Can someone please explain port tuning to me?

Discussion in 'Home Theater Projects' started by Anthony_I, Oct 1, 2003.

  1. Anthony_I

    Anthony_I Stunt Coordinator

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    What really is the point of it? to have a constant added frequency when your speaker is working?

    And how do you know what frequency you want it tuned to.

    If the manufacturer suggest a port length and you figure out what its tuned to, could you use 2 or more ports that were all tuned to equal the suggested frequency? what problems could come of not tuning it/using the port length suggested by the company?

    Also if i use a ported box and use the sizes suggested by the company bassbox shows my excursion far exceeding its limits.
    Thank you [​IMG]
     
  2. Neil Joseph

    Neil Joseph Lead Actor

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    The tuning of the enclosure is important in that the enclosure takes over from the driver at the tuning frequency basically protecting the driver from bottoming out.

    Moving to DIT/Advanced area where my friends there will correct and add to my mini theory.
     
  3. Neil Joseph

    Neil Joseph Lead Actor

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    Read more about this in the FAQ area... here
     
  4. Anthony_I

    Anthony_I Stunt Coordinator

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    If i understand it properly, this means that if my box is tuned to 20HZ, then there is a constant 20Hz freq added to the bass? adding more bass to the overall frequency?
    OR, that when a 20Hz freq is played it is "enhanced" because thats when the port starts to resonate and emit a 20Hz sound? essentially adding a boost to that frequency?


    and how do i know how i should tune it?
    should i use something that is close to what the manufacturer uses (there is no specific freq used, but there 2 different designs have different tuning freqs...should i try and match this with mine if i port it?)
    theirs is tuned to 34.34 Hz
    I can achieve about 35Hz using 2 ports in my box.

    are there advantages to using more than one port?
    should the port face the listening direction? or away? perhaps at the ceiling or floor?
     
  5. Brian Bunge

    Brian Bunge Producer

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    Assuming you're referring to your PG sub, keep in mind ALL of their recommendations are for car audio use. For in home use you can throw those recommendations out the window.

    Yes, there are advantages to using more than one port. There are also disadvantages. But I don't have time to go into either right now. [​IMG]
     
  6. Anthony_I

    Anthony_I Stunt Coordinator

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    ok that link is great...but i dont understand al ot of what is said [​IMG]
     
  7. Dave Poehlman

    Dave Poehlman Producer

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    I've always based it on a driver's T/S parameters. You want to tune a box to give you a maximally flat response. The swinging speaker diagrams in that article are great at describing it:

     
  8. Neil Joseph

    Neil Joseph Lead Actor

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  9. Brian Bunge

    Brian Bunge Producer

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    Keep in mind that tuning is independent of the T/S parameters of the driver. Tuning is a function of the box volume, port diameter and port length (as well as number of ports). But you do need to tune the enclosure based on the driver you are using.
     
  10. Anthony_I

    Anthony_I Stunt Coordinator

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    so how would i figure out where i want to tune it?
    what if i tune it really low.... like lower than any frequency thats going to be put out.... like 5Hz (for example), this would keep the driver from moving too far since it will never go that low.?? but then the vent wouldnt make a difference would it?
     
  11. Neil Joseph

    Neil Joseph Lead Actor

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    If you tune it too low then you sacrifice output at the usable frequencies, in other words, the output starts to dip at a high frequency rather than the proper 3dB cutoff point. What you should get is a sharp rolloff (cliff) where the output drops off steeply at round the upper teens or the lower 20's (Hz) but if you tune it too low for the driver, you will see a gradual dropoff from a much higher frequency like 30-40 Hz instead.
     
  12. Anthony_I

    Anthony_I Stunt Coordinator

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    this is just theroyetically (bad spelling huh) speaking but if i use a 1" port that is 6" long, then it tunes the box to 14.66Hz, this makes my F3 about 21Hz (though bassbox lists the F3 as 16.73Hz)

    is there any type of "general" tuning frequency, or a way to figure out what my optimal tune would be?
     
  13. Patrick Sun

    Patrick Sun Moderator
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    But the trade-off for using that 1" wide port is the port noise (it'd probably whistle and wheeze) that would be present a moderate to high volume levels.

    Optimal tuning has to do with setting goals for what kinds of frequency response and output you want from the subwoofer.
     
