Axiom Home Page Forums General Discussion Advice from Axiom Owners VP 100 or VP 150 paired with M3ti?

Comb filtering? I thought that was a video thing...

No Ken, it's an audio thing in this context, but an over-exaggerated thing. Unless someone's listening in an anechoic chamber the reflections in the average listening room cause comb-filtering all over the place when the reflections of the same sound reach the listener at different times, so I don't worry about a little more caused by multiple speakers.

Unfortunately, with my TV, they won't be widely separated. 27" CRT.

No, it wouldn't be "unfortunately" Ken, because those who worry about comb-filtering would want the speakers to be as close to each other as possible, and your separation would be considered "wide". Again, I don't think there's any significant problem with that sort of twin speaker setup.

For those of you who are curious, "comb-filtering" is a fancy way of describing "phase-cancelations."

Suppose you are in an anechoic chamber, listening to two small single-driver speakers driven by the same sine-wave signals. If one speaker is placed further from you than the other by exactly half the wavelength of the sine-wave sound, then the sounds from the two speakers reach you at a 180-degree phase shift. That will result in a severe cancelation of sound.

For example, the wavelength of 250Hz sound is about 4 feet. So, if one speaker is 2-feet more distant from you than the other, the 250Hz sound will cancel out, creating a big dip in the frequency response at 250Hz.

Now, consider the 750Hz sound, with wavelength of 1.3 feet -- for that frequency, the 2-feet distance between the two speaker represents 1.5x wavelength and will induce a 540-degree phaseshift (360+180), again resulting in a big dip in frequency response at 750Hz.

Similarly, the next dip will happen at 1250Hz, 1750Hz, etc... In the end, the entire frequency response will look like a comb.

Damn you're good!!

Hi Sushi,

All true, and you don't have to be in an anechoic chamber to experiment with comb filtering and its effects. Just sit exactly in the sweet spot between your main speakers, and play a sustained pink-noise test signal in stereo (no center, no surrounds).

Now, while you're staring at the midpoint between your two front speakers, move your head to one side a few inches--don't turn it to the side--and then back and a few inches to the other side. You'll hear the mid and high-frequency response change noticeably and fluctuate as you move your head and body a few inches either way.

Regards,

Can you draw a picture of that?

Alan,

Damn you're good, of course!

One important point in this experiment: You have to use a MONO pink noise, so that the two speakers receive an identical signal. Some test CDs/DVDs include "de-correlated" or "de-synchronized" pink noise (in which the actual instantaneous wave forms sent to each channel are always different from each other -- it produces a very diffuse feel without any imaging whatsoever). The experiment won't work with it.

Incidentally, I had actually done this exact experiment with my VP150 alone, to examine the possible comb-filtering among the drivers, especially between the two outside tweeters. And boy, Ian is right!!! At least in my room, the noise timbre is VERY uniform until I go almost 30-degrees off axis, where the highs start to roll off as expected.

I LOVE this very "unusual" center channel -- it defies conventional wisdom, yet it performs!!!

You might want to try Alan's test when there is no one around.
Not unless your family and/or roomates already know you are a bit nutsy about the audio thing lest you look like Stevie Wonder with your eyes closed, head swaying back and forth listening to pink noise.