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Thinking about building a box, need some help.

Discussion in 'Car Audio / Security / Electronic Accessories' started by trappd-in-859, Oct 20, 2007.

  1. trappd-in-859

    trappd-in-859 New Member

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    I know this has probably been discussed here before; but I need some help with

    building a subwoofer box. Currently I have a JL 13w1v2 subwoofer in a store-

    bought sealed box. The bass it puts out is pretty good for a low power sub, but

    I'm looking to be able to hit some lower frequencies.

    I know the basics from doing some research on the net. Using 5/8-1'' MDF,

    bracing, how to seal the box etc., but I having trouble understanding how to

    actually port the box and to how large the box, and port would have to be. Below are the specs of my subwoofer:

    Free Air Resonance (Fs): 20.9
    Electrical “Q” (Qes): 0.4060
    Mechanical “Q” (Qms): 6.9366
    Total Speaker “Q” (Qts): 0.3840
    Equivalent Compliance (Vas): 6.131 cu. ft. / 173.51 liters
    One-Way, Linear Excursion (Xmax)*: 0.475 in. / 12.1 mm
    Reference Efficiency (no): 0.375%
    Efficiency (1W/1m)**: 87.71 dB SPL
    Effective Piston Area (Sd): 105.774 sq. in. / 0.0682 sq. m.
    DC Resistance (Re): 4.114 ohm
    Nominal Impedance (Znom): 4 ohm
    Thermal Power Handling (Pt): 200W
    Driver Displacement: 0.05 cu. ft. / 1.42 liters
    Net Weight: 11.21 lbs. / 5.08 kg

    Here is also a link to the JL website if you would need more info.: JL Audio Products: Subwoofers / 13W1v2

    The car this is going into is a 2001 Chevrolet Monte Carlo if that gives you any idea of the trunk space I have. TIA for all replies.
     
  2. TommyTheCat

    TommyTheCat Gonzo Scientist

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    Why not follow the recommended box layout: JL Audio: Products

    PS: just use 3/4" MDF, probably wont need bracing for that size of box.
     
  3. trappd-in-859

    trappd-in-859 New Member

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    Thank you for that link tommy. I was trying to find something like that on their site but I guess I wasn't looking in the right place.

    The site is saying that you have to tune it to 26hz. Does that mean that I have to adjust something on my amp, or is that just how low of a freq. my sub will be able to make audible?
     
  4. TommyTheCat

    TommyTheCat Gonzo Scientist

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    The reason you make a ported box is so you create an resonator. Just like a pipe, or flute, or some other resonator - you must adjust the volume of the tube to "tune" it to a certain frequency.

    Do you remember high school physics dealing with a mass-spring system and how it forms a resonant structure? Well, in our case, the spring is the volume of air inside the box and the mass is the mass of air in the port. Once we create this resonator we use the sound pressure it creates and add that to what our speaker provides in order to increase efficiency in some band/range of frequencies.

    Below these freq's (near tuning freq) you are going to get a lot of destructive interference, a loss in output, and a shit-load of excursion which can damage your subwoofer. So make sure your amp has a "subsonic filter" or "high-pass filter" that you can set to slightly below your tuning freq. This will limit the amount of signal your amp will try to provide below the freq you set it at.

    Bottom line: You tune your box. But you must set a filter below or close to where you tune to protect the sub.
     
  5. neonmike

    neonmike Senior Member VIP

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    :ditto:

    i am not sayin that it is hard to build a box but if it is your first time around ported boxes are not the easiest to do. if you have any sort of reputable shop around you i would suggest looking into having them do it, probably wont be more than 100 bucks or so that what i usually charge.
     
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