Neck: Density, Stiffness & Contribution to Tone

[arie]

Originally Posted by printer2

A guitar neck is a column that we do not want to bend (oscillate back and forth), we want the guitar top to bend. So how do you get a column to resist bending? Make it stiff.

Little car and a big car meet on the road in unfortunate circumstances. The little car bounces off while the big car gets changes vector. Next little car and a semi, little car, you guessed it, semi rolls to a stop. Next a train and a small car, the conductor slows down to see what that was.

Looking at the energy of the string and the neck, well there is not much in the way of impact. The mass of the string is much smaller than the mass of the neck. You could set up a jig the shape of the back of the neck on a heavy oak workbench and clamp the neck down allowing space for a couple remaining strings that you can pluck. Will the the tone change much now that you have a couple hundred pounds of mass hanging of the neck? Probably not.

But I am just guessing here, and that is not good enough for me. I'll be right back.

I could not hear much difference, hard to say given the nature of the test though. I took a guitar I made with a poplar neck that is very resonant, can feel the neck vibrate when you pluck a string. Took a longer radius block and clamped the neck with quick release clamps to a 100+ lb thick glass top coffee table. Now plucking the string I feel absolutely no vibration off the neck.

Try to be consistent in plucking the open string and listen carefully, pop the two clamps off and pluck again. No change that stands out, mind you I am only using one open string and not fretting it along the neck so can't say it is the same everywhere. But given that the neck used to vibrate and clamped it did not and I did not hear a change (but I must admit I do not have golden ears).



Maybe with a better guitar a change may be more apparent. Maybe even change the bridge on this one may help (I want to do it and one day will get to it. Hey, I taught fluid mechanics also. ).

ever see players who tape $.05 and $1 coins, lumps of metal, and fishing weights to the back of the head stock? the claim is increased sustain.

[murrmac123]

Originally Posted by charles Tauber

This was exactly Kasha's thinking back in the 1960's and what Schneider implemented. He went so far as to use a steel H between neck and body, embedded "inertial weights" in the head under the head veneer and then balanced the whole thing by adding counterweights in the end block.

And still the guitars sucked ...

[LouieAtienza]

Originally Posted by Viking

Yeah, that's what I'm shooting for.

Often, when I think of these things, I tend to go to an extreme in my mind to illustrate a point. Imagine that one end of the strings of a guitar are attached to a 1000LB concrete block, the other to a lightly built acoustic guitar top. Forget for a moment the practical aspects of building such a thing. How much of the energy of the vibrating string is spent at the concrete block end of things? Pretty much none, right? Almost all of it would be spent on the soundboard's end of the string. That's what I'm referring to here. A more massive, more dense neck should mean more of the energy is available at the point in the system where it will do the most good.

There would obviously be an upper limit to how useful this could become without encroaching on the playability of the instrument. If the neck is too heavy the balance of the guitar is lost.


Part of the way my thinking is evolving on some of these issues, is that the term "overbuilt" is really only applicable to certain aspects of the system. Those aspects that are SUPPOSED to vibrate should be made as lightly as possible(top, back, bracing). Those aspects of the system that are not supposed to vibrate can be made heavier and that will in fact allow the energy budget to be spent in those places where it will produce a maximum of acoustic return.

This would be why those like Somogyi(at least, in the way that I am thinking) use a heavy upper transverse brace firmly anchored to the sides, double sides, and the L shaped inside heel block. Thick, heavy structures to ensure the structural integrity of the instrument, AND, to minimize the amount of energy lost to those parts of the system, leaving that energy in the places where it will do the most good.

Is this a valid way to view the system?

I'm certainly noticing that when you ask a question of 10 guitar makers, you're going to get 15 different answers. Makes me believe that all these different schools of thought represent local minima in this complex system of variables. And further, that a global minima may not actually even exist.

