Hi Ajax,

I did take a look at that thread and those objections are specious. Comb filtering occurs all the time when we listen to two or more speakers operating together. It's only audible with test signals and it is simply not a significant factor with programme material.

Do you hear stereo and surround enthusiasts running around complaining about comb filtering? I doubt some of those posters have ever listened to comb filtering and can explain the phenomenon, let alone ever having participated in controlled double-blind listening tests.

If you want to hear comb filtering, get a test CD that has a broadband pink-noise test signal on it and play it back over your two main stereo speakers (turn off the others; they will only obscure the results). Set it for a fairly loud level, about 80 dB SPL or a bit more, and sit exactly between your two main speakers in the sweet spot at your normal listening location. Concentrate on the upper frequency "hiss" part of the signal. Now slowly move your head and body a bit to the left then back again, then to the right. You should hear parts of the high-frequency hiss get a bit duller, then brighter, then duller repeatedly as you shift to the left and right. That's because alternate cancellation and reinforcement of the signals is occurring, with tiny nulls followed by tiny peaks, like the spikes of an inverted comb, hence "comb filtering."

It's extremely difficult to detect with music and impossible with 5.1-channel home theater.

This is not to say there may be other as-yet undetected effects that could occur and may indeed be audible using two center channel speakers in parallel. But I won't predict anything until I do some blind listening comparisons with different placement.

Regards,

Alan Lofft,
Axiom Resident Expert (Retired)