Help me finally finish my project!!!!! Need to calculate length of port!!

Discussion in 'Home Theater Projects' started by SteveSpoon, Dec 21, 2005.

  1. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Ok, FINALLY going to finish this thing up.

    12" Dayton DVC sub

    250 watt Partsexpress plate amp

    Partsexpress 1" thick MDF cabinet (yes, 1"!!). Internal measurements are 18"x18"x18"

    Partsexpress port. Port is 3" in diameter.

    Question: How long should this port be? In the directions, it asks for a value of "FB", or "tuning frequency". How do I know what to "tune it" to? Most of the use from the sub will be movies.

    Any suggestions? Thank you.

    Steve


    [​IMG]
     
  2. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Here's another pic:

    [​IMG]

    Steve
     
  3. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Also, when it says "port length", does it mean the entire distance of the port (which includes the flare), or just the length of the port before the flares?

    Or, for simple simons such as myself, should I just button the box up and leave it as a sealed box? [​IMG]

    Seems like an awful big box for sealed, though.

    steve
     
  4. SammyXP

    SammyXP Auditioning

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    Download a program like WinISD and experiment with various alignments. That 18x18x18 box is a little small for that driver, actually. But it's probably worth giving it a try since you've got it built. Running it sealed would give you an F3 around 43Hz and put you down -14.6dB at 20Hz.

    You'd theoretically be able to pick up more than 2dB at 20Hz with a 13.3" long 3" flared port (tuning to 20Hz), but would more than likely end up with a lot of port noise (WinISD beta shows a Vent mach of 0.50 with one 3" port). Two 3" ports would seem the obvious solution, but then you'd be talking about a 29" long port that wouldn't fit in your box.

    Leave it sealed with a few pounds of polyfill and enjoy it. If you want more extension, you'll have to give up some space and go with a bigger box.

    Nice snowmobiles!
     
  5. johnBee

    johnBee Auditioning

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    Hello Steve,

    On the flared ports, do not count the flares. Just the straight part of the ports.
    Do you have the WinIsd program downloaded? If not, google WinIsd, and download it. I believe that driver is in the database so all you will have to do is play around with different tunings and box sizes.

    Also after amp, driver and brace displacment you will probably be around 2.6f^3 net internal volume. IMHO the box is too small and the port will be too long. For that size cab at around a 25Hz. tune with a 3" flared port, the port will need to be around 28".[​IMG]
    You should really go sealed with that box. I have a Dayton DVC in a 2.6f^3 cab, and it is great. Excellent SQ but it is restricted in the low end, but mine is just used in a 2 channel system.
    Hope this helps
     
  6. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Too small of a box????? Are ya' kidding me, this thing is huge (and very heavy!!). [​IMG] Heck, my Jensen 12" sub box has a port.

    Maybe I will just button it up and see how it sounds. I am so far out of the loop when it comes to figuring this stuff out that I'm just beating my head against the wall anyway.

    And in all honesty, I kinda "cringe" when it comes to drilling a big hole in the cabinet for the port.

    Case closed............we're gonna bolt the amp and woofer in, seal the top, and see how she sounds.........$%$% the torpedoes, full speed ahead!! [​IMG]

    Thanks for the help.

    Steve
     
  7. SammyXP

    SammyXP Auditioning

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    I don't doubt that the box looks and feels like a monster! But with a Vas of 9.9 cu ft, your 3 cu ft box is sort of scratching the surface of what that driver could really do.

    Don't sweat it, though. You could double your output (+3dB) at 20Hz with a box that was twice as big, but if your space does not allow, then there's not point in entertaining the thought.

    Button it up and let us know how you like it. Don't forget the polyfill. With an undersized box, it becomes even more critical. Shoot for 48oz, if possible.
     
  8. ThomasW

    ThomasW Cinematographer

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    For flared ports use one of the box modeling programs, then a 1" to the length it says.

    And yes you include the flares in the that length.
     
  9. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Did somebody say sleds????!!!!!!! [​IMG]

    Steve

    [​IMG]
     
  10. Eric Hargrove

    Eric Hargrove Stunt Coordinator

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    Thomas,

    You obviously have more knowledge on the subject than I. Can you please tell me why you would add an additional 1" to the port length? Thanks.
     
  11. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Can I use for polyfill the stuff that comes in a bag from Walmart? White fluffy stuff?

    Thanks.

    Steve
     
  12. Robert_J

    Robert_J Lead Actor

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    It was determined about 4 years ago over at the PE board that the optimum sealed size for the 12" DVC is 1.2cf. Anything larger and it will bottom out easily.

    -Robert
     
  13. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Why would it bottom easily? Seems like it would bottom easier (quicker) in a ported enclosure since there is less "resistance" for the sub........it's not compressing air.

    Steve
     
  14. Robert_J

    Robert_J Lead Actor

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    It depends on the frequency of the sound being sent to the sub. From Fb and above, the port provides resistance to the driver. Send a tone that is below Fb and the port offers no resistance. The driver acts as if it is running free-air and is easily driven past xmax.

    -Robert
     
  15. Eric Hargrove

    Eric Hargrove Stunt Coordinator

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    Thanks Robert. I didn't realize this. It looks like I inadvertently tuned the last box I built too high. Can I put a flared port closer to the back of the enclosure than a straight port? I seem to remember the port should be at least one port width away from the back wall. I am hoping I can fudge this a little and lower the tuning. Thanks, Eric
     
  16. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Hmmmmmmmmm........I really don't know what to do now.

    Steve
     
  17. ThomasW

    ThomasW Cinematographer

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    A sealed box functions like a acoustical HP (high pass) circuit. This means it limits the outout below a specific frequency. The smaller the box, the higher the frequency.

    You can certainly run your driver in that box sealed. Just be careful not to overpower it. IOW don't turn the knob to "11" ......:wink:
     
  18. Catalin Capota

    Catalin Capota Auditioning

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    Is there no infor that comes with that sub? You can usually contact the manufacturer for a suggested enclosure design.

    The dimensions of the port and the enclosure will have a big effect on the subwoofers performance. If you can find any reliable info for the port dimensions I would say that you should just stick with a sealed enclosure. It is the most forgiving as far as flexibility with different subwoofers. Once you change the design to a ported or bandpass you really need to make sure that it's designed properly.

    Also when designing it you should count the length of the port in total, including the flared part. The port should also be placed on the same as side of the woofer to create something called 'accoustic coupling'. The subwoofer will sound much better/louder in this configuration.. and placing the enclosure up against a corner or upside down to fire into the floor will greatly improve it's output.

    Good luck.. looks like a great enclosure.
     
  19. SteveSpoon

    SteveSpoon Stunt Coordinator

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    Thanks again for the info. I think I am just gonna button it up and see how it sounds.

    Steve
     

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