  14. Neil Joseph

    Neil Joseph Lead Actor

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    What program are you using to calculate the enclosure specs? 1" sounds like a very small (ie. narrow) port especially singly ported but without knowing anything else about the driver specs, one cannot say for sure. You do want to avoid having too small a port to prevent port noise.

    How about providing all of the driver specs for us..... for example Qts, vas, Fs, Re, Qms, Qes or supply the driver make and model.
     
  15. John_MackieBass

    John_MackieBass Stunt Coordinator

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    Hmm, at the tuning frequency the cone should move very little, even if the frequency being played is 15hz. At and under the tuning the frequency the box "unloads", the port begins to REALLY blow A LOT of air, and the driver will "bottom out" given enough power.

    The advantage of ported boxes is a theoretical +3db gain in output around the tuning freqency due to the rear wave of the driver being allowed to interact with the front wave, higher power handling at and above the tuning frequency, and deeper bass extension than a sealed box.

    Disadvantages include somewhat less sound quality because the driver does not have a "spring" of air behind it like in a sealed box to make the cone tighter. Under the tuning frequency the cone will act like it is in free air and will bottom out.

    Port size selection is completely dependent on the driver and the amount of power you are going to be using. If you are going to build a box for a 6.5" driver with a 8mm XMAX getting 50rms, the port area (length*width or Pi*Diameter) can be MUCH MUCH MUCH smaller than if you were using my 18" RE XXX with a 32mm XMAX getting 1400rms here: http://www.geocities.com/etude316/au.../DIYsubs08.htm

    Notice my port area is MASSIVE (5"x14")!!!!!! At the tuning freqency and below...the port literally feels like a box fan. I can feel the port air 10ft away!!! I should have gone with a larger port but I ran out of depth in the box!!!

    [​IMG]

    In car audio applications I don't think its as important, but in home theater/audio I think ported enclosures are almost always necessary!! Granted you can get "ok" output from driver with a low FS or a large diameter. An 8" in a sealed box would have a VERY hard time competing to the same 8" in a well tuned ported enclosure in Home Theater applications.

    PORTED = [​IMG] [​IMG]
     
  16. Anthony_I

    Anthony_I Stunt Coordinator

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    well like i said im not going to be using a 1" port, that was simply for sake of argument.

    but here are the T/S params for my driver..

    FS 23.640 Hz
    RE 7.24 Ohms
    QMS 8.190
    QES 0.499
    QTS 0.470
    VAS 117.7087 L
    Mms 135.9013 Grams
    CMS 318 uM/Newton
    BL 8.6628 Tesla-M
    SPL 91.74 dB
    Sd 511 Sq. Cm.
    Xmax 25mm Point-to-Point

    hope that helps.... i think i may want to add a port if i can get deeper bass extension and a better sound so to speak.

    It is currently running at 114W output of my stereo, but i will be adding a plate amp in the near future that will bring it up to 245W... my speaker is 250W DVC. (wired in series for 8 Ohms)

    I only have 16.5" at the most to deal with as well (that is internal measurement from top to bottom) and about 3.5" in dia. to work with. and that is from top to bottom since im using a truncated edge prisim.

    the specs state i should use a single port that is 4" wide by 12" long, however that is for a box that is 0.9 or 1.2 Ft^3. my box is 3.0 Ft^3.

    also i would like to use dual ports for symmetry.
    (almost triangle shape but not quite) and bassbox 6 pro is my software of choice.
     
  17. Brian Bunge

    Brian Bunge Producer

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    Since you've got BassBox then you should simulate a ported enclosure for your driver that has 3ft^3 of internal volume. Play with the tuning frequency until you have a response curve that you like (I'd tune no lower than the driver's Fs) and check to see what type of port lengths you get with both 3" and 4" diameter ports. Also, keep in mind that dual ports will be longer than a single port so that has to be taken into consideration as well.
     
  18. Anthony_I

    Anthony_I Stunt Coordinator

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    now i realize that a flat response is best, but how do i know what kind of response curve i might "like"???
    this is my whole problem in this matter.

    what happens if tuned belw the FS?
    What exactly is the FS (in terms i can understand) and what happens when a speaker plays below its FS?

    ----EDIT----

    What i have now is 2 ports, 12" long and 2" wide. this tunes my box to 20.89Hz, slightly below my FS, but bassbox says that the vent air velocity will not reach 10% of mach... therefore no vent noises. and my driver will begin to exceed its Xmax (12.5mm) at around 15Hz.
     
  19. Jack Gilvey

    Jack Gilvey Producer

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