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Bass cab modification question - port design

Discussion in 'Amps and Cabs [BG]' started by flyguy909, Jul 3, 2014.

  1. flyguy909


    Mar 3, 2011
    Coventry, CT
    Sorry if this is the wrong place for this...

    I am modifying a GK 2x12 MBE cab by replacing both speakers with Eminence Delta 12LFA's. I am researching whether the Delta's will play nice in the cabinet as originally designed... I've downloaded cab and port design software and punched in the Thell Small params.. etc.

    The original cab has a 5 x 9 inch cutout in the bottom corner for a rectangular vent with a length of about 3". The info with the Deltas say a cab tuned to around 42hz will work best. So after entering the cab volume and taking into account 2 drivers, one way to tune this cab to 42hz is to use a 4" pipe about 3.6 inches long as a port. According to these utilities the existing rectangular vent/port is not even remotely close to what it needs to be.

    So I'm thinking of overlaying the rectangular cutout with a piece of plywood and cutting a 4' hole for the port and using a 3.6 inch long 4" plastic pipe.

    Asking anyone that knows... am I in the ballpark here?

  2. RoadRanger

    RoadRanger Supporting Member

    Feb 18, 2004
    NE CT
    Why ruin a perfectly good cab? Maybe sell it and order up a custom with the drivers you want? I have two of them and they work fine as-is.
  3. will33


    May 22, 2006
    That's one way to do it, but a single 4" ID pipe won't have enough area, meaning the air velocity in it will be too fast, it'll make weird chuffing noises and mess with the tuning "wandering" at higher power levels.

    For a pair of 12's with decent xmax like that, I'd use 3 of them. Or four 3" pipes for the 212 cab.

    One sort of rule of thumb is to set the power input to whatever level makes your speakers hit xmax and look at the air velocity graph. You're looking to stay 20 meters per second or less at the excursion hump that comes about an octave above the tuning frequency, to insure no chuffing and also to keep the tuning from wandering due to constriction/high air velocity that comes with high power levels if you use too few and/or too small of ports. Using ports with flared ends can allow you to go a little high with the air velocity before chuffing.

    Alternately, you could use the port that's there and "lengthen" it by adding a couple pieces of plywood to the back of it to make it extend deeper into the cab, like making a longer tunnel. 9" or 10" depth may get you there as the wall of the cab will act as a bit of the port, making the tuning a bit lower than WinISD predicts.

    Being that your short port dimension is 5", don't go any closer than 5" from the back wall with the rear of the port, as the back wall will then act as an elbow and become part of the port, making it "act" longer than it is, again making the actual tuning lower than the program prediction.

    You can test the actual tuning by laying the cab on its back, putting a few kernels of uncooked rice on the cone, and playing sinewaves through it and fishing around the frequencies until you find where the rice stops "dancing" as the cone is held nearly still.

    Search up "sinewave tone generator" and you'll find several websites that generate the tones. Or, there is a phone app called True Tone that does it on phones. Handy as you can take the phone to the cab rather than having to set up the rig next to your PC. Test tones are loud and annoying, may want to do it in the garage or backyard, at least if you have a wife or a baby. :)

    Can use the same thing to see where the cab is tuned now. I'm guessing 55-60hz.
    AlexanderB likes this.
  4. What is it about the stock cab that you don't like? Replacing the stock drivers with the Delta 12LFA'a and retuning will produce pretty significant changes to the tone: bass, mids and lower highs. Is the Delta even the best driver to use?
    AlexanderB likes this.
  5. AlexanderB


    Feb 25, 2007
    I strongly second will33's advice on how to do a "real life" check of the tuning frequency. Low "ceiling" tunnel ports are much harder to predict the outcome from, compared to circular vents. On the other hand, the more laminar air flow in these flat ports will make it seem "ancoustically longer" than an equally long, circular vent of equal cross section. In other words, for boxes with limited room for deep ports, a flat port can allow a deeper tuning for a given length.

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