How low to go?

Discussion in 'Archived Threads 2001-2004' started by PaulDF, Oct 2, 2002.

  1. PaulDF

    PaulDF Second Unit

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    I am playing with the numbers to build a 250-300l tempest. Tuned to 19.5 hz. My question is, how low do I need to go? About the best I can come up with is 110db at 20 hz. Is 20 hz too low? This is for both music and HT. I am assuming that lower is better and with a nice flat plot line. What about 33 ms delay? Is that a problem? Thanks to you all!
     
  2. Jeremy Stockwell

    Jeremy Stockwell Supporting Actor

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    Paul,

    Well, it depends . . .

    Everything's a sacrifice. Tuning to 19.5Hz will give you some extra SPL's probably in the 25-35 Hz range, but you risk bottoming the driver sooner with the growing number of DVD's that have loud levels of sub 20Hz content. Be sure to look at the "Cone Excursion" graph here. Below your tuning point, the driver becomes unprotected by the air in the enclosure and will reach xmax quickly. Tuning a couple of Hz lower (16-17Hz) will give a little more protection to the driver, but you will lose some SPL's up higher.

    I'm building a 280-290L Tempest Sonotube now and have yet to finalize my tuning point, but I'm leaning towards 17Hz.

    Yes, you can tune an enclosure too low, but you sound like you're in a reasonable range.

    You will find differing views here about Group Delay. You might want to do a search. What little I understand is that the lower the frequency, the less effect GD will have. You probably don't want a 33ms at 40Hz, but the same delay at 15Hz wouldn't be as much of a concern.

    JKS
     
  3. Michael R Price

    Michael R Price Screenwriter

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    Paul, you will get a peak at the tuning frequency if you use such a large box and high tuning. It's great that you're aiming for a large size (stick with it), but you may want to tune it in the 16-18Hz range to avoid boominess, protect the driver from unloading under tuning, and to extend response into the 16 Hz range. You will still have plenty of output in the 20-30Hz area.
     
  4. PaulDF

    PaulDF Second Unit

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    Okay, so 300l and 16hz. Do you all agree this is the most appropriate tuning? I definitely don't want boominess, I don't want the bass ALWAYS there, just when it's supposed to be, and LOUD! I also want protection against unloading. I think this is the setup for me! My tempest will be here any day now...
     
  5. PaulDF

    PaulDF Second Unit

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    By the way, thanks Jeremy and Michael!
     
  6. 300L @ 16hz looks pretty damn good to me!
     
  7. Allen Ross

    Allen Ross Supporting Actor

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    what about port placement, I am trying to get the smallest foor print from the tempest I can, That maens moving the ports to the top or even on the side, should I avoid this and just go with the 24" square with dual 4" like on the Adire specs?
     
  8. Bill Fagal

    Bill Fagal Stunt Coordinator

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    Allen,
    You might consider slot porting, like the Basszilla. (I know, it's not really a subwoofer, but it's a good example pic.)
    I understand that loading the port into the groundplane pressure zone like this can have some efficiency and FR benefits, though you should probably test the slot-port's real-world tuning, since it may be lower than your model port dimensions predict.
    Bill
     

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