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  #16  
Old 01-19-2021, 04:34 PM
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Mark Hatcher Mark Hatcher is offline
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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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  #17  
Old 01-19-2021, 05:54 PM
charles Tauber charles Tauber is offline
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Quote:
Originally Posted by Mark Hatcher View Post
I think you are right when you say you gain some stiffness like that.
Okay, I'll bite, now that two people have asserted that.

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?
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  #18  
Old 01-19-2021, 08:08 PM
yellowesty yellowesty is online now
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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
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  #19  
Old 01-19-2021, 08:57 PM
charles Tauber charles Tauber is offline
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Originally Posted by yellowesty View Post
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.
And what is the proven downside to not contouring bridges and bridge plates? If there is no upside and no downside, there is no compelling reason to do one or the other.

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.
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  #20  
Old 01-20-2021, 03:54 AM
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Quote:
Originally Posted by charles Tauber View Post
Okay, I'll bite, now that two people have asserted that.

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?
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.
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  #21  
Old 01-20-2021, 07:54 AM
JonWint JonWint is online now
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Originally Posted by Victory Pete View Post
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.
"Stiffness" is only determined by the structural member dimensions and its Young's modulus (E).

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.
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  #22  
Old 01-20-2021, 08:25 AM
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Quote:
Originally Posted by JonWint View Post
"Stiffness" is only determined by the structural member dimensions and its Young's modulus (E).

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.
I disagree. If you tighten trussrod as much as possible counteracting the pull from the strings, the neck will be stiffer just as a neck with no strings and a loose trussrod will be less stiff.
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  #23  
Old 01-20-2021, 08:37 AM
Alan Carruth Alan Carruth is offline
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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.
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  #24  
Old 01-20-2021, 09:06 AM
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Quote:
Originally Posted by Alan Carruth View Post
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.
Even Neil Young agrees, Larry Cragg explained it in an interview.
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  #25  
Old 01-20-2021, 10:32 AM
redir redir is offline
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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.
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  #26  
Old 01-20-2021, 12:10 PM
charles Tauber charles Tauber is offline
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Quote:
Originally Posted by redir View Post
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.
Quote:
Young’s modulus, numerical constant, named for the 18th-century English physician and physicist Thomas Young, that describes the elastic properties of a solid undergoing tension or compression in only one direction, as in the case of a metal rod that after being stretched or compressed lengthwise returns to its original length. Young’s modulus is a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression. Sometimes referred to as the modulus of elasticity, Young’s modulus is equal to the longitudinal stress divided by the strain. Stress and strain may be described as follows in the case of a metal bar under tension.
https://www.britannica.com/science/Youngs-modulus

Quote:
The bending stiffness (K) is the resistance of a member against bending deformation. It is a function of the Young's modulus E, the area moment of inertia I, of the beam cross-section about the axis of interest, length of the beam and beam boundary condition.
https://en.wikipedia.org/wiki/Bendin...by%20a%20force.


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:
Originally Posted by Victory Pete View Post
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.
Victory Pete is asserting two things. First that tightening a truss rod increases the (bending) stiffness of a neck. Second, that the increase in stiffness changes the response of the guitar to increase its "volume, sustain and tone".

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.
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  #27  
Old 01-20-2021, 01:17 PM
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Victory Pete Victory Pete is offline
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Quote:
Originally Posted by redir View Post
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.
You are right, thanks for thinking outside the box.
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  #28  
Old 01-20-2021, 08:24 PM
Gordon Currie Gordon Currie is offline
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Quote:
Originally Posted by Victory Pete View Post
Even Neil Young agrees, Larry Cragg explained it in an interview.
If renowned luthier Neil Young says it, it must be true.
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  #29  
Old 01-21-2021, 10:02 AM
Alan Carruth Alan Carruth is offline
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So how did Neil Young measure it?
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  #30  
Old 01-21-2021, 12:29 PM
KingCavalier KingCavalier is offline
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I think he starts the process "Down by the river"
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