Putting ports on the bottom questions

Discussion in 'Amps [BG]' started by V63, Mar 7, 2014.


  1. V63

    V63 Supporting Member

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    Sep 13, 2008
    I'm starting a do-over on a cab build but my question now, is:
    If I don't what a rear ported cab, nor see the ports, and I want the cabinet to be as shallow as possible, is putting the ports on the bottom the best way to go?
    Are there any problems or quirks associated with this type of porting?
    How high off the floor does it need to be so as not to restrict air flow?

    Thanks
     
  2. CL400Peavey

    CL400Peavey Supporting Member

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    It's a bad idea. The floor acts like a port extention, and the floor covering will effect your sound. Port on the front. Cover with grill cloth.
     
  3. V63

    V63 Supporting Member

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    What about putting it on a 45 degree angle? Does this work?
    I'd like to get it shallow without compromising port function.
    I could squeeze them into the corners to get artificially lengthen them right? Is there math for this to know how much depth I could save?
     
  4. Downunderwonder

    Downunderwonder

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    You're pushing stuff uphill with a stick. Slot port gives maximum port area for dimensional uptake. Use grill or cloth. The aesthetic in a cab with tall feet to allow any bottom port to function is looney.
     
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  6. V63

    V63 Supporting Member

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    If I need two 4" round ports 6.86" in length what size slot port would I need?
    The cab would be about 20"X26"
    Could I put the slot off-side to accommodate a grill frame?
    How do I adjust it to verify tuning?

    Thanks
     
  7. Johnny Crab

    Johnny Crab ACME,QSC,Fame/Hondo/Greco user & BOSE Abuser Gold Supporting Member

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  8. JimmyM

    JimmyM Supporting Member

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    Andy is of course right, but for a bottom port, giving it a little bit of room to breathe is necessary. Don't need much...maybe an inch...the real cab experts could fill you in on it. It would work, and I've played a cab with the port on bottom that worked quite well, but it does need breathing room.
     
  9. bgavin

    bgavin

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    No matter where the vent is located, it should have a minimum of one diameter distance between the vent and any obstruction.

    If using 4" ID PVC pipe, this means at least 4" off the floor.
     
  10. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

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    Downfiring ports are fine if done right.

    With a downfiring port, your cross-sectional area transitions from being the are of a circle (the round port) to being the surface area of a cylinder (the perimeter of the port times the height above the floor).

    What we want to avoid is a bottleneck that can cause turbulence.

    So, I suggest going with flared ports. The perimeter of that cylinder will now be at the outer diameter of the flare, and so will be significantly larger than the perimeter of the port itself. And, the flare smooths the airflow in and out of the port.

    One inch tall feet should be sufficient if you're on a hard surface, assuming a port diameter of no more than 4 inches. On shag carpet or grass, you'd probably want to put a board underneath the cab.

    The effective length of the port will be increased by the proximity of the floor, by how much I can't really say, but would guess from 1 to 2 inches for a 3" or 4" diameter port. If the downfiring port is near a cabinet edge, the increase in effective length won't be quite as much.

    I've done prototype and beta-test cabs with downfiring ports with no problems when following the above guidelines, and in a couple of weeks will probably build and test another prototype with downfiring ports. Unfortunately I didn't save enough data from my previous trials to give a clearer picture of the effect on the tuning frequency; I just remember expecting it to be a few Hz lower than the modeling program predicts, and it was.
     
  11. Downunderwonder

    Downunderwonder

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    Angling won't do squat.

    Compare winISD results with slot vs tubes and dig around for discussion on actual results vs winISD predictions.

    A slot can be started back flush with the baffle.
     
  12. bgavin

    bgavin

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    What is being overlooked is consideration for the oscillating air mass in the vent.

    This slug of air extends beyond the vent (each end).
    This is the basis for the one diameter distance guideline.
    The intent is to keep this air mass from colliding with the floor.

    The cross sectional calculation is entirely valid, to ensure the port does not exit into a confined space.
     
  13. JimmyM

    JimmyM Supporting Member

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    If the bottom port cab Duke built that I played is any indication, a flared port takes care of that. And I played it on a rug and a hard surface and it sounded killer on both ;)
     
  14. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

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    Thank you sir!

    That cab has gone through at least four generations of major revision since you tested it, and I'm now back to almost the same driver complement you had but in a more compact package.
     
  15. JimmyM

    JimmyM Supporting Member

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    Man, you need to let that thing run free in the wild, Duke!
     
  16. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

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    I hope to bring it to the Seattle GTG at the end of the month, and it may have a little brother tagging along. Seems to me the format is better suited for small gigs than for big ones.
     
  17. V63

    V63 Supporting Member

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    I have a Mac so winISD is a problem :(
    The slot port idea may be the way to go. I'll have to look for a calculator online to figure it out. Has anyone worked this out already for Eminence's mid-sized 3015 cab?
    http://www.eminence.com/pdf/Kappalite_3015_cab.pdf
     
  18. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

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    If there's sufficient cross-sectional area and geometrical smoothness that turbulence doesn't set in, the only significant result is an extension of the effective port length and therefore a lowering of the tuning frequency. The "slug of air" is fluid enough that it doesn't mind changing direction or shape; any resulting loss of energy is negligible.

    Adhering to the one diameter spacing guideline keeps the port tuning frequency much more predictable.

    Consider a slot port as a squashed round port: If the effective "diameter" is the average of its width and height, there seems to be no ill effects from ignoring the one "diameter" rule of thumb regarding distance to the internal walls of the enclosure. And the net result of the proximity to these walls is what we might expect - a lowering of the effective tuning frequency (though this isn't the only mechanism at play there).
     
  19. V63

    V63 Supporting Member

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    Found these numbers for a slot port:
    2.5"X 17.08" 5.04 long -would it work?
     
  20. Raf Seibert

    Raf Seibert

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    Clearly, those math classes in school didn't really resonate with you, did they?
     
  21. fdeck

    fdeck Supporting Member

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    My latest speaker has a side port, enabling it to be as long as possible within the dimensions of the box.
     

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