Why does my sub "burp"!?

Discussion in 'Archived Threads 2001-2004' started by james e m, Sep 18, 2001.

  1. james e m

    james e m Second Unit

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    In some instances my Acoustic Research S112PS makes a "burp like noise" while watching a DVD. For instance on Fight Club at the very beginning when the camera comes to the end of the gun in Ed Norton's mouth it makes this strange noise. It also happens in the title sequence of Toy Story 2 right before the Toy Story 2 logo appears on screen. Can my sub just not handle it? When I shut the sub off the main speakers seem to handle it fine, but they can't handle the real low end like the THX intro. What should I do?
    james
    http://mirkwoodhometheater.tripod.com/
     
  2. John Gates

    John Gates Second Unit

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    James,
    Yours sub is probably bottoming out the driver during those segments. If it is a popping sound, it's bad for the driver. If this is the case, you should back off the volume of the sub until you no longer hear the popping sound in those areas. Long-term, the bottoming can destroy or damage the driver.
    John
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  3. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    What you're probably hearing is port noise. This happens with deep bass sometimes on my Polk PSW350, and sounds like a kind of flapping sound (or a burp as you said). It's nothing to worry about as far as anything being damaged.
    I've heard of people using foam plugs to dampen this effect, but I couldn't tell you for sure what that does for you. Perhaps someone else here knows more...
     
  4. Kevin P

    Kevin P Screenwriter

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    An air leak can also cause a sub to burp or sound strange. Put on a low frequency test tone (the AVIA LFE sweep works great for this) and feel around the sub to make sure that you aren't getting any air coming out from around any seams or cracks in the enclosure (other than the port).
    If air is leaking around the edge of the driver or the amp plate in back of the sub, the screws might need to be tightened. Otherwise, the sub might have a faulty seal.
    KJP
     
  5. james e m

    james e m Second Unit

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    It doesn't sould like a "popping" sound, it does sound like it's "flapping". Should I turn the bass down? The sub handles the Avia LFE track fine. There is no way that I can get to the woofer itself to check it out, the enclosure won't allow it.
    james
     
  6. John GB

    John GB Stunt Coordinator

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  7. Patrick Sun

    Patrick Sun Moderator
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    If it's a sub with a port on it, what's happening is that those DVDs have low low bass frequencies under 20Hz recorded at high levels, and when the sub is required to play the low bass frequencies (probably under 16Hz), the driver "unloads" due to its EBS (Extended Bass Shelf) alignment which does extend the frequency down to the box tuned frequency (combination of the port used and the enclosure volume and driver). But once the signal contains lower bass frequency content at high volume levels, the port doesn't provided enough resistance to keep the driver "in check" and subsequently flaps or farts on demanding bass passages.
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  8. John Garcia

    John Garcia Executive Producer

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