#16
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I don't believe it is wise to build any more tension into the top than you need to. I think you are right when you say you gain some stiffness like that. The problem is you may voice the top to that stiffness and with time the top settles into its new position and you loose the tension and your top deadens.
This is also why I don't use the top and back to hold the sides in position.
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Mark Hatcher www.hatcherguitars.com “"A conclusion is the place where you got tired of thinking". Steven Wright |
#17
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By what mechanism does gluing arched braces to a flat top using a gluing caul that is flatter than the braces increase stiffness? While doing so will increase the tension and compression of the components being glued, how does that relate to increasing stiffness? |
#18
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As long as all the elements (the braces and the soundboard, in this case) are operating in their linear mode (stress proportional to strain), then gluing a contoured brace to the soundboard using a flat gluing caul won't change the overall stiffness.
The resulting soundboard (and contoured brace) shape will be that which equalizes the strains in the two elements. That will result in a shape which is less pronounced than the original brace shape and will lock opposing strains into both elements. It sounds like a bad idea. Any environmental change (temperature, humidity, aging of the materials, etc.) that changes the stiffness of one element more than the other will rebalance the strains, resulting in a changed shape to the soundboard. That's unlikely to improve playability. And we most certainly want to construct our musical instruments such that the components function in a linear mode. Otherwise, the instrument will, itself, create anharmonic (aka dissonant) sounds (don't call them "harmonics," they won't be). On a similar topic, this is a reason to contour the back of the bridge and the top surface of the bridgeplate to match to soundboard contour. There is no upside in locking unnecessary strains into our builds. Last edited by yellowesty; 01-19-2021 at 08:12 PM. Reason: elaboration |
#19
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As I alluded to previously, one can make rational academic arguments on both sides of the issue. Empirical proof of either is more difficult to establish. Last edited by charles Tauber; 01-19-2021 at 09:03 PM. |
#20
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#21
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Keeping a truss rod "very tight" will change the relief. The stress in the truss rod will not affect stiffness. You can build a prestressed top system but it will not increase stiffness. |
#22
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#23
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Victory Pete wrote:
"It is like keeping a truss rod very tight with a straight neck, it makes the neck more stiff and improves volume, sustain and tone." Are you sure? Tightening the truss rod puts the neck itself in compression. Loading a column in compression lowers it's resonant frequency, which goes to zero at the stress that causes the column to buckle. This relationship has been used as a reliable method of non-destructive testing in aircraft truss structures. I suspect that, in most cases, the load of a tightened truss rod is nowhere near the buckling stress of the neck, but without data it's hard to say. Some of those electric guitar necks are pretty long and skinny. Also germane is the question of how you know this. How are you measuring volume, sustain and tone? Two of those, volume (or power, at least), and sustain are amenable to some sort of objective measurement, which should settle the whole question relatively easily. Have those measurements been made? We all hear what we expect to hear. |
#24
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#25
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It sounds like it's sliding in into an argument of technical terms. I'm not an engineer but it sounds like the term 'stiffness' might be part of the engineers manual and that it is an actual property of in this case the top of the wood.
So if you measure the top for stiffness you get a number. Then no matter how you brace it you are not changing the number or the measured properties of the top. However of course you are also bracing the top which makes it stronger for the sake of handling over 100 pounds of string tension. So if you suspend a floppy unbraced top across two supports at each end and put a 5 pound weight in the middle it will deflect say 1/2 inch for example. Now brace the top and do the same test. It only deflects .03 inches now so its 'stiffer' in the layman's sense of the word but the Young's Modulus of the top has not changed. The engineers can correct me if I'm wrong. |
#26
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The top material has its modulus of elasticity (Young's modulus), the bracing has its own. The top material has its own bending stiffness, the bracing has its own. When you join them - such as by gluing them together - the bending stiffness of the "assembly" is a combination of its parts. As is often stated here, the area moment of inertia, and hence, bending stiffness, of a beam of rectangular cross section is proportional to the cube of its height and directly proportional to its width. That is, if you double the width of that "brace", it is twice as resistant to bending, but weighs twice as much. If you double its height, it is eight times as resistant to bending, but weighs twice as much. Similarly, the bending stiffness - a function of the geometry of the "beam" - of a guitar top, for example, is heavily influenced by the thickness of the top. The top's modulus of elasticity - a property of the material - is not affected. To be clear the "stiffness" we are discussing is its resistance to bending, aka "bending stiffness". This discussion has to do with mechanical structure. Quote:
The second assertion has to do with changes in response based on assumed changes in mechanical structure. I'm not going to attempt to dissuade him from those unsubstantiated assertions. Last edited by charles Tauber; 01-20-2021 at 12:28 PM. |
#27
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#28
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If renowned luthier Neil Young says it, it must be true.
__________________
-Gordon 1978 Larrivee L-26 cutaway 1988 Larrivee L-28 cutaway 2006 Larrivee L03-R 2009 Larrivee LV03-R 2016 Irvin SJ cutaway 2020 Irvin SJ cutaway (build thread) K+K, Dazzo, Schatten/ToneDexter Notable Journey website Facebook page Where the spirit does not work with the hand, there is no art. - Leonardo Da Vinci |
#29
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So how did Neil Young measure it?
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#30
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I think he starts the process "Down by the river"
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