I had the pleasure of playing Paul Heumiller's [of Dream Guitars] McConnell, which has a Brazilian Rosewood neck, and it was by far the most amazing guitar I've ever played. I don't know how much "tone" was contributed by the neck, but definitely it sustained seemingly forever! Having Brazilian rosewood back and sides couldn't hurt either.

Overall, I didn't feel much of a balance issue when playing, though you know the weight's there. The guitar in entirety was still lightweight.

I would tend to agree that the moving parts are most important to keep lightweight, to maximize responsiveness. But I'm not so sure that non-moving parts should be so massive as to totally impede vibration. If a part can't vibrate, than it can't contribute to the tone (though it would contribute to sustain.)

Your thinking on guitar construction is similar to what Paul Reed Smith illustrated on YouTube. A very thick graphite rod is inserted into a Peruvian walnut neck to give maximum stability. Then, the sides and back are braced such to minimize movement. My verdict is still out because I'm still unsure if I like the sound... If the sides and back are braced to minimize vibration, then why even bother offering different woods other than for decoration?

[Alan Carruth]

Ever play one of the Parker 'Fly' solid bodies? They weighed about three pounds, had good tone and plenty of sustain. The softwood body and neck were stiffened with .005" thick unidirectional carbon/epoxy pre-preg. The added stiffness improved the sustain, and the light weight moved any resonances up in pitch to where they would not be a problem, as well as reducing the amplitude. There are lots of ways to skin cats.

[Viking]

Originally Posted by charles Tauber

As my fluid mechanics teacher used to say, "An art is a science with too many variables."

I love that saying. Part of what I'll be doing is to catalog quite a bit of data of the various components as I build. Weight, stiffness, dimensions, etc. At some point in the future, I'll see about applying some artificial intelligence to the mess of variables. Neural nets can be good at cutting through the clutter and honing in on the important elements. Though it's also possible that they simply over-fit to the noise and nothing good comes from it. But I can also make my own determinations based on my observations.

Originally Posted by LouieAtienza

If the sides and back are braced to minimize vibration, then why even bother offering different woods other than for decoration?

By my way of thinking thus far, the back is a part of the vibration equation. It should be allowed to vibrate. It should be allowed to contribute to the tone produced. The side species doesn't matter much(in terms of tone production) as they are their for structural purposes and to define the size and shape of the air cavity. Again though, based on the way my thinking is evolving on the subject.

Originally Posted by Alan Carruth

There are lots of ways to skin cats.

I totally agree with you. As long as you've got a good marinade handy, the method of skinning the cat is irrelevant!

Originally Posted by HCG Canada

Another thing to remember about necks is that dimensional stability is extremely important to ensure consistent unchanging action and hence playability. This is one reason why mahogany is such a standard choice. A website called "The wood database" (google it, it will show up right away) has information on different wood species' average properties. Compare the dimensional stability (expansion/contraction along and against the grain) of rosewood, mahogany, and other woods.

I think I've seen that database before, but wow. That is an awesome resource. Question though. Which of the common metrics that they record would be correlated to what we would refer to as "dimensional stability"? They list "Janka Hardness", "Modulus of Rupture", "Elastic Modulus", "Crushing Strength", and "Shrinkage". Elastic modulus seems like it would be the closest to what we call dimensional stability.

Originally Posted by arie

page 20 of john greven's 2011 gal discussion might shed some light:
http://www.grevenguitars.com/pdfs/VoicingtheGuitar.pdf

An ironic observation... He states in his article: "Use the intemet as an information resource with great caution. There is much nonsense and misinformation contained therein." And then makes it available via the internet.

[charles Tauber]

Originally Posted by Viking

Which of the common metrics that they record would be correlated to what we would refer to as "dimensional stability"? They list "Janka Hardness", "Modulus of Rupture", "Elastic Modulus", "Crushing Strength", and "Shrinkage". Elastic modulus seems like it would be the closest to what we call dimensional stability.

"Shrinkage". He's explained the terms well, so there is no point to my restating them:


http://www.wood-database.com/wood-ar...nal-shrinkage/
http://www.wood-database.com/wood-ar...of-elasticity/

[John Arnold]

Elastic modulus seems like it would be the closest to what we call dimensional stability.

