Vent mach limits - need advice

Discussion in 'Home Theater Projects' started by Neil Joseph, Apr 10, 2004.

  1. Neil Joseph

    Neil Joseph Lead Actor

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    I want to design a box. The dilemma.....

    - I want to tune it as low as I can (preferably around 20Hz but not higher)
    - I don't want the box to take up half the home theatre (5 cubic ft is the maximum, no sonotube for this project)
    - I need the ports to fit in this box without elbows

    Now I messed around with various port diameters.

    Question, does pvc tube come in 2.5" diameters? Is a vent mach of 0.13 acceptable or is it too close to the limits? If this is the case then I need to go with 2 wider ports of 3" and yielding port lengths of around 16.5" as opposed to two smaller ports 2.5" wide and only 12" long. In other words...

    - 2x2.5" ports @ 12" long with vent mach = 0.13

    or

    - 2x3" ports @ 16.5" long with vent mach = 0.09

    I need to work out these numbers then figure out my optimum box length, width, and height for this 12" driver.
     
  2. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Call it a hunch, but I think a 2.5" port is way too small for a 12" driver.

    I built a 110L box for a 12" driver, tuned it to close to 20 Hz, and used a 4" PVC pipe for tuning. It measured around 18".

    Since it's tuned so low, I have rarely had issues with port noise (if at all), but every sim said I'd have serious turbulence issues.
     
  3. Neil Joseph

    Neil Joseph Lead Actor

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    That's why I would use two 2.5" ports and not just one.
     
  4. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Heh, sorry about that. I didn't notice you mentioned 2 2.5" PVC ducts. :b

    Anyway, based on my limited experience, the lower the tuning, the less important turbulence seems to be. 95% of the time, my sub ports are basically dead silent, with no air coming out of them.
     
  5. Brian Bunge

    Brian Bunge Producer

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    Neil,

    Keep in mind that those vent mach numbers are at peak power, which your sub will not be running at all the time. Also, many people have stated they do not feel that WinISD is all that accurate. Since you mention vent mach I assume this is the program you are using. I gave up on it a long time ago and have used Unibox almost exclusively for a couple of years now. If you have the latest version of Excel I'd suggest switching to it instead.

    Also, what driver and what amp are you using? This may help us to better assist you with your questions.
     
  6. Neil Joseph

    Neil Joseph Lead Actor

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    I dont have the amp yet but I will be shooting for 300-500 watts.

    Here are the numbers...

    qts=0.36
    vas=56L
    fs=26hz
    re=4ohm
    xmax=0.028m=2.8cm
    z=8ohm
    qms=8.77
    qes=0.37
    spl=93.9dB
    pe=900w
    bl=18
    dia
    0.305m=12"
    sd=0.048sqm=0.517sqft

    If you could plus in the numbers for me in a 4 or 5 cubic ft box (net volume) tuned to 20Hz and see how the port lengths work out for 3" and 2.5" port diameters I would be grateful and curious how it compares to my numbers. Also, does this program produce the mentioned port noise (vent mach)?
     
  7. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Neil, the specs look great. Can you tell us the brand of the driver? Xmax looks especially impressive. [​IMG]
     
  8. Neil Joseph

    Neil Joseph Lead Actor

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    The driver is a MaxCaliber 12150
     
  9. Neil Joseph

    Neil Joseph Lead Actor

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    OK, this is what I am going to do unless told otherwise...

    - 4 cubic ft net volume (4.92 cubibc ft gross approx)
    - dual 2.5" ports (3" outside diameter) at 14.5" long each
    - tuning at 20Hz

    I will brace the box and ports around the midpoint of the box. My estimate, hopefully is to get an in-room -3dB point around 17.5Hz (keeping my fingers crossed).
     
  10. Greg Monfort

    Greg Monfort Supporting Actor

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    When I use golden ratio dims plugged into MJK's worksheet I get a ~flat response to ~35Hz, then rolling off to a ~31Hz Fb if heavily stuffed. If I extend it into a long straight ML-TL, I can get a nice shelved response with a ~18Hz Fb, though it too needs to be heavily stuffed. Due to the long, well damped pipe, the vent can be a single 4" dia. x 15" long.

    Low Vas drivers are nice for small sealed or vented down to Fs, but pushing them lower isn't their forte. If it were me, I'd use some other driver/alignment better suited to HT.

    Let me know if you're interested in building the pipe. BTW, I didn't find this brand in a search, do you have a link?

    GM
     
  11. ThomasW

    ThomasW Cinematographer

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  12. Greg Monfort

    Greg Monfort Supporting Actor

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    Thanks TW! Not a very informative site, is it? Anyway, I see from the other thread that I'm not the only one who thinks this is a poor app. for this driver. Factor in that with no Le listed, it's probably safe to assume it's high, which as you know is great for car audio, but not for an LFE channel.

    GM
     
  13. Neil Joseph

    Neil Joseph Lead Actor

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    That image above is not my driver. Mine looks like this...

    [​IMG]

    I notice on the website they do not have this model listed which would be a vintage Caliber model CA12150. I do have the spec sheet at home which I will post on the site later tonight.
     
  14. Mike Keith

    Mike Keith Second Unit

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    I have found that anything over 18m/s port speed and you will hear port noise at Fb. Most still don't consider this a problem, as this will be a rare event, but IMO any port noise, even once is too much, it just sounds awful. If you are using WinISD change the Preferences to have the Vent Velocity read in m/s (Meters per second) most are use to this format when talking about Port Velocity.
     

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