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  1. #1
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    HELP! Who's up for algebra?
    Who's up for algebra? I have a tool on one of my websites that I need to update. I've spent about six hours trying to reverse engineer a formula for my purposes and I keep going in circles.

    Is it possible to solve this for S_v?

    S_v((S_v(10x1.84^8)/V_b/((F_b/0.159)^2)) - 0.823(S_v)^0.5)
    + 0.823(S_v)^0.5
    = 1921*(V_d ^ 2 / V_b)

  2. #2
    Troll Killer and best Snooper!
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    Tell me what it does and I might take a stab at it.

  3. #3
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    At the risk of identifying one of my websites:

    S_v = surface area of port (Square inches)
    V_b = volume of enclosure (Cubic inches)
    F_b = tuning frequency (Hertz)
    V_d = displacement of woofer. (Cubic Inches)

    It's for determining the minimum port size of a subwoofer to eliminate (unwanted) noise. My calculator makes a few assumptions about port noise elimination, it's not 100% accurate in that regard.

    Everything within the first set of parentheses is the accepted port formula, though solved for L_v.

  4. #4
    Troll Killer and best Snooper!
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    Once upon a time I did algebra problems coz they relaxed me. Can't say I've done that in a while though. Rather than have me try it (and possibly mess it up, coz it is tricky what with all the nested formulas), why don't you post it over on Google Answers and pay somebody a couple of bucks who can get it right?

    And that advice is free!

  5. #5
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    Google answers...

    Tricky is right: I thought I had it last night after 20 pages of "showing my work", so I wrote a little code, and got a really big number. It made sense for a while: 4.5 cubic foot box, tuned to 23.75hz with a woofer that displaces 99.2 cubic inches, linear.

    Plugged it in on paper and found that I had forgotten that the second term on the left side of the equal sign also needed to be divided by Sd.

    That's when I went to bed.

  6. #6
    ABW Ambassador sjangro's Avatar
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    >> Once upon a time I did algebra problems coz they relaxed me.

    wow, that one would have won the prize in the "what do you do that's geeky?" thread.

  7. #7
    Troll Killer and best Snooper!
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    I'm only half geek Scott.

    The other half is nerd.

  8. #8
    ABW Ambassador sjangro's Avatar
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    Quote Originally Posted by Rhea
    I'm only half geek Scott.

    The other half is nerd.
    And what do you do for excitement, Diff EQs?


  9. #9
    Troll Killer and best Snooper!
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    I'm a biology nerd, not a math nerd. I play with snakes.

  10. #10
    Full Member David_NLC's Avatar
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    Hey weisinator,

    You want to solve for, not isolate, right?

    You could try Dr. Math:
    http://mathforum.org/library/drmath/..._equation.html

    If you know some Mac users check with them too, I had an algebraic equation solver - freeware - back in my B-School days...just can't remember the darn name now.

    Here's a link to a whole slew of math programs:
    http://www.educational-software-directory.net/math/
    Best,

    David Rogers
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  11. #11
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    Thanks for the Dr. Math link, a few of the simpler questions help make sense of this monstrosity.

    I think I'm going to try a different approach.

    My goal is to get everything but S_v, determine it and go from there.

    I think I should get a few sets of those numbers (all but S_v) and work with it from there. I think it will make more sense after I get rid of variables, replacing them with actual numbers, see how changing variables effects the whole piece.

    Everything will be easier from there.

  12. #12
    Affiliate Manager DavidVanHook's Avatar
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    Rhea, do you think antisense RNAi is the next big thing competing with stem cells research

  13. #13
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    I feel like an idiot. I found the formula via google. I have to convert from a round port (solves for minimum diameter) to a square port, but still.

    V_b is eliminated somewhere.

  14. #14
    Troll Killer and best Snooper!
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    David, I probably don't known any more about RNAi than you do. I've only recently heard about it too. It's very promising (and very cool to think about). Since stem cell research in the US is being repressed by special interest groups I'd have to say that RNAi may have the greater potential to solve some very nasty medical problems.

    Biology is an enormous subject and everyone (even at the undergrad level) specializes, albeit informally. My specialization is marine biology, more specifically I'm very interested in crustaceans, even more specifically the swimming crabs. If I'd been fortunate enough to pursue a graduate degree in behavioral ecology my hope was to study the differential morphology of swimming crabs and walking crabs and how that affects their behaviors, metabolisms, reproductive strategies, etc. So unless you're really, really interested in crab behavior I'm not even close to being an expert source of information.

    But if you ever need to know how to safely handle a North American, Atlantic crab, feel free to ask!

  15. #15
    More Cheesier Than Ever Cheesehead's Avatar
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    Algebra will be of no help here. You have an equation with squared, 3/2 power, and square root terms for the variable you are solving for. It's not quadratic in form nor is it even in a form where quadratic substitution can be used. Even something as simple as

    x^2 + x^(3/2) - x^0.5 + 5 = 0 could not be solved using any algebraic techniques. This "simple" equation could be solved numerically however by graphing
    y=x^2 + x^(3/2) - x^0.5 + 5 and zooming in on the x-intercepts. The equation you provide would require an exact symbolic solution in terms of the other variables so a numeric method could not be used.

    You said you found the solution? Could you post the link?
    This World is Not My Home
    We're gonna go inside, we're gonna go outside, inside and outside. . . And then we're gonna go go go and we're not gonna stop til we get across that goalline! Quotes from the movie Rudy, 1993

  16. #16
    Pimp Duck popdawg's Avatar
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    I haven't felt this stupid in a looooooooooooong time. :sad:
    ================================================================
    Been away, now I'm back. Not as much, but I'm back & starting from scratch. Where I was, was fantastic. Where I am now, less so. Things have changed, become harder. So have I. Game ON!!!
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  17. #17
    ABW Ambassador buy_online's Avatar
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    JBL Pro has bunches of stuff on boxes and Thiele parameters/calculations/tools as well. You might try them. I have files full of design stuff in the back, I'll try to find something that might be useful for you.

    Fred

  18. #18
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    Quote Originally Posted by Cheesehead
    You said you found the solution? Could you post the link?
    http://www.ajdesigner.com/phpvent/su...m_diameter.php

    It takes woofer diameter in centimeters, xmax in millimeters and tuning frequency. I reworked it to use inches. It's close with smaller woofers, though a bit exponential with bigger woofers.

    Quote Originally Posted by buy_online
    JBL Pro has bunches of stuff on boxes and Thiele parameters/calculations/tools as well. You might try them. I have files full of design stuff in the back, I'll try to find something that might be useful for you.
    You got PM.

  19. #19
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    I wrote down the equation wrong, screwing up one of the exponents, before wracking my brain.

    The solution ends up: S_d = (V_d^2 * (F_b/.159)^2 * 1921)^0.5

    That equation at AJ designer makes the port MUCH TOO LARGE.

    Thanks to everyone, though.

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