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Discussion in 'Home Cinema Building DIY' started by stegalv, Jan 15, 2004.

  1. stegalv

    stegalv
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    can anyone explain in everyday terms e.g a box with an f3 of 37hz does it mean the bass out of the sub won't go lower than 37hz, to make the f3 lower is it the driver specs,volume of box,lower or higher qtc,
    also the -3db figure always being qouted what is the significance of it, are the frequencies below 87db not really useable, i have been looking on numerous sites which keep qouting these sorts of figures but don't explain the signifcance in laymen terms

    ps gary g sent your regards to julie
     
  2. GaryG

    GaryG
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    The f3 point is the frequency at which the volume (loudness) has dropped by 3db, in other words half of the maximum volume.

    Lowering the f3 frequency depends on the type of box. If it's a sealed box you can increase it's size until the box has no significant effect on the driver at which point the driver parameter s govern the f3 point (this is usually the case with an infinite baffle).

    If it's a ported box then you can change the box volume and the tuning of the port to lower the f3 frequency (up to a point).

    You should have little difficulty in getting an f3 of around 20hz with most of the drivers discussed in your previous thread if constructing a ported box.
     
  3. stegalv

    stegalv
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    the techs at parts express have told me the drivers we discussed would work best in a sealed box of 3cuft but winisd and unibox say a larger box volume(around 4cuft),regarding above if a sealed box has a f3 37hz does that mean lower frequencies wont be heard
     
  4. GaryG

    GaryG
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    3ft^3 looks good to me for a Q of .707 (unibox no leaks no fill). If you can accomodate a larger box then you will gain in efficiency and low-end response. For example, a 6ft^3 will increase your output from 99.5db @20hz to 101.7db, that's nearly the equivalent of doubling the amplifier power, or using a second driver.

    You will hear the frequecies below f3, they're all just points on a continuum. A sealed box rolls off at 12db per octave, so as the frequency drops the output drops, just like turning the volume down. Bear in mind that you will have room gain at the low frequencies so thing aren't too bad, it you stick it in a corner you will get a boost of around 9db at the low end, combined with any boost on the amp and you should be fine.

    That MkIII driver models very nicely, much better than the Tempest. If it had been available before I ordered the Tumults I would have bought that instead. In fact I would have bought four of them, actually I'm still thinking about it. The Tumults are MASSIVE and weigh a ton, not sure if I can do what I want to do with them because of the weight.
     
  5. stegalv

    stegalv
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    hi gary,4 tumults millionaire or what,decision time i am so far up my own backside don't know when im crapping or talking ,i am braindead from formulas and figures and dozens of forums an sites, sealed is best 3ltr 4ltr 6ltr, no ported is king, go for passive,my room is small,medium,large,aaarrrrggghhhh,
    right bottom line,just been told house needs re-roofing, but come to far to stop, so need a sub i am happy with for lowest cost,i am not a audiophile just want a sub that is not boomy but when the monster bangs his foot down i know about it, so would your last suggestion of the dvc 15" and 250 watt amp in a 6 cu ft box do it, bk are doing the 200w amp with a 10" driver and passive or are these not large enough/to hard to build, lastly at a push cpc are doing the same 250w amp as parts express for 89 quid and get two 15"dvc/two 12" titanic mk11,phew what do you think of the options.
     
  6. GaryG

    GaryG
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    I meant 4 Titanics, in preferance to the Tumults. Now I've got them they're proving difficult to work with because of the size and weight.


    Right to your situation.

    1. Stop reading now, you'll drive yourself mad.

    2. Go for the 'biggest bang for the buck' option. A parts express DVC 15 and 250w plate amp. The DVC is more sensitive than the Titanic MKIII so you will get more output from the 250w amp, which is well below the power handling of either of them, another reason to choose the DVC over the Titanic.

    3. Go for a ported cabinet to get the most out of your 250w. I don't have access to the modelling software at the moment but the DVC will need a slightly bigger cabinet than the Titanic but should still be relatively small (<8ft^3).

    4. Bear in mind that the T/S parameters will not be an exact match for the driver you will get so you may need to adjust the port tuning after you've built the sub if you've got a hump in the response.

    5. Get on and do it! The worst that could happen is that your port tune may be slightly out and give you too much low end boost!!! I'm sure you wont complain at that.
     
  7. stegalv

    stegalv
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    thanks gary going to order those parts, do these look right,
    winisd pro/unibox 8.1 ft^3 , 1 port, 4" inside diameter , 6" long,f3 22.42 hz,but unibox recommends sealed box does it matter which box type, are these figures theorertical and need tweaking if so to what ,and how much volume does the driver and amp take up, is there a problem with the driver reaching it's max travel.should the port be flared at the face of the box, that's praobably all i need to know to start building (till i think of something else). if it's not to much trouble could you answer above questions
    by the way is this a hobby of yours or work, you seem well clued up.
     
  8. GaryG

    GaryG
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    8.1 ft^3 looks good, check with Parts Express for the exact volume of driver and amp, 0.5 ft^3 is probably reasonable.

    Forget the sealed box, unibox calculates for all it's box types.

    Flared ports are best, they reduce the air turbulance through the port. Have a look at the ports here:
    http://www.partsexpress.com/pdf/catalog04/151pec03.pdf?
    For maximum flexibility you might want to go for a pair of 260-389's and adjust the ports between 19hz (13.75") and 24.5hz (7.5"). Two ports give you plenty of headroom to stop 'chuffing' at high volume, the adjustable ports allow you to tailor to your room response, the 19hz will give you a flatter response when you factor in 'room-gain', the 24hz tune will give you a boost of around 3db (double the volume) at 30hz.

    Peak driver excursion is around 13mm at 30hz at 250w, which gives you a couple of mm of leeway on xmax.

    Yes, it's only a hobby (although my wife would say obsession). Now, get cracking and you'll find out what the SVS crowd are shouting about!
     
  9. Exeter Man

    Exeter Man
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    Sounds like it's going to be a cracker :smashin:

    Really good performance for the sort of money you're talking about.

    Regards
     

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