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Thread: Introduction to Comb Filter Effects

  1. #11
    Member Sponsor [Technical Expert] DonH50's Avatar
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    Yes. Even if you don't move your head. There is debate over how our brain processes the information and if it matters. I am not really competent to engage in that debate.
    Don Herman
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  2. #12
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    Comb filtering is around us all the time in real life. An example is when we listen to loved ones around the home. Whenever there are two paths to your ears, and one is delayed as would be with reflections in your home, there will by definition be comb filtering. Fortunately, there are many reflections in the room and they all combine to fill the holes. In addition each ear hears a different comb filter due to different path lengths to each ear and you see that in reality this is not at all a simple situation.

    Fortunately reflections have existed since we moved into caves and the brain has had plenty of time figure out how to evolve and not only ignore it, but use these delayed reflections to good effect. This is why your loved ones don't sound different in different parts of your home. And why you learn to ignore reflections from your seatback. See the top article in this link on audibility of room reflections: http://www.audiosciencereview.com/fo...ticles/&page=5

  3. #13
    Quote Originally Posted by DonH50
    Number three is coupled with the room treatment debate; reflective surfaces add signals that can cause comb filter effects. Note that studies have shown that if the delay is small, we cannot really isolate the signals, and if the delay is large, we hear them as separate sources. Psychoacoustics is not my field so you’ll have to ask somebody else to explain that.
    No explanation at hand, but there is lot of literature relating to echo thresholds, which is the point at which the reflection is perceived as independent sound event. These thresholds are different for different signals, and range from about 1 ms to about 80 ms.

    Blauert et al. (2005), “Acoustical communication: The precedence effect”, Proceedings FORUM ACUSTICUM BUDAPEST, OPAKFI Budapest

    Damaske 1967/68, “Subjective investigation of sound fields” (in German), Acustica, vol. 19, p.190

    Kuhl (1978), „Spaciousness as a component of total room impression“ (In German), Acustica, vol. 40, p.167

    Litovsky et al., “The precedence effect”, J. of the Acoustical Society of America 1999, vol. 106, no. 4, pt. 1, p.1633


    How much comb effects matter is subject to psychoacoustics, something outside my field.
    The amplitude (or modulation depth) of the comb filter depends on the level of the reflection. The lower the reflection level, the lower the amplitude and the coloration decreases. Coloration further depends on whether a single or multiple reflections are present and in the case of multiple reflections, whether these are regularly spaced in time or not. Irregularly spaced reflections result in lower amplitude of the comb filter and the perceived coloration becomes less with increasing number of such reflections.

    Quite a lot of information about this can be found in

    Salomons, “Coloration and binaural decoloration of sound due to reflections”, PhD Thesis, Delft University, 1995

    http://repository.tudelft.nl/view/ir...-eb5d6cc04fbf/

    provides the PDF.


    Binaural decoloration is a mechanism a microphone does not have, so a simple in-room frequency response won’t tell you anything about the audibility of those comb filters. As Amir says (good article, hits the nail on the head!), in rooms there are many reflections, irregularly spaced in time, and that’s a good thing, which in general does not require further action.

    Klaus
    Speakers: Klein + Hummel O500C, Electronics: Funk MTX preamp, Rane PS1 phonostage, Analog: Michell Gyrodec, SME 309, Shure V15VxMR, Digital: Tascam CD-RW4U, Tascam MD-801R

  4. #14
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    David Griesinger has studied reflections extensively from the perspective of room intelligibility & envelopment - search for Griesinger & envelopment
    Manufacturer digital products www.Ciunas.biz
    "The greatest obstacle to discovery is not ignorance – it is the illusion of knowledge." - Daniel J. Boorstin.

  5. #15
    Member Sponsor [Technical Expert] DonH50's Avatar
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    Thanks for the background guys. I used to know (remember) more but decided for this thread I'd leave Haas (precedence effect, at least back when I last looked at it) out of it simply because I don't know enough to personally comment. Calculating the effects are easy; describing what we do with them is outside my area of expertise. Really appreciate the references! - Don

    p.s. It looks like I could have saved myself a lot of work by just linking to Amir's article!
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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