Changing Speaker boxes

Discussion in 'Home Theater Projects' started by Williams16, Sep 16, 2006.

  1. Williams16

    Williams16 Extra

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    Well, I have two boxes and two speakers, what I want to do is to take out the speakers and put them in a dual box with the exact volume of the first two boxes.

    But I have a problem each box have two vent exits so I have four exits, when I change all I don´t know if I need to put the four exits exactly like the originals in the dual box.

    I need help, also I´m learning DIY. I hope I will have good advises from you.

    Thanksss a lot ...
     
  2. Dave Poehlman

    Dave Poehlman Producer

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    Are you looking to keep the same number of ports? If you're looking to reduce the number of ports (which will affect port length), it'll probably require some math.

    As far as I know, there's no real restriction on where to place your ports.. or in what orientation. I recall reading someplace that you should avoid putting them within their own diameter from a driver, box edge, or another port (something to do with proper air flow).

    Personally, I prefer porting out the back. My idea is, it'll diffuse any sound coming out of the port since, ideally, a port is strictly there to regulate airflow.. not provide sound.
     
  3. Brent_S

    Brent_S Second Unit

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    Box tuning is a function of the box volume, desired tuning frequency, port diameter, port length, and number of ports...the math Dave mention lets you solve for X when you know the other constants. Any of the modelling programs available around the 'net will do the vent math for you...my personal favorite is WinISD Pro from www.linearteam.org. You can also use the quick&dirty port calculator on Precision Ports web site... http://www.psp-inc.com/psp-inc.com/c...calculator.cgi

    More advanced theory gets into air velocity and port resonsances. Too much air velocity (under sized port diameter) and the port makes chuffing noises. Too long and the port may resonate like a coke bottle at a frequency that's within the passband of the given speaker.

    What are the particulars of the speakers (subs?) you're trying to combine?

    -Brent
     
  4. chuckg

    chuckg Supporting Actor

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    Not quite. The port produce a great deal of sound! As the response of the speaker falls off at low frequencies, more of the sound output is produced by the port. At higher frequencies, the port does little and the speaker produces the sound level.

    But, there is no penalty for having the port on the back. The only reason I dislike it is because too many speakers end up close to walls or other things that block the sound from the port.
     
  5. Dave Poehlman

    Dave Poehlman Producer

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    But isn't any sound coming out of the port out-of-phase with the sound coming from the driver since it's coming from the back of the driver?
     
  6. Brent_S

    Brent_S Second Unit

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    When the port is producing sound, the driver is producing virtually nothing and vice versa, so there's not a phase problem. To get the overal frequency response and output of a ported enclosure, you sum the output from the driver and port separately, if measured nearfield. There's probably a more scientific way to say this.

    At the frequency most ports are tuned to, their location relative to the driver is irrelevant...unless you've got a REALLY BIG enclosure. I think the LDC recommends staying within 1/4 wavelength...even an 80hz tune gives you 3 feet to play with.

    As chuckg says, the only real problem with rear port placement is with wall reinforcement. However, that's usually only a problem for main speakers where the port frequencies will throw off the overall spectral balance of the speaker...easily adjusted by pulling the speaker away from the wall a bit.
     

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