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Thread: String pressure on soprano top

  1. #1
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    Default String pressure on soprano top

    I want to build a jig to experiment with sound board thickness.
    Does anyone know how much pressure in pounds is applied to the sound board by the string pressure on a soprano uke?

    thanks
    rick

  2. #2
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    Check out David Hurd's website. It's on there
    http://www.ukuleles.com/HouseKeeping/sitemap.html
    My Real name is Terry Harris

  3. #3
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    And dont stop reading David Hurd's website at the string force charts.
    Liam Ryan.
    Cairns, Australia.
    Stump Jump Ukulele Co.
    Stump Jump on Facebook

  4. #4
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    If you're trying to find the "perfect" thickness you might be out of luck. Even in my very limited experience wood is very variable - I've found that adjacent slices from the same piece of timber can flex very differently from each other at the same thickness, enough to be noticeable by finger pressure.

    I'm now working on the theory that I work the top until it "feel" right when I flex it, and different parts of the same top require more or less work to achieve an even feel. I believe that some builders use deflection jigs to check the board at different point, which I suppose to be the precision version of what I'm attempting by feel, but they also write that each top is different.

  5. #5
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    Quote Originally Posted by ProfChris View Post
    If you're trying to find the "perfect" thickness you might be out of luck. Even in my very limited experience wood is very variable - I've found that adjacent slices from the same piece of timber can flex very differently from each other at the same thickness, enough to be noticeable by finger pressure.

    I'm now working on the theory that I work the top until it "feel" right when I flex it, and different parts of the same top require more or less work to achieve an even feel. I believe that some builders use deflection jigs to check the board at different point, which I suppose to be the precision version of what I'm attempting by feel, but they also write that each top is different.
    I'm not sure you'd want and "even feel" across the entire sound board. Some areas need to be stiffer, others need to flex more.
    Chuck Moore
    Moore Bettah Ukuleles
    http://www.moorebettahukes.com

  6. #6
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    It's tension, not pressure. And there's a very strong torque element unless you're using a tailpiece.

    Read Roger Siminoff on deflection testing of acoustic guitar tops. He designed an amazingly clever jig to measure deflection under real conditions simulating the torque and tension on bridges. He also found a very string correlation between measured deflection under load and tap tuning to pitches around the top both with mandolins where the deflection measurements do not involve torque and flat top guitars where torque is very important. He'd measure deflection with mando tops in two places, but on flat tops in four.

    And yes, if you're going this deep into it, you have to graduate tops and carve braces to suit the wood, not a set of plans. Traditional luthiers do the measurements with fingertips and ears. Some "scientific" luthiers set up jigs and work to Chladni patters or tap tones or deflection measurements. Both paths can lead to great instruments.

  7. #7
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    Quote Originally Posted by Moore Bettah Ukuleles View Post
    I'm not sure you'd want and "even feel" across the entire sound board. Some areas need to be stiffer, others need to flex more.
    Understood, and maybe when I've built a few more tops ... My current challenge is to get all parts of the top to flex at least enough!

    The point I was trying to get across is that at an even thickness some parts of the top will be stiffer than others, and that this will vary between pieces of wood. So just measuring the thickness doesn't give you a useful answer. Measuring the deflection at a set of points seems like a good solution, though rather too sophisticated for a hobby builder like me.

  8. #8
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    Thank you all for your replies. Though I am still at a loss as what to do. My idea, which I did not explain fully, was to be able to gradually thin the top down, testing it on this jig for the start of deflection and then brace it just enough so it would not deform under normal string tension. Perhaps not a well thought out plan. But I have one soprano I built with a very thin top and it has a wonderfully big sound but the top looks like a great design for a roller coaster.
    If there are any further thoughts on this issue I would be glad to hear them.

  9. #9
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    Quote Originally Posted by rickpauly View Post
    Thank you all for your replies. Though I am still at a loss as what to do. My idea, which I did not explain fully, was to be able to gradually thin the top down, testing it on this jig for the start of deflection and then brace it just enough so it would not deform under normal string tension. Perhaps not a well thought out plan. But I have one soprano I built with a very thin top and it has a wonderfully big sound but the top looks like a great design for a roller coaster.
    If there are any further thoughts on this issue I would be glad to hear them.
    Fan bracing? Normally OTT for sopranos, but with a super-thin top it might help.

    My guess is that the top has to deform a tiny bit, otherwise it would be too stiff to resonate properly. But I really don't have enough experience to say for sure.

    If I wanted to try the experiment I might make a jig out of a thick piece of MDF with a body-shaped cutout and enough length to fit tuners at the appropriate point. Manufacture top over-size and guess at a bracing pattern which strengthens the points where your roller-coaster uke has deformed. Fit braces which are narrow but far too tall. Screw top to the jig (maybe you need a rim on top so that it's held all round at the edges of the body shape. Attach a long vertical pointer to bridge. Now string up. Deflection is measured by distance pointer moves from vertical.

    Then you could thin down the braces and check deflection again, and also tap the top to hear how musical it sounds. Continue reducing height of bracing until you're satisfied or the top explodes.

    From what I've read ultra-thin tops tend to be loud but have poorer tone, unless you do something complex like lattice bracing. But this is hearsay, not based on my experience.

  10. #10
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    Thanks for your relpy and ideas. I need to give this a lot more thought.

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