No. Elastic modulus is a measure of stiffness. In general, stiffer woods are denser. Denser woods are usually less stable, but there are plenty of exceptions.

[Viking]

Originally Posted by charles Tauber

"Shrinkage". He's explained the terms well, so there is no point to my restating them:

http://www.wood-database.com/wood-ar...nal-shrinkage/
http://www.wood-database.com/wood-ar...of-elasticity/

Originally Posted by John Arnold

No. Elastic modulus is a measure of stiffness. In general, stiffer woods are denser. Denser woods are usually less stable, but there are plenty of exceptions.

Okay, so when we say dimensional stability(shrinkage) around here, we're talking about how the wood reacts to changes in moisture content. Right? And the elasticity would be how a piece of wood bends upon the application of stress. So for a neck, it needs to be stable enough to withstand a modest change in moisture content and strong/stiff enough to withstand the tension of the strings.

I'm really grateful for all the hand holding. All your points of view are appreciated.

[charles Tauber]

Originally Posted by Viking

Okay, so when we say dimensional stability(shrinkage) around here, we're talking about how the wood reacts to changes in moisture content. Right?

A fundamental property of wood as a material is that it is both anisotropic and hygroscopic. To successfully use it as a material in any fine woodworking application requires a good working knowledge of its properties, which include how it changes size and shape as its environment (moisture content) changes.

And the elasticity would be how a piece of wood bends upon the application of stress.

Technically speaking, one applies forces ("loads") to an object. Stress is a result, a calculated quantity, as is strain. (Strain is a measure of how much a material deforms under load.) A measure of elasticity is the slope of the linear portion of a stress vs strain graph/plot for that material.

Deformation (due to loading) can be "elastic" or "plastic". Elastic deformation is that in which the material returns to its original size/shape after the load is removed. Plastic deformation is that in which the material's size and shape is permanently altered as a result of the applied load, even after the load is removed. One form of plastic deformation is called "creep". Creep is plastic (permanent) deformation that occurs as a result of the application of loads for a long period of time. Usually, the magnitude of the load that causes creep is less than that required to cause immediate plastic deformation. That is, a load that when applied for a short time results in elastic deformation, if left for a long time causes permanent deformation (creep).

Guitar bodies undergo creep due to the perpetual forces the tensioned strings apply to the body. (Glues, a different material with different properties, can also creep, particularly "white" glues.) For example, the need for a neck reset is not a result of lack of neck strength, plastic deformation of the neck, or failure of the neck joint. It is due to geometric changes in the shape of the body that occur over a period of time due to the applied forces: creep. It is not, for example, due to sudden failure.

So for a neck, it needs to be stable enough to withstand a modest change in moisture content and strong/stiff enough to withstand the tension of the strings.

Yes. However, if the wood is sufficiently dried, prior to use, and reasonably cared for after completion, moisture-related stability issues are few and far between for any of the woods commonly used for guitar necks. Similarly, strength is rarely an issue. That leaves stiffness - resistance to bending - which can definitely be an issue, which brings us full circle back to Modulus of Elasticity.

I'm fairly certain, however, that many very fine guitar makers don't know lots of these terms or the numerical values relating to the woods they are using. I'm not going to go so far as to say they are irrelevant, but one can certainly become overly fixated on them, losing sight of the forest for the trees, so to speak.

[John Arnold]

The popularity of mahogany for acoustic necks is due to its relatively light weight and its superior stability. There are plenty of woods that are stiffer, so it tells me that a premium is placed on those properties.

[Alan Carruth]

Woods tend to have different shrinkage rates depending on the grain orientation: usually tangential shrinkage is larger then radial for a given drop in R.H. This is why logs tend to develop radial checks as they dry: the circumference is getting smaller faster than the diameter. Woods with a large radial to tangential shrinkage difference tend to be less stable than woods with more uniform shrinkage: they will warp and cup more.

