OK I ran the tests, I got a negative VAS???HELP!!!

Discussion in 'Archived Threads 2001-2004' started by Geno, Sep 14, 2002.

  1. Geno

    Geno Supporting Actor

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    I ran all the tests [ for a 10" high excursion woofer] at the suggested:
    http://www.eatel.net/~amptech/elecdisc/spboxad3.htm
    Every other # looks about right but the last measurement was the Vas and it showed me a negative #. someone please help me.
    do I have to go thru all that testing again????
     
  2. What were the numbers you plugged into the program?
    Perhaps someone here can find a faulty number.
     
  3. Geno

    Geno Supporting Actor

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    here is the DATA OUTPUT:

    fs - 25 Hz
    Qts .374
    Qes .431
    Qms 2.833
    Vas -.057
     
  4. not the output, what is the input data that yielded those output numbers. Did I also read right and you have a (-) Fs as well?
     
  5. Geno

    Geno Supporting Actor

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    no, my bad, fs = 25 HZ

    from the top here is my input
    speaker resistance = 8 ohms
    resistor resistance = 8 ohms
    test voltage = 3 volts
    free air resonance = 26 Hz
    resistor voltage at resonance = .35 volts
    F high = 36 Hz
    F low = 17 Hz
    resonant freq in enclosure = 25 Hz
    internal volume = .75 FTcubed
     
  6. The free air resonance should be lower than the enclosure resonance. That is why you have a negative Vas.
     
  7. Geno

    Geno Supporting Actor

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    do the other #s look about right. Iv gotta be doing something wrong in my testing
     
  8. The only strange one is the Fs.

    Did you measure exactly an 8ohm resistor?
     
  9. Geno

    Geno Supporting Actor

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    Fs ?
    8 Ohm? yeah it was 8 Ohm, marked and tested
    Im gonna retest when I have time this week. Bummer, I really wanted to start designing the box this week.
    Ive been bugging guys at my shop to help me build it but Ive been such a slacker that theyre getting peeved. dang it [​IMG]
     
  10. Greg Monfort

    Greg Monfort Supporting Actor

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    According to the site you should be using a 0.05ft^3 sealed cab for a 10" Vs the 15x larger 0.75ft^3 cab you used. As already noted, the sealed cab Fs should be >Fs. Acoustically, your cab seems to be large enough that the trapped added air mass and baffle area are lowering the sealed Fs if your measurements are otherwise accurate, hence the negative Vas number.
    Also, sealing must be 100% or the results are worthless, so it's easier and probably a bit more accurate IMO to use the added mass method. Here's a nifty little program for calculating driver measurements, plus it has more supporting tech data than most DIYers will ever need, but then it was written by a Dynaudio engineer. [​IMG]
    http://home10.inet.tele.dk/cfuttrup/dpc049.exe
    GM
     
  11. Geno

    Geno Supporting Actor

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    thanks, I installed the program but cant find it on my computer, do you know default location?
     
  12. Greg Monfort

    Greg Monfort Supporting Actor

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    It 'defaults' to wherever you tell it to. If you don't choose, then it goes to wherever your last download went. [​IMG]
    Click on Start, then Find, then Files or Folders, type in: dpc049.exe in the Named field, then click Find Now. it should open a window extension and list the location/path.
    If for some reason you don't find it, download it again and either write down where it goes or do like I do and make a folder for it.
    GM
     
  13. Geno

    Geno Supporting Actor

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    thanks but i dont understand half the equations, im gonna retest with new enclosure and see if I get new results
     
  14. Greg Monfort

    Greg Monfort Supporting Actor

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    What does that matter? All you do is fill in the blanks with your measurements, just like at a website.

    GM
     
  15. Geno

    Geno Supporting Actor

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    uh yeah, right
     
  16. Greg Monfort

    Greg Monfort Supporting Actor

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    That's how it works for me and many others who use it. Measure.txt explains all. Oh well....

    FWIW, I punched in your measured specs for Fs, Re, Fh, and Fl, and either your Fs, and/or Re, Fl/Fh are off. This is one of the reasons I like this program, if there's an offset, it tells you.

    Anyway, Qe can be off a lot if any of these are, so measure carefully. If I accept your measurements and adjust Fs down to 24.7Hz to suit, then Qe rockets to 0.766, a 77.7% over the website calc.. Of course this will also affect the Vas calc..

    Good luck with it.

    GM
     
  17. Geno

    Geno Supporting Actor

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    so basically, all of my specs are off???
    like I said Ill just retest and rrepost.
    the program doesnt say what Ive done wrong so I cant think of anything else to do.

    maybe I could just send one of you this driver and you can test it for me :0
    sorry, im an impatient kind of guy. Ill get back to y'all
     
  18. Geno

    Geno Supporting Actor

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    greg, your program was very confusing. I kept getting division by zero and then it would crash. I dont see how it tells you what you are doing wrong either.
    sorry [​IMG]
     
  19. Greg Monfort

    Greg Monfort Supporting Actor

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    Well, you don't say where it does this, so I don't have a clue what you're talking about. I just inputted some values to calculate Qm, Qe, and nowhere did I get a division by zero. The error message shows up as "fres is skewed: x.x Herz". if x.x is anything other than 0.0, then one or more of the inputted data is incorrect. For instance, you measured Fs as 26Hz, Fl = 17Hz, Fh = 36Hz. Well, Fs = (Fl*Fh)^0.5, or 24.74Hz, so one or more of these specs is wrong.
    Once you have accurate data, only then can you move on to calculating the rest, just follow the instructions in MEASURE.TXT. If you want simple, or at least about as simple as it gets, buy a PE woofer tester. [​IMG]
    GM
     
  20. Geno

    Geno Supporting Actor

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    awwww i see ok makes more sense, I guess my meter just isnt that accurate [​IMG] i dont mean to be harsh, but ive been watching movies with no sub for so long, im kinda impatient
    like I said, Ill get new #'s up when its possible
    yikes $200 for that PE thing? im trying to put all the sub $ into construction
    guess ill have to learn to twiddle my thumbs for now
     

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