Multiple vents / length per...

Discussion in 'Home Theater Projects' started by ColinM, Oct 2, 2003.

  1. ColinM

    ColinM Cinematographer

    Dec 9, 2001
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    This is something I've never really understood - -

    In WinISD, If I model a sub (Shiva) with 1 port, 4" dia, 5ft3 tuned to 20hz, length = 13.62.

    2 ports in the same config, all else being equal, yields "vent length 30.17". I have asked before, and all responses say that that's 30" for EACH port.

    Now, looking at some recent production subs, like SVS's box types or the VTF-3, there are multiple ports, but not nearly enough room for vents longer than maybe 18" each. Of course I don't know the driver parameters, but would those parameters be enough to be able to have short multi ports?
  2. Phil~E~Guy

    Phil~E~Guy Extra

    Jan 30, 2003
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    Hi Colin,

    WinISD has two programs, beta and alpha pro. The alpha pro version will show the tuning graph. You must maintain port velocity below 5%. Port air velocity greater than 5% could add noise.

    So here is your answer (no port size or length is being said here).

    One port may need 5" to maintain 5%.
    Two ports may need 10" to maintain 5%.

    Two ports would let out to much air, port air velocity would rise above the 5%. To decrease port air velocity to 5% would require more port length (both ports). This increases resistance of air flow through the ports.
  3. Patrick Sun

    Patrick Sun Moderator

    Jun 30, 1999
    Likes Received:
    They just accept a higher tuned frequency for the enclosure in their design as a trade-off for the form factor.

    The more cross-sectional area is used (multiple ports), the longer the port will be for the same target tuning frequency. That just how the math works out.

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