How do you get an accurate measurement of port length

Discussion in 'Archived Threads 2001-2004' started by Trenton McNeil, Jan 30, 2001.

  1. Trenton McNeil

    Trenton McNeil Second Unit

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    using Flared Ports? I'm using the Adire Audio flared 4" port set, and want to be sure I'm using the right length.
    Do you meausure from end to end, or from where the flare
    begins, or somewhere in between?
    Also...2nd question: Should you subtract the port volume
    from the enclosure volume to get accurate tuning?
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  2. David Judah

    David Judah Screenwriter

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    I haven't used flared ports, but Brian Steele addresses them at his site http://www.diysubwoofers.org/portcal.htm .
    And for the second question, the answer is yes. Subtract the volume of the port using the outside diameter in your calculations and everything else inside (terminal cup & MDF). There will be some small variances due to calculation inaccuracies, but it shouldn't change the tuning too much.
    Good luck,
    DJ
     
  3. Greg Monfort

    Greg Monfort Supporting Actor

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    Brian's info doesn't address flared lengths, but 90deg flanges at the entry/exit. For flared, this is what I have:
    Lv = (8466*R^2)/(Vb*Fb^2) - 1.463*R + 1
    where:
    Lv = total vent length (end to end)
    Vb = box volume (cu.ft.)
    R = Radius (in.)
    Fb = desired resonance frequency
    I'm surprised Adire doesn't have instructions.
    GM
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  4. Patrick Sun

    Patrick Sun Moderator
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    When you get the Precison Port from Adire, it does comes with a sheet of instructions for determining the proper length of the flared port assembly. And it's the same formula that GregM listed.
    For example, if you need a conventional port to be 19" long, then with a flared port, the total length should be 1" longer for a total length of 20" from END-to-END of the assembly.
    The flared port assembly is 18" from end to end. This includes one 12" port section and 2 flared ends. So this would be an effective 17" long 4" port. If you need a port longer than 17" then you need to order an additional coupling ring and another 12" port section and cut it down to size to get the right overall length.
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  5. David A. Frattaroli

    David A. Frattaroli Stunt Coordinator

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    I used the formula in Ray Alden's book, Advanced Speaker Design. I got the book for $3 at Radio Shack and used it build my mini-monitors.
     
  6. Greg Monfort

    Greg Monfort Supporting Actor

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    Note that the formula in Alden's book is for a straight pipe, and one 90deg flange (baffle), which isn't accurate for flared vents.
    This, BTW, is the best beginners book I've seen. The LDC is more comprehensive, but harder for a newbie to fathom, and a bunch more $$.
    GM
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  7. Marc_D

    Marc_D Extra

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    How does the formula change for multiple ports?
    Double radius?
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  8. Greg Monfort

    Greg Monfort Supporting Actor

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    How does the formula change for multiple ports?
    Double radius?
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    Hmm... maybe... normally there's an additional end correction for multiple ports with straight pipes. IOW, two vents are a bit longer than 2x due to frictional losses, but the flares may obviate the need. If there's one for flared, I don't know it. I've never used multiple flared, so have never researched it.
    Based on the straight pipe formula though, I assume it would be this (don't double R):
    Lv = (8466*R^2*Np)/(Vb*Fb^2) - 1.463*R + 1
    where:
    Lv = total vent length (end to end)
    Vb = box volume (cu.ft.)
    R = Radius (in.)
    Fb = desired resonance frequency
    Np = number of ports
    I take it that the instructions don't enlighten? Where's Dan W. when we need him? [​IMG]
    GM
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