Port length to diameter ratio?

Discussion in 'Archived Threads 2001-2004' started by Mark Tranchant, May 21, 2002.

  1. Mark Tranchant

    Mark Tranchant Stunt Coordinator

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    I'm considering a sub project at the moment. I don't yet want to get bogged down in a detailed discussion of what I'm trying to do until I know it's feasible - but I will need to tune a relatively small box (1.3 cu ft) to a very low frequency (22Hz).

    For a 3" diameter port, the port length will be about 36", a ratio of 12:1, and will require a couple of 90deg bends to fit in the box. I am aware that I need to allow for the port volume within the internal box size.

    I've come across various suggestions for a limit to this ratio from 2.5:1 to 4:1. What is the audible or physical effect of having such a long port? Has anyone ever built such an enclosure and gotten satisfactory results?
     
  2. Patrick Sun

    Patrick Sun Moderator
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    The port will stop functioning like a slug of air (resistance of mass), and start introducing pipe resonance to the output. I try to keep port lengths to 5x-6x or less of its diameter.
     
  3. Mark Tranchant

    Mark Tranchant Stunt Coordinator

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    Yes, I know that a PR would be ideal, but I don't have room for it in my high-WAF threshold design as it stands, nor do I have a good source for the radiator (UK suggestions, anyone?). The alternative would be to go electronically-assisted sealed, or possibly an isobaric design, but I'd like to avoid the extra cost.

    I'm after as low a cut-off frequency as possible, combined with "musicality" and compact size (basically a box up to 2ft³). I already have a 100W amp to drive it with, and am not bothered about high output level capability.
     
  4. Patrick Sun

    Patrick Sun Moderator
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    You may as well go sealed, then, and pray that room gain will help in the bottom end.
     
  5. Mark Tranchant

    Mark Tranchant Stunt Coordinator

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    Aha - I've found a source for a passive radiator in the UK. That'll help - and I think I've found a way to incorporate it without drastically reducing WAF. [​IMG]
    The plan is to use a MacAudio Mac Fire 250 subwoofer driver (available from Maplin) in a pseudo-sixth order (electronically assisted vented box) arrangement. Theory behind this is from Kaufman's "Enhanced Sound" book - I'll add details to my web site if and when I get around to this.
    In theory, I should be able to get an f3 of about 22Hz in a very small (1.3ft³) enclosure. [​IMG]
    Thanks for the suggestions and advice.
     
  6. Dave Poehlman

    Dave Poehlman Producer

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    Looks like a job for an ELF&#8482 subwoofer:

    To quote a friend:

     
  7. Greg Monfort

    Greg Monfort Supporting Actor

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    >For a 3" diameter port, the port length will be about 36", a ratio of 12:1, and will require a couple of 90deg bends to fit in the box.
    >I've come across various suggestions for a limit to this ratio from 2.5:1 to 4:1.
    ====
    WRT a round pipe aspect ratio, there really isn't any. All that matters is that the cross sectional area be sufficient to keep vent mach low and the length needs to be short enough to keep pipe harmonics out of the sub's BW and low in amplitude.
    Awhile back this approximation was derived:
    Max Length = 13560/(20*Fb)
    Where:
    Max Length is in inches
    Fb is the tuning frequency in Hz
    So for a 22Hz Fb, ~30" is the practical limit.
    ====
    >What is the audible or physical effect of having such a long port?
    ====
    If the pipe is long enough its harmonics will be low enough in frequency to affect the sub's output, usually in a negative way.
    ====
    >The port will stop functioning like a slug of air (resistance of mass), and start introducing pipe resonance to the output. I try to keep port lengths to 5x-6x or less of its diameter.
    ====
    FWIW, the bends don't stop the resonant mass, and pipe harmonics are there regardless of shape. What happens with >90deg bends is that new, higher frequency harmonics are created, increasing out of BW 'noise' that can sometimes be loud enough to comb filter with the upper BW drivers.
    I've found that using 45deg elbows don't do this, even if two are joined to make a 90deg bend, at least I can't hear them and that's all I care about. [​IMG]
    All that said, back before decent PRs were available I've built subs/speakers with far longer vents (up to 8ft long) and used acoustic resistance to damp the harmonics, but that's all ancient history. [​IMG]
    GM
     
  8. Magnus Lindqvist

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    That PR doesn't seem to have any spider according to the picture, it's probably difficult to mass the PR without getting the leveling problem (is leveling the right word??).

    The price is almost to good to be true for us europeans!

    /Magnus
     
  9. Greg Monfort

    Greg Monfort Supporting Actor

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    'Balancing' is probably the word you're looking for, but using discs attached to the rear shouldn't tend to cock it at high excursion as long as some attempt is made to center them, and with a surround that's a significant percentage of the area I don't see the need for a spider.

    GM
     
  10. Magnus Lindqvist

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    One could make a disc that have a diameter as big as the MDF disc and this way keep the mass as close to the center as possible (keep the offset distance to minimum),good idea Greg!

    /Magnus
     
  11. Mark Tranchant

    Mark Tranchant Stunt Coordinator

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    Dave
    Yes, what I was referring to as "electronically assisted" appears to be what you describe as ELF. It can also be applied to vented boxes, although you need to change the box dimensions. See Kaufman's book.
    Jack
    What PR specs are needed? Moving mass and moving area only, isn't it? Could I derive the mass adjustment needed by putting it in the box and finding the resonance of the PR, perhaps at two masses (x and x+n when n is the known added mass and x is the unknown PR mass), and solving to find the required added mass?
    The woofer isn't that good for straight sealed or vented use, but the combination of parameters happens to work well with the equations for the "ELF" vented box to give a deep bass in a small enclosure.
    Magnus
    I'd be using the PR horizontally, facing upwards, so the lack of a spider shouldn't matter so long as I place the weights symmetrically or centrally.
    Greg
    Thanks for the vent advice.
     
  12. Magnus Lindqvist

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    Mark, PR's can not be used horizontally, only vertically!!

    /Magnus
     
  13. Michael_UK

    Michael_UK Stunt Coordinator

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  14. Mark Tranchant

    Mark Tranchant Stunt Coordinator

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    Magnus - why? I can see that the PR would end up "biased" downwards, which would reduce the excursion I could use, but is there any other reason why a PR must be vertical?
     
  15. Magnus Lindqvist

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    Mark, yes the PR will sag heavilly because of the mass.

    You could put the PR on the front and the active driver downwards (if the MacAudio driver is suitable for downfiring).

    /Magnus
     
  16. Greg Monfort

    Greg Monfort Supporting Actor

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    >One could make a disc that have a diameter as big as the MDF disc....
    ====
    Yes, though of course you'd have to cut into pieces to install it. Chances are though that it wouldn't be the right amount of added mass, so using metal slugs or similar screwed or bolted (using a threaded insert) allows for eay adjustability. Hopefully they provide some means as an accessory to make it easy.
    ====
    >....but is there any other reason why a PR must be vertical?
    ====
    How can it reproduce a sinewave if there's no zero (centered) position? [​IMG]
    GM
     
  17. Mark Tranchant

    Mark Tranchant Stunt Coordinator

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    If the surround behaviour is linear to its limits, it should work just as well horizontally albeit with reduced maximum excursion; the zero position would be its "rest" position. That's just theory, though...
     
  18. Jack Gilvey

    Jack Gilvey Producer

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