Can bass traps have any negative effect on sound or only positive?

Discussion in 'Home Theater Projects' started by Todd smith, Aug 8, 2004.

  1. Todd smith

    Todd smith Supporting Actor

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    I am a little confused about what bass traps actually do. They tame bass peaks (correct?), but does that mean the overall bass volume will decrease when playing movies or music? Or will it just tighten up the bass and still be just as loud? Can bass traps only help sound, or can they harm it as well?
     
  2. Leo Kerr

    Leo Kerr Screenwriter

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    It's been a while since I read up on it - though I'd recommend to anyone doing DIY acoustics to read The Master HandbBook of Acoustics by F. A. Everest. Published by Tab Books / McGraw Hill. I'm most familiar with the 3rd edition; aparently there was a recent 4th edition, so your milage may vary.

    Anyway, my understanding of things like bass traps and Helmholtz resonators is that they 'tune' certain frequencies in a room. But, like an equalizer, you can boost or cut any given frequency too much and make the problem worse than before.

    You can also add enough volume of 'stuff' from your acoustic treatments to fundamentally change the modes of the room in the first place.

    (Again working from distant memory,) I think the role of a bass trap is to take a hypothetical situation. You've got 'the perfect subwoofer.' You place it in your room, and the very best place (compromise between position and function and SAF(!)) yields up a pretty sharp boost of the 60Hz mode in the room. Exciting modes is never really a good thing - after all, who really wants (especially) the 60Hz mode excitied?

    So you build a bass trap tuned to 60Hz. Or two. Or three. Depends on how bad the mode is. In essence, you're building in a... passive equalizer that can be fairly sharp in its tuning, without, hopefully, adding a lot of other troubles.

    Your milage, of course, may vary from EPA estimates...

    Leo Kerr
     
  3. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Todd,

    > They tame bass peaks (correct?) <

    Bass traps do two things: They flatten the response at low frequencies, which means they reduce the effects of both peaks and nulls. They also reduce modal ringing, which is sort of like low frequency reverb that muddies up the sound generally and reduces the clarity of bass instruments.

    > but does that mean the overall bass volume will decrease when playing movies or music? <

    Adding bass traps usually increases the perceived level of bass in a room. A lot of people don't realize that nulls are more of a problem than peaks. Peaks are rarely more than 3-6 dB, but nulls are typically 30 or more dB deep in an untreated room. So by reducing the severity of nulls, the level and quality of bass is increased.

    > Can bass traps only help sound, or can they harm it as well? <

    As best I can determine it's not possible to have too much absorption at low frequencies. Though you can definitely have too much absorption at mid and high frequencies. I have 31 bass traps in my living room home theater, and as I've added more over time the low end has continued to improve.

    --Ethan
     
  4. ChrisWiggles

    ChrisWiggles Producer

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    We're also confusing different kinds of "traps."

    Ethan is talking about broadband bass absorption, which is just (hopefully) absorption in the bass frequencies, usually thick fiberboard, etc.

    Leo is talking about targeted absorbers/resonators which 'absorb' specific, limited, targeted frequencies.
     
  5. BruceD

    BruceD Screenwriter

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    I (and many others) continue to disagree with Ethan on this point.

    I think most users find that peaks are much more destructive to good sound quality in our HTs than nulls. It is simply not true that modal peaks are only 3-6dB. Many graphs submitted by members in these forums show peaks of 9-20dB for various bass modal peaks.

    There is just no getting around the massive boominess of a persitant modal peak. It simply destroys the system's capability to provide a enjoyable HT experience because you must lower the sub's calibration SPL to keep this peak from drowing out all the other bass.

    While nulls mean you may miss a specific frequency they simply don't rise to the same destructive potential as a peak.
     
  6. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Chris:

    > Ethan is talking about broadband bass absorption <

    I wasn't being even that specific. Just trying to explain why it's not likely that adding bass traps would ever do anything but improve the sound.

    Bruce:

    > There is just no getting around the massive boominess of a persitant modal peak. <

    I don't really disagree with that, but let me try putting it another way. It's not difficult to reduce resonant peaks to an acceptable level with bass traps. Assuming they're effective at the frequencies of interest. But it takes even more absorption to counter the effects of nulls, so to my way of thinking that's the more difficult task. And I do continue to insist that nulls are very damaging, at least when they align with musical notes in the key of the music.

    --Ethan
     
  7. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Say Bruce,

    I just looked at your equipment list and saw no mention of bass traps or other acoustic treatment. I'm curious - what have you done to reduce the modal peaks in your room?

    --Ethan
     
  8. Wayne A. Pflughaupt

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    I can answer that one for you, Ethan! It’s under the “Tweaks” heading: “Behringer BFD 1100P Parametric EQ (sub only).” [​IMG]

    Yes, we are aware, it doesn’t actually “reduce modal peaks,” it just deprives them of energy. But it gets the job done.

    Regards,
    Wayne A. Pflughaupt
     

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