Port chuffing at low end... fix?

Discussion in 'Home Theater Projects' started by Jordan_Brulotte, Nov 14, 2004.

  1. Jordan_Brulotte

    Jordan_Brulotte Stunt Coordinator

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    Since completing my diy tempest, I've been putting it through the paces. As I do not have the rest of my HT setup I've been testing it mostly with music. I recently played the first song on the album "Songs for the deaf" off the Queens of the Stone Age album. It's not quite a song but more of an intro... but my god does it have insanely deep bass. I am positive that it is playing down to nearly 15hz with power.

    This test showed a fairly annoying flaw in my design. I have 2 4" ports tuned to ~18hz, with flares on the outer end. But when the sub get's waaay down I get a lot of port noise. I first thought it was the tinsel slapping, but I'm quite sure it is the air in the tube. I then tried a tone generator, and once again it does seem to chuff at ~22hz and down, even at fairly moderate volumes.

    I was thinking that putting a high pass filter set at 20hz would help. Is there any way to stop or fix this without increasing the port diameter? I built the port flares by hand so increasing the diameter would be a MAJOR pain. The flare radius is quite large, and was not done with a router.

    For 99% of music the sub will still perform quite well, but HT is my main concern. I threw Matrix: revolutions in and went to the conversation with the deus ex machina. At a comfortable listening level (sorry no spl meter yet... will get one soon) there was some noticable chuff but it wasn't too bad.

    Please help! [​IMG]
     
  2. Jordan_Brulotte

    Jordan_Brulotte Stunt Coordinator

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    I did a spectrum analysis of the "song" and it has most of it's power in the 20-30hz range and a huge amount of 9hz bass near the end. I think that explains it. But it definately showed me the port problems I have. Since my cone excursion is highest at 25hz, I'm sure that this is the problem. Click HERE to see the waterfall graph. Warning image slightly large.

    Help!
     
  3. Mattak

    Mattak Stunt Coordinator

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    I also have a tempest with two 4" flared ports tuned to 18~19hz. I've never noticed any port noise running tones from 15-70hz. I did have some leakage around the sub so I redid the gasket and that went away...
     
  4. ThomasW

    ThomasW Cinematographer

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    Yeap that cut has a bunch of bottom end.

    At what SPL's are you getting the chuffing?

    It's best to use flares at both ends of the ports. Perhaps you can add after market flared ends to the inside ends of the ports?

    Hopefully the internal ports ends are spaced the diameter of the port away from any internal surface.

    Have you actually measured your tuning frequency?
     
  5. Jordan_Brulotte

    Jordan_Brulotte Stunt Coordinator

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    I don't have an spl meter but I'm going to try and get my hand on one today. Not much selection in my town I'm afraid.

    Will leaks in the cabinet add to the perceived chuffing? If so I'll go around the cabinet again and re-seal.


    When I designed the cabinet I made sure to keep the port ends away from any internal bracing or walls. However the 2 port ends are within a half diameter of each other. I never stopped to think how they would interact with each other. [​IMG]

    I am going to get some pics up soon of my cabinet and the internals.
     
  6. Mattak

    Mattak Stunt Coordinator

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    Yes, leaks can sound pretty bad. If you didn't caulk the internal seams of the enclosure you might want to try that, and make sure you have some type of good seal around your driver and ports.
     
  7. Aaron Gilbert

    Aaron Gilbert Second Unit

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    I agree with Mattak, any air leaks can sound quite bad, and very similar to port chuffing. I'd be very surprised to hear chuffing with dual 4" flared ports on a Tempest. As Thomas mentioned, it is best to have the internal end flared as well, but even if not, that's still a lot of port area. Also, your chuffing should be worst at the tuning frequency, and decrease both above AND below that frequency. If your noise stays just as loud as you continue to sweep below where you think the box is tuned, I would highly suspect air leaks.


    Aaron Gilbert
     
  8. Jordan_Brulotte

    Jordan_Brulotte Stunt Coordinator

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    Great advice guys! I'm going to check for leaks. When I built the cabinet I lined every seam with acoustical sealant. But I'm going to check everything.
     
  9. Mattak

    Mattak Stunt Coordinator

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    Def. check the driver seal itself, especially around screw holes...
     

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