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Thread: Radiused backs and strength

  1. #1
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    Default Radiused backs and strength

    Hi, all,
    I don't believe I've posted before. My name is Viktor, I've built several guitars and basses and am working on my third concert ukulele.
    Just curious about this:
    I like the look of a slightly radiused back, but I'm skeptical of the notion that the arch makes the back stronger. (I've heard this a number of times.) An arch may resist downward force, but not lateral force (from the side). Downward force is an issue on archtop guitar tops (or violins, etc.), but there seems to be no downward force on the back any of these instruments, including ukuleles.
    I believe that arching the back introduces tension (this is an important element in violin construction, for example)--but I really don't see how it would make the back stronger? Do you believe it makes the back more rigid (and reflect better)?

  2. #2
    Join Date
    Dec 2012
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    Sheffield, England
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    I think most people introduce an arch not simply for strength, but to resist changes in humidity and therefore a protection against the wood drying out, shrinking and cracking. And the (in my view) misguided belief it somehow reflects sound better.

  3. #3
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    Nov 2013
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    Trois-rivieres,Quebec
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    I normally make my tenor backs wit a 20 ft radius. The last 2 I used 15ft. It seems to me that these backs are less responsive.

  4. #4
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    Question for Alaine: Why would you want the backs responsive. The early elite instruments of a well known ukulele brand ran into trouble because of this very thing - the back 'stealing' energy from the front. You want all the power and energy in the front hence laminated sides which kill vibration transfer to the back. The back needs to be stiff and profoundly radiused. If you want to learn more about design I am running a zoom event next Saturday on Facebook. This is not SSP since you will have to hunt around to find the details.

  5. #5
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    Nov 2013
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    A responsive (or live) back is a good thing on guitar, So my thinking was that it should be good also for Ukulele! But I have been wrong before!
    I said it seems to be less responsive because when tapping and recording the frequencies it was very difficult to see a difference between the top and back curves. Tapping the top or tapping the back gave almost the same peaks, which was not the case with my previous flatter backs.
    So may be the reason is that these backs are more reflecting than responsive which, like you say, might be a good thing. I am still learning!
    What does SSP stand for ? This event will be on your Facebook group?

    Alain (without e)

  6. #6
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    I have a much better sense of what constitutes good tone in an archtop guitar than I do in a ukulele. Like Alain, my sense is that responsiveness is good in a guitar (for tone, and perhaps complexity of tone), but that it has to be balanced with stiffness/reflection for volume. Is volume the more important concern in a ukulele?
    Red Cliff's response about resisting cracking makes perfect sense to me. Thanks!

  7. #7
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    Maybe the sting instruments that are not held against body while being played are best for back response eg: the violin family Cello Double base etc..that is why they have a sound post under the bridge to transfer the vibes to the back of the instrument thus increasing the volume...maybe a double back and sound post would also work on a uke.
    http://ukulele-innovation.tripod.com ebay i/d squarepeg_3000 Email timmsken@hotmail.com

    If you can believe that moving images and sound, can fly through empty space across the universe and be seen and heard on a box in your living room ?.. then you can believe in anything.

  8. #8
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    There are two schools of thought, the traditional (i.e. live back) and the more modern non-live back i.e. Smallman. I guess there are degrees between the two extremes. You can be sure that there's no wrong or right, it all depends on what you consider to be 'the best'. We tend to have different ideas on that.
    As for flat and arched backs. I've made many of both. I can't say that I've come to any firm conclusion of one being better than the other. Nor have I had a flat back crack. However flat backs (over time) do tend to sink and show a bit of concavity after being subject to the inevitable fluctuations in humidity. They don't tend to recover when brought back to higher humidity either. Not sure that the slight concavity is much of a problem though.

  9. #9
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    Did you read my response? This is not theory it is practical. The company concerned had to sell at cost a whole huge batch of instruments and go back to the drawing board. I value the real thing over theoretical considerations. And yes, if I was building a flamenco guitar I'd want the whole thing to be shuddering as a played it! I haven't said this for a long timer - ukulele are not guitars.... The soundpost only works because by it, the plates are coupled and the whole instrument resonates. It's why you have to have a different 'carve' for the front and back - if they were the same you'd get 'wolf' notes.

  10. #10
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    Sorry Pete, but I have to disagree. Live backs can clearly cause issues with resonances and wolf notes if not careful. Solid sides and stiff back can mitigate some of that. But it is possible to have a live back, thin sides and still have a great instrument without wolf notes. In fact the classical guitars through history (e.g. hauser etc) that people tend to associate with the best sound (to their ears) tend to be traditionally braced with live backs and thin sides. The more modern approaches ( reinforced sides, double tops, laminated stiff backs etc) have increased volume, but haven't on the whole produced better sounding instruments, at least not to my ears. And that isn't theory, but is practical. I think taking one example where it hasn't worked and ignoring the thousands where it has is being very selective of the evidence. If it works for you then great, but there is more than one way to skin a cat.
    Last edited by Red Cliff; 09-19-2020 at 09:58 AM.

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