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Does port positioning matter?

Discussion in 'Amps and Cabs [BG]' started by Myth_103, Sep 17, 2008.


  1. Myth_103

    Myth_103 Supporting Member

    Ok, I know to never position a port directly behind a driver, but other than that does it really matter? I'm about to cut the face of the cab (speaker baffles and port holes), but I only have room for the 6" ports on the bottom right and top left. Will this pose a problem? It's very much like the aguilar GS 412, EXCEPT, instead of having a tweeter at the top I'd had my second port. So yeah.. very bottom. very top.
     
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    No.
     
  3. Myth_103

    Myth_103 Supporting Member

    Sweet. I like straight to the point answers. Appreciate it.
     
  4. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    What do you expect to gain out of cutting an additional port? The size and depth of a port can be important.
     
  5. For a more detailed explanation you can check Acme website:
    http://www.acmebass.com

    Click on "Products" and then on the link called "An explanation of the (un)importance of port placement"
     
  6. No, the outside world doesn't care where they are but it is commonly said that for accuracy you would want to place them at least one port diameter away from any interior walls. I really don't know how much this will affect the tuning though :meh:
     
  7. spode master

    spode master

    Jan 21, 2007

    I think thats mostly true, but I do think for someone designing a speaker system that would relate to some of the path lengths of the port in its non operational range.

    If the port is long, some of the mids can arrive later and cause cancelations, or dips and boosts.

    Putting a port in the rear can make port noise less apparent as well us higher order port resonance. Depending on wavelengths involved the energy from the port might not be added back into the effective radiated pressure (say if the port is behind a baffle wall).

    I don't no how Bill will feel about any of that. But for Live Bass rigs, its normally not going to cause much difference. Unless you do something strange. So no it doesn't really matter.

    Just remember not to let a rear firing port be blocked by anything like a wall, or other solid objects.
     
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If you have mids coming through the port the cabinet design and/or construction is defective. There should be nothing much above 100 Hz going through any port, and the minimum 11 foot wavelengths involved render port location moot.
     
  9. spode master

    spode master

    Jan 21, 2007
    Just to play devils advocate.

    I know that there is a big phase shift that happens around port tuning, isn't it possible to have a rear firing port have a fraction of a critical wavelength to cause the energy from the front radiation of the driver recombine 180 degrees out of phase?

    I have built ports that have had out of pass band resonances (Asymptotic Looking peak and dip). If the port is rear firing can't that help minimize the effect of Higher port resonances?

    Normally I find this problem happens when you are trying optimize a cabinet with a driver that has a really High Xmax.
     
  10. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A cabinet where this might be a problem would be impractically large.
     
  11. At the tuning frequency, the vent is 90 degrees away from the cone. The output from the vent is prime, that from the cone is nil. The 90 degree lag is the excitation time between cone movement and that of the air mess in the vent.

    As frequency varies around Fb, so does the phase. The upper and lower resonances are 180 degrees. The lower resonance is a push-pull relationship. Driver goes in, vent mass goes out.

    At port frequencies, the wave forms are spherical. This makes the box (and vent) into pressure devices. Andy's analogy of blowing up a balloon makes the point quite clearly. No matter where you blow the balloon, the whole thing inflates. Same withe vent location.
     

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