the signal to go around your head.
Similarly, the signal from the front right-channel speaker reaches your right ear, then, a millisecond later reaches your left ear. At high very frequencies, these signals alternately cancel and reinforce each other because the wavelengths are so short. You can hear this effect by playing pink noise and sitting exactly between your two front speakers (in stereo). Move your head slightly to the left and then to the right. You will hear the high-frequency part of the signal become slightly louder then softer as you move your head laterally to the left and then to the right. Note: It's very subtle and tricky to detect but it's there and it occurs all the time as you listen to any material throught more than one speaker.
Seems like I went into tedious detail after all! But the point is, this effect is simply not audibly significant. It does not prevent our enjoyment of stereo or multi-channel playback. People who rant on about phase-coherent loudspeaker designs are simply not aware of these basics of acoustics and loudspeaker playback.
Likewise, running two center channels in some installations has real advantages in anchoring dialog and vocals firmly at the middle of the screen for large front-projection installations. If can work very well when there are placement difficulties for a single center channel speaker.
Regards,
