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port tube question

Discussion in 'Amps and Cabs [BG]' started by Metal Mitch, Oct 10, 2003.


  1. Metal Mitch

    Metal Mitch

    Jul 14, 2003
    NJ
    Is there an easy way to extend the length of existing port tubes?

    Playing around with WinISD it looks like I can get one of my cabs to perform better by extending the port tube length from 5" to 8". There's plenty of clearance inside the cab, but I haven't found any prefab 8" tubes and thought it may be easier to just glue on an extension.

    Would this be feasible, or am I going too far out on a limb here?
     
  2. BruceWane

    BruceWane

    Oct 31, 2002
    Houston, TX
    As long as you maintain the same inside diameter and it doesn't rattle, anything goes. I've built prototype cabinets with external ports made of PVC pipe so I could try different tunings and actually hear what sounded best. What I usually find is that there isn't a huge difference in the overall sound, but different tunings will accentuate different frequency ranges which makes some ranges of notes sound a little stronger than others. And if you tune it so that there's a pretty sharp bump in the response, it can actually make notes in that range sound a little off pitch.
     
  3. Metal Mitch

    Metal Mitch

    Jul 14, 2003
    NJ
    Hey, thanks Bruce. So you've built an "octopus cab" eh? ;)

    I'm actually tuning it to level off the bump and give it a relative +5db at 30Hz. :D
     
  4. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Look out, cuz tuning for that might cause the speaker to get beat on pretty hard.

    A lot of cabinets are set up as much for cone control as for response. It's a compromise. If you mess with the tube you might make it flap pretty hard at some frequency.

    You can check by watching the cone.
     
  5. Metal Mitch

    Metal Mitch

    Jul 14, 2003
    NJ
    Notanaggie, thanks for the reply. I replaced the stock driver, and I'm not tuning lower then the new driver's Fs... if that's what you meant?
     
  6. notanaggie

    notanaggie Guest

    Sep 30, 2003
    When you put the driver in a box, the driver's resonance is not the system resonance. The box and speaker work together and the natural (closed) resonance is higher.

    So I would have no idea whether your tuning is above or below the "new" frequency.

    But, if the box tuning is low, the speaker may be moving pretty far to reproduce power at the frequencies just above the area that the tube/box takes over.

    You can see easily if you use a signal generator and tune through the range. In many cabs tuned low, the speaker starts to really boogie, then at a lower frequency it almost seems to stop.

    That "stop point" is where the cabinet/port takes over. An engineer would say the box system "presents a high impedance to the speaker" there.
     
  7. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I'm assuming the cab volume is OK.

    If the speaker has an Fs in the high 20Hz range, and he's tuning it to 30Hz, in theory the speaker cone movement should be fine. My understanding of vented designs is that speaker excursion is at it's greatest at frequencies below the cab's tuning frequency. Could be a problem if you intend going lower than the B note on a 5 string, but other than that, is should be fine.

    Back to the original question - I've soon ports in stores that are adjustale for length. They're basically a trombone type sliding device. the theory is that you use a small screw to lock it in place once you've got the desired length. If you can find one that's the correct diameter, it could be an option. Click HERE.
     
  8. notanaggie

    notanaggie Guest

    Sep 30, 2003
    """My understanding of vented designs is that speaker excursion is at it's greatest at frequencies below the cab's tuning frequency."""

    Well, it depends on the design. Greatest is probably true due to there being no restriction on movement below the box tuning, but there is usually a max movement above box tuning too.

    IF the speaker has the ability to put out, air-movement-wise, at all frequencies above where the resonance takes over, you are OK.

    Typically the lower you want to tune the box the worse it will be.

    But, it depends on the allowable speaker movement. Some speakers are 'tight" and can't move far, even though they may have a good F(s). Might just be due to a voice coil issue, or might be suspension.

    More efficient speakers tend to have higher F(s) and lower allowable movement, just from the practical physics of the way they work.

    For the hifi folks that may not be an issue, but it can be for high power bass amplification. if the speaker excursion can be exceeded by the user power level before the assist from the port comes in, you have a problem.