Quote Originally Posted by Kevio
The interference occurs when one of the drivers is closer to you than the other. In this situation, due to the limited speed of sound, you get two copies of the same sound arriving at your ear at slightly different times. The delay translates into a phase difference and the two copies partially cancel each other out resulting in what we typically perceive as timbre change. The technical name for the phenomenon is a comb filter. But keep in mind that none of this happens when the distance to the two drivers is approximately equal as is the case for most of these ear height tower speakers.

It certainly is an issue in stereo playback - that's why we have a listening sweet spot (were the distance from our ears to each speaker is the same).

Also recognize that this comb filtering happens in our natural environments as sound bounces off walls and reaches our ears through multiple paths. Along with reverberation, It is part of what gives our hearing a sense of space. So as with 2nd harmonic distortion, a comb filter scenario can add to and enhance our enjoyment of recorded music.
I'm well aware of comb filtering - in the dual center channel example, the acoustic centers of the two speakers in question is what I'm wondering about - while we may measure some artifacts, how bad is it really going to be at the listening position, and would other perceived benefits of such an arrangement offset the negatives? If you're head is "in between" the axis of each speaker? Vertical dispersion in this setup is going to be fairly poor, but you're not likely to drift higher or lower than either speaker. Your post even hinted at it - "when the distance to the two drivers is approximately equal". While not measuring perfectly equal at the listening position, in light of all the other interference and distortion sound picks up between the time it leaves the speaker and the time it hits your ears, I would contend the interference effects in the dual center arrangement are tolerable, and overall not that significant. (But still easy to avoid if you buy a ONE good center channel, which would of course be preferable).


I know in speaker building, we try to keep the acoustic centers as close as possible, less than a wavelength of the frequency of the xo as a rule of thumb. The D'Appolito arrangement (woofer, tweeter, woofer) is well documented for its minimal vertical dispersion, but so long as your head is in between the woofers (close to tweeter level, give or take several inches) you're ok. I'm half expecting a similar effect here, just haven't seen any measurements of such a setup.