Subsonic filter ?

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Systems
Systems Posts: 14,873
edited December 2003 in Car Audio & Electronics
How far below the tuned frequency of a ported box do I want to set the subsonic filter to? The port will be tuned to 35 hz acccording to the box plans. I want to be safe cause I cant afford another sub. Thanks
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  • PoweredByDodge
    PoweredByDodge Posts: 4,185
    edited November 2003
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    dont hold me to this but i believe you set it "AT" the tuning frequency.
    The Artist formerly known as PoweredByDodge
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  • delvryboy
    delvryboy Posts: 62
    edited December 2003
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    Originally posted by miked909
    How far below the tuned frequency of a ported box do I want to set the subsonic filter to? The port will be tuned to 35 hz acccording to the box plans. I want to be safe cause I cant afford another sub. Thanks
    actually this sub/suspension/enclosure dependent....usually anywhere from the tuning frequency to 10hz below.....your best bet would be to use testtones at every freq below tuning to see where the sub starts to reach max excursion....set the ssf slightly above that

    i personally have my ssf set to 30hz and the tune on my box is at 34hz
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  • Jethro
    Jethro Posts: 28
    edited December 2003
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    Where you set your SSF will be somewhat dependant on how much power you are running, how large your enclosure is, and how much Xmax your driver has.

    If you are running enough power to push your sub close to Xmax, then set your SSF at 1/3 of an octave below tuning (@ 30 hz in this case).

    If you are only running like 200 watts to a sub with 16+ mm of Xmax, you can probably get by with setting it a little lower - just be careful.
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  • geolemon
    geolemon Posts: 67
    edited December 2003
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    You don't need to set it right at the tuning frequency. 10hz below is probably a better rule of thumb, but there are some influencing factors.
    None of it matters in the scope of "just be careful, and listen for your sub's limits, and set the subsonic based on those." :cool:



    Bear in mind, the port is simply a delay device, essentially. Delay in airflow.

    When a sub moves in and out, the signal generated off the front of the cone are 180 degrees out of phase with the signal off the rear of the cone.
    Picture in slow motion...
    The cone moves out, and compresses the air in front of the cone... and decompresses the air behind the cone.

    If it weren't for your enclosure, the pressure would bleed into the vacuum, and you'd get almost no sound. Hence the need for a box.

    A sealed box is like a garbage disposal.. it simply contains and destroys that rear wave energy, so it can't escape to cancel out the front wave energy... but it's all wasted in there.

    The port is a tube with a known air mass and resistance... so the pressure fluctuations inside the enclosure can travel through this port... but they are delayed by that static amount of time... so that by the time a pulse of compressed air travels out of it, say... it's just about in time for the cone to move out again and compress the air in front of the cone again.
    In reality, it's never going to be a perfect phase alignment, but your "tuning frequency" is the frequency where, if played, the cone and energy escaping out the port are going to be as close to "in phase" with each other as they can get, so they combine constructively rather than cancel out.

    However, as you travel away from the tuning frequency in either direction, they become less and less "in phase", eventually you'll get to the point where the sub again might as well not even be in an enclosure.

    At low frequencies, this is bad, because the sub literally behaves like it's not in a box at all... the port acting simply like a hole...
    Rather than nearly twice the pressure forces inside the box compared to a sealed box, you end up with literally none, if you go too far, and it's really easy to bottom out a sub essentially free-air like that.
    Hence the need (ah... "need" is such a strong word :p) for a subsonic filter.

    My 10W6's are going to be celebrating their 9th birthday in 2004, they've been in a very large 32hz ported environment for most of their life now, and I've never had the need for a subsonic filter.
    I'd be willing to bet 25hz will be fine for you, but you probably wouldn't miss much setting it to 30hz either, which would be even safer.
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  • PoweredByDodge
    PoweredByDodge Posts: 4,185
    edited December 2003
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    ... on a side note, welcome to our new forum member geolemon who apparently 'knows his ****' and is proving to be a valuable and helpful asset to this forum. :)
    The Artist formerly known as PoweredByDodge
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  • delvryboy
    delvryboy Posts: 62
    edited December 2003
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    Originally posted by PoweredByDodge
    ... on a side note, welcome to our new forum member geolemon who apparently 'knows his ****' and is proving to be a valuable and helpful asset to this forum. :)
    definitly props in that area....i have learned some very valuable things from the man...i have seen him turn some of the most useless threads into a excellant learning experience:)
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  • geolemon
    geolemon Posts: 67
    edited December 2003
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    Oh, thank you both... how complimentary-
    Your compensation checks will be mailed out shortly... :D
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