You also have to consider the shrinkage rates of the different woods used in the neck. If the fingerboard shrinks more along the grain than the neck with a given change in humidity you'll see seasonal changes in the relief, for example.

[Jim.S]

Hi Nick, great thread. By asking the sort of questions you are asking you seem to be going down the road of guitar design rather than just building guitars. In the early days builders worked on design by feel, they learned methods that yielded instruments that worked well in the way they hit our ears and became Masters. They taught apprentices that also learned by feel from their Masters experience.

Understanding what Charles had to say in his last post would be a handy thing and he makes a good point here...

Originally Posted by charles Tauber

"I'm fairly certain, however, that many very fine guitar makers don't know lots of these terms or the numerical values relating to the woods they are using. I'm not going to go so far as to say they are irrelevant, but one can certainly become overly fixated on them, losing sight of the forest for the trees, so to speak."

Many of us nowadays in the boom of hand built (folk or steel string) guitars don't have a Master to work with day in day out (I know I could have used one). So although Charles is right about becoming "overly fixated" on terms or numerical values, having a firm understanding of this stuff will benefit anyone who is designing guitars. "More than one way to skin a cat".

Jim

[Viking]

Originally Posted by Jim.S

Hi Nick, great thread.

Why thank you.

Originally Posted by Jim.S

By asking the sort of questions you are asking you seem to be going down the road of guitar design rather than just building guitars.

In my mind, I don't see a difference between one or the other. They seem like a synonymous pursuit. I know that they are not, but that's not how my brain works.

It's my nature. I'm a why guy. To quote the Merovingian from the Matrix(one of my favorite movies), "Why is the only real source of power". If one can understand the why of any one thing they might want to pursue, it is a small matter to change it to fit a new or different purpose.

And then there is the simple fact that this whole pursuit is just so fascinating. All these intricate parts, painstakingly crafted to fit in precise ways, taking on a form that is more than the sum of it's parts, and then magically made to come alive by feeding it the energy of a vibrating string. As far as I am concerned, music and musical instruments represent one of mankind's most important and lasting achievements.

Originally Posted by Jim.S

In the early days builders worked on design by feel, they learned methods that yielded instruments that worked well in the way they hit our ears and became Masters. They taught apprentices that also learned by feel from their Masters experience.

Understanding what Charles had to say in his last post would be a handy thing and he makes a good point here...

Many of us nowadays in the boom of hand built (folk or steel string) guitars don't have a Master to work with day in day out (I know I could have used one). So although Charles is right about becoming "overly fixated" on terms or numerical values, having a firm understanding of this stuff will benefit anyone who is designing guitars. "More than one way to skin a cat".

Jim

I'm sure one can make guitars that sound wonderful without being so interested in this level of detail. But that's not how I'm built. I have too obsessive a personality.

[Jim.S]

I am a bit the same and maybe I can be a bit too obsessive at times. I reckon you know that there is a load of information out there in books and on DVD but if you have not already bought them I would suggest getting copies of Alan Carruths Free Plat Tuning DVD (not just for his tuning methods but he explains stuff about the coupled resonances), Somogyi's The Responsive Guitar and Gore/Gilet Contemporary Acoustic Guitar Design and Build. You will have to empty a fair bit out of your wallet but I think worth it.

Jim

[Viking]

Originally Posted by Jim.S

I am a bit the same and maybe I can be a bit too obsessive at times. I reckon you know that there is a load of information out there in books and on DVD but if you have not already bought them I would suggest getting copies of Alan Carruths Free Plat Tuning DVD (not just for his tuning methods but he explains stuff about the coupled resonances), Somogyi's The Responsive Guitar and Gore/Gilet Contemporary Acoustic Guitar Design and Build. You will have to empty a fair bit out of your wallet but I think worth it.

Jim

I've already got Somogyi's stuff. I've heard of the Gilet material, but yeah, I won't be able to plunk down any money for new materials for a while. Maybe for Christmas.