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Port size/number for new cabinet

Discussion in 'Amps and Cabs [BG]' started by Sundogue, Nov 15, 2004.

  1. Sundogue


    Apr 26, 2001
    Wausau, WI
    Thanks to all of you who helped with my cabinet building (from a previous post).

    I built it over the weekend (4 hours each, on Saturday and Sunday). The only thing that is left is routing out the edges, sanding the crap out of it, and...

    I either need a port of 6" X 7.5" long, or two 3" X 3.75" long ports.

    Which would you go with, and why?

    Also, when given the length of the port, is that measured from the inside or outside of the speaker baffle? I put in a hefty 1.25" thick baffle. So, do I add 1.25" to the length of the port?

    Any suggestions?
  2. doesn't matter. I think one of those flared ports would look cooler and reduce (inaudible) port noise.
  3. Port is measured end to end. If flush with baffle, then from that flush point to end of port.

    Leave 1 diameter clearance behind the port and the rear wall. Too close interferes with the port operation.

    Number of ports makes no difference. To calculate two ports, divide the cabinet volume in half and calculate one port for the half-volume. This is accurate, where doubling or halving the port volume is not accurate.

    The cabinet volume for tuning purposes is the net volume, after subtracting for the drivers, port volume, and bracing. This will get you a ballpark tuning. Accurate tuning requires a computer sound card and digital volt meter. Easy stuff, but takes a small bit of effort.
  4. Sundogue


    Apr 26, 2001
    Wausau, WI
    Thanks for the info. That's what I was looking for.
  5. I think another question is whether you want it rear firing or front firing. Personally, I like rear firing for corner loading and for the drummer's benefit.
  6. Sundogue


    Apr 26, 2001
    Wausau, WI
    It's already designed for porting in the front (which I prefer).

    It's just for tuning my cab, not for giving any directional sound (which, I do believe, is irrelevent for low frequencies).
  7. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Sundogue...congrats on your new half cabs...I hope that you understand that posting pictures is mandatory! :D
  8. Sundogue


    Apr 26, 2001
    Wausau, WI
    Yes, I plan on posting some pics of my combo cab when it's done.

    I've got some pics already that I'll include, of the bracing I employed, as well.

    It's not as light a cab as I hoped for, but it is smaller and more manageable. Being that it is a combo contributes to this, as the head adds to the weight.

    Very solid cab. I used 3/4" plywood and glued a 1/2" piece to the 3/4" baffle board, for a 1.25" thick baffle board. Along with the bracing it is really solid.

    I also angled the back, at the bottom, for wheels. It is not the typically large angle found on most "tilt back" combos. A very small area just to allow for wheels. This was taken into account when calculating volume, but it also adds to the rigidness.

    Granted, while this is not a purely scientific test, if you tap on the cab at various spots, there is hardly a drop off in the areas that have no bracing, whereas before I braced it, the open areas of wood sounded significantly more "hollow". Think of how one might tap on a wall to find where studs are located. Where the wall is solid, it sounds solid. Where there is no stud, it sounds hollow and resonates more. The bracing I used ties all six sides together (though there are a lot of "open" areas cut out in the bracing).