Question about cabinet venting

Discussion in 'Amps [BG]' started by daveihde, Apr 10, 2014.


  1. daveihde

    daveihde

    Joined:
    Jul 17, 2006
    Am building a small vented 10/6 bass cab and have a design question. I apologize in advance if this has already been discussed elsewhere, but I couldn't find it. Eminence's suggested design called for a 3.5" port at a certain length. I already had some 3" PVC, so I did the math to calculate what the same area would be and applied that to two smaller, shorter ports, instead of one longer, bigger one. I have since been listening and not getting the response I expected, so wonder if I was mistaken in my approach. Using WINISD to model the box, whenever I change from 1 vent to 2, the length specified INCREASES instead of decreases, the opposite of what I would expect.

    Being a horn player, I understand that a longer, bigger pipe equals lower frequencies. Am I getting this wrong, or is the physics of venting counter-intuitive; that longer = higher?
  2. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

    Joined:
    Nov 24, 2008
    Location:
    Preston, Idaho
    Disclosures:
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Same diameter but longer vent = lower tuning frequency
    Same length but smaller diameter = lower tuning frequency, and possible earlier onset of vent noise (chuffing)

    So when you go from one 3.5" diameter port to two 3" diameter ports, the port length needs to be increased to maintain the same tuning frequency, because the two 3" diameter ports have more cross-sectional area (larger effective diameter, if you will) than the single 3.5" diameter port.
  3. BassmanPaul

    BassmanPaul Gold Supporting Member

    Joined:
    Aug 25, 2007
    Location:
    Toronto Ontario Canada
    As I recall, it's the volume of the port that matters. Thus if you reduce the diameter of the port you have to increase the length to compensate. If the port becomes too narrow, you'll end up with port "chuffing" where the tightness of the port restricts air flow.
  4. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

    Joined:
    Nov 24, 2008
    Location:
    Preston, Idaho
    Disclosures:
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Nope! But don't feel bad - that's an easy one to get wrong. What you say is what intuition would tell us, but in this case our intuition is mistaken.

    In the name of science, I recommend this experiment: Buy two equal-volume bottles of beer, one with a long skinny neck, and one with a short fat neck. Empty them at your leisure. That is the first and most important step. Then, blow across the top of each of them. You'll hear a lower pitch from the beer bottle with the long, skinny neck, because that long skinny neck is tuning it to a lower frequency.
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