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Thread: Comb Filtering

  1. #1
    Founding Member OSN's Avatar
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    Comb Filtering

    I have read a little bit on the topic, and would like to hear about some people's experiences with comb filtering. Wikipedia defines it below:

    Quote Originally Posted by wikipedia
    In signal processing, a comb filter adds a delayed version of a signal to itself, causing constructive and destructive interference. The frequency response of a comb filter consists of a series of regularly-spaced spikes, giving the appearance of a comb.

    ....

    However, comb filtering can sometimes arise in unwanted ways. For instance, comb filtering will occur where two loudspeakers are playing the same signal at different distances from the listener.[1] Comb filtering occurs in acoustics. In any enclosed space, listeners hear a mixture of direct sound and reflected sound. Reflected sound, taking a longer path, constitutes a delayed version of the direct sound and a comb filter is created where the two combine at the listener.[2]
    I am generally interested in the effects of driver multiples in close proximity reproducing the same frequencies, such as a small array or MTM/TMM. I have a few mids in multiples and a few tweeters that I will be testing out in the car, and would like an idea of what I should be concerned with.

    So I have a couple of questions to start things off:

    -is it as simple as ensuring the distance from the drivers to the listener are the same?

    -is there a formula or methodolgy to predicting at what frequencies comb filtering is expected? I assume it has to do with quarter wavelengths and the distance between driver centers or something along those lines.

    If anyone would like to demonstrate with an example, I'd be interested in determining the issues I may encounter with a pair of these side/side or stacked, playing from 315 Hz and up.

  2. #2
    Tester Extraordinaire ErinH's Avatar
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    a center console is a prime cause for comb filtering as is the dash (especially with drivers firing in their direction).

    13560(in/s) ~ speed of sound
    Use that to divide by frequency to get distance or distance to get frequecy. Take a 1/4 of that to get 1/4 wave.
    For example, your console (just because I mentioned it already) is 28" from the driver.
    13560/28"/4 = 121hz.
    With that, I'd say you could start expecting issues around 120hz. I'm not 100% if it's going to be a cancellation affect or grouping affect; probably has more to do with a spcific frquency and it's wavelength.
    I love this site:
    http://zonalandeducation.com/mstm/ph...lanation3.html

    I also believe incidence angle is going to play a role, too (ie: at what angle is the soundwave hitting the surface and is that angle enough to cause the wave to go back at itself).
    Last edited by ErinH; 02-09-2011 at 10:25 AM.
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    Founding Member OSN's Avatar
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    So play my subs up to 150 Hz, is what you're saying? :p

    I was thinking more along the lines of things that are unique to playing with multiples
    in close proximity, if I am trying to keep them equidistant. I've always thought that a driver- on-axis- with narrow dispersion pattern in upper frequencies sounded more coherent to me, like the Tang Band w3 bamboo. And yes, they are excursion limited, but if I could either increase surface area or cross them over higher. I was hoping to keep as much of the vocal range on them as possible, as that is an upside of their's, in my estimation. I just don't know if that's going to be my best option when I put it to the test.

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    Tester Extraordinaire ErinH's Avatar
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    I'm wondering if lobing would be more a factor in this case. I'd have to look that up again as I forget what the terms are in reference to sometimes and need a gentle reminder. ;)
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    If you can really, REALLY grasp these TWO concepts, most of audio becomes silly-easy :

    1. The generation of an acoustic signal from a point source, PLUS a delayed version of that same source, creates a "comb" in the frequency response. I'll do the math a bit later (the interesting point is where, in frequency, the first null is found). Different distances to the listening ear means : signal plus delayed-signal.

    2. A reflection is iDENTICAL to point #1 above This becomes stupid-obvious when you analyze reflections with simple "image theory".

    How powerful are these two concepts? Let's see :

    They completely explain the off-axis response of any single driver (we should all recognize by now that any single driver is nothing more than an array of point sources distributed across the surface of the cone. What happens when the listening ear is NOT the same distance from all those tiny points?)

    They completely explain the on-axis, and off-axis, response of any array of drivers (although the math can get a bot hairy, the underlying principle is simple).

    They completely explain the frequency response resulting from reflections near the speakers.

    ... and so on.

    But while i'm writing, time for an old quiz :

    1. Is it possible for TWO sources in a straight line (kind hard not to be, for only two points) to be the SAME distance to your ear?

    2. is it possible for THREE (or more) sources in a straight line to be the SAME distance to your ear?

    All you need is a pencil, paper and semi-functioning brain to figure it out

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    Quote Originally Posted by ErinH View Post
    I'm wondering if lobing would be more a factor in this case. I'd have to look that up again as I forget what the terms are in reference to sometimes and need a gentle reminder. ;)
    lobing and combs are manifestations of the same thing

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    The answers are yes and no.

    But depending on how small the drivers are, 3 could get really close.

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    Quote Originally Posted by werewolf View Post
    But while i'm writing, time for an old quiz :

    1. Is it possible for TWO sources in a straight line (kind hard not to be, for only two points) to be the SAME distance to your ear?

    2. is it possible for THREE (or more) sources in a straight line to be the SAME distance to your ear?

    All you need is a pencil, paper and semi-functioning brain to figure it out
    Simply in a straight line or in a straight line on a flat plane?

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    Founding Member OSN's Avatar
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    Quote Originally Posted by winslow View Post
    The answers are yes and no.

    But depending on how small the drivers are, 3 could get really close.
    This is my vote. 3 or more would require an arc for same distance from listener.

  10. #10
    Tester Extraordinaire ErinH's Avatar
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    Quote Originally Posted by werewolf View Post
    1. Is it possible for TWO sources in a straight line (kind hard not to be, for only two points) to be the SAME distance to your ear?

    2. is it possible for THREE (or more) sources in a straight line to be the SAME distance to your ear?

    All you need is a pencil, paper and semi-functioning brain to figure it out
    If you mean actual physical distance, stacked in the same straight line:
    1) no. not unless you're talking engineering terms (ie: good enough). we know mathemeticians don't believe in anything other than absolute. ;)
    2) no. same thing.

    If you mean physical distance, with the drivers on different paths/angles/azimuths:
    1) yes
    2) yes


    If you're talking about perceieved distance in terms of sound:
    1) depends on bandwidth and how they're set up (ie: are they playing at the same time, reflections/room, and are they level matched to your listening position)
    2) ditto



    whenever you have a question, we all have a tendency to try to think ahead rather than right at the answer. lol.
    Your ears: The best tools you have... and they're free, too!

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