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tuning non-typical port shapes

Discussion in 'Amps and Cabs [BG]' started by Crockettnj, Apr 30, 2006.


  1. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ
    Thought this woudl be a agood topic for cab builders discussion.

    there are scads of DIY calculators and programs that will aid someone in tuning a round port. there are end corrections for flared round ports. Ditto for tuning a square/rectangular port . There are also easily obtainable calcultors for slot ports.

    HOWEVER, i have been unable to find a calculator for tuning slot ports that are triangular in cross section.

    I am off to see what vance has to say about this, and to find a generic cab tuning / port tuning equation. I am guessing i can then fit any shape to this, since it should be a function of cross sectional area and length. (but then again different cross sectional shapes will have differing resistance to air flow... is it significant?
     
  2. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ
    eee gads, no one?
     
  3. ibz

    ibz

    Apr 14, 2005
    Columbus, OH
    What would be a benifit of having a trianguilar port vs. more normal port shapes?

    There might not be many calculators for these ports because its not really done that much...

    But I don't know a whole bunch about cabinet design, I'm sure someone might be able to shed more light on it.
     
  4. Actually triangular or nearly triangular ports are quite common. They are used because they fit nicely into the corners of a cabinet and allow a larger port area to fit into a smaller baffle plate without having to switch to a rear ported design. Many speaker parts outlets sell molded plastic port inserts in those shapes.

    Unfortunately I don't have the generic port area/length based tuning equations to offer. My wild guess is that unless the length/area ratio were something really extreme (something I would not expect to see in a fairly standard bass cab), the specific shape would be of little consequence - however I am not a speaker system designer - maybe someone with that background can enlighten us.

    Peace,
    S
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    5th grade geometry is all you need. Once you know the area of the round port (pi x R squared) you convert that to a triangle.
     
  6. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    For us DIY'ers, there is another useful trick -- cut the port a bit too long, measure its tuning frequency, and adjust to suit.
     
  7. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ

    ultimately that's the plan. Mostly I figured this would be an interesting topic for discussion.

    I checked The loudspeaker design cookbook, and Vance specifically leaves out all ports other than tubular.

    From what Bill posted above, it's more the x-sectional area and not the x-sectional shape that is relevant. Am i misinterpreting? or is there a difference, but it is slight enough to be Jiggered with when you fine tune? I thought there would be a difference since the port is located along 2 cab walls, as opposed to the round which is NOT contiguous with any cab walls.
     
  8. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    To the first approximation, cross sectional area and length are the relevant parameters. In terms of being located against walls, this is where "end effects" creep in, and unless you can find a published correction factor, it's best to assume that you will be doing some adjusting. You will get close with your calculation by treating the interior end of the port as "flanged."
     
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    You're overcomplicating things. The port resonance is determined by the volume of air it contains, area x length, so long as the shape isn't taken to extremes. Proximity to walls, or lack of it, is irrelevent.
     
  10. The slot port offers you the largest amount of port area that consumes the least amount of baffle board space.

    By definition, a slot port uses three cabinet side walls to enclose it's air volume. The fourth wall is the inner port wall. The correction factor for slot port total length is 1/2 the height, added to the port length.

    Doing the math with a 2" height and a 10" depth, the effective depth is 11" (half of 2" height + 10").

    I can fine tune a standard slot port by making the inner panel replaceable. That goes out the window when using an oddball shape. At that point, you are into succesive iterations of cut/install/measure until you find the tuning you want. No thanks.
     
  11. joelb79

    joelb79

    Mar 22, 2006
    Lansing, Michigan
    Basicaly, the same math you use to tune a round port is the same math you need for a triangle shaped port.

    If you have a 4" equallateral triangle as a port, it would be the same port area as a 2" square port. Use the same program to calculate that port, and there you go. Usualy, a 4x10 cabinet with 2 of those ports would be the same as using one 4" square port. For that volume of air, that would be enough port area to keep the air wizzing sound down to a minimum.

    Any questions?
     
  12. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    An even simpler way of saying this - calculate a square/rectangular port first. Then chop it in half to create the triangle frontage. Doing so halves the port area so to compensate, double the length of the port to get back to your original area.

    I'm with Bgavin, slot ports are my preference.
     
  13. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    To first approximation, you're right. The port is a mass on a spring. The mass is determined by the volume of air, and the spring constant by the port area and box volume. I think that the derivation at my web page lays this out correctly.

    But the air mass is not a perfect cylinder of air. The end correction factors are what determine the effective mass. Design software like WinISD gives two choices -- flush or free ends, but a port that is up against a wall would not be represented by either of those choices. How much it matters is debatable, especially assuming that a person is going to measure & adjust anyway. So I would agree with your assessment that I am overcomplicating things. ;)

    And I'm definitely with bgavin on planning the design for the port to be adjustable. You can always glue the panel in after you are happy with the tuning.
     
  14. A slot port cabinet is nowhere close to the work in building a bass horn, but I'll be damned if I'm going to do that work only to find out the permanently glued slot is tuned wrong.

    As pointed out above, once the ideal tuning is determined, the slot wall is permanently glued into place. When measuring at 1w/1m small signal, it is easy to use clay to seal the edges of the slot wall during testing.

    The other advantage to a slot port is it can be sized exactly to 0.045 MACH just be varying the height. Other than rear mounted ports, the slot port is the least interference with the other parts on the baffle.
     
  15. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If I was to use a slot port I'd place it midway on the baffle, so that it would stiffen the baffle as well as the side walls. You could screw it to battens inside the box, so that the depth of the slot could be easily altered.
     
  16. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I don't want to appear cavalier, but how much adjusting are we talking about here? My simulated port frequency has never been more than 2 or 3 Hz out either way, certainly not enough to warrant re-building the port or the box.
     
  17. joelb79

    joelb79

    Mar 22, 2006
    Lansing, Michigan
    That all depends on the size of the cabinet. Its a little more complicated than that.
     
  18. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Where'd you pull that number outa? Isn't it just a matter of keeping it below a certain value?
     
  19. tplyons

    tplyons

    Apr 6, 2003
    Madison, NJ
    Hey, who wants to build me a cab where the ports spell "TIM"?
     
  20. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    What I want is a port that's big enough to stow my cables, tuner, toll change, earplugs, aspirin, etc.

    While we're at it, how about adding those LED's like that one Fender combo amp.
     

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