Official Luthiers Forum!

Okay, with a full belly(lunch!) and fresh perspective...

John, I think your issue is that yo think we're dancing around the subject and not "giving" the information you want.

Problem is, somethings can't be taught. What I wrote last night is one of those "things". It can't be taught, but it surely can be learned. I gave you the tools to go ahead and learn, what can't be taught.

I'm not asking for thank you's, or even an acknowledgment from anyone, but at least, don't be so ungrateful.

[QUOTE=grumpy] Okay, with a full belly(lunch!) and fresh perspective...John, I think your issue is that yo think we're dancing around the subject and not "giving" the information you want. Problem is, somethings can't be taught. What I wrote last night is one of those "things". It can't be taught, but it surely can be learned. I gave you the tools to go ahead and learn, what can't be taught. I'm not asking for thank you's, or even an acknowledgment from anyone, but at least, don't be so ungrateful. [/QUOTE]

Whew!

I obviously didn't communicate my thoughts very accurately, and not just to Mario.

First of all, in an ideal situation, the data would be coming from everybody building, not just the 'successful' builders with long waiting lists for their instruments.

Second, the way (public) science works is that after the data is published, it's entirely likely that somebody will come along and say: "We followed your method and couldn't replicate your results. Something else must be going on here."
It's very possible that after publishing deflection, resonance/spectra, Chaldni patterns, etc of a successful guitar, another builder could say:"Mario- I tried to build a duplicate of your great guitar, using your numerical data, and folks say it sounds terrible. So, either my listeners like very different guitar sounds, or something is going on that cannot be measured using our techniques. Let's change our focus to figuring out those other factors."

It's not just the 'one-way street' that some folks here imagine. You don't necessarily look for progress only from those who are already famous and accomplished- at least in science.

In the 'some things can't be taught' department: I heard this a number of times during a course I took (after I'd paid the $4K, not before!). On the 'can't be taught' list :
-French polishing
-evaluating wood for selection
-judging correct thickness for top/back woods
-judging bracing size/strength/taper

Since I spent my professional career as a teacher, I needn't say that I don't agree with this approach!

Cheers
John

<shrug>

A music teacher -can- teach you the notes, which one to use and when, but only you can learn to make great music.

My school teachers taught me the alphabet and words and sentence structure(to two languages, at the same time <g>), but that won't make me into a writer, in any language.

Some things cannot be taught. But all can be learned, if we first learn to learn.

If you go through life searching for the teachings to everything, you'll miss the best teacher of all. Yourself.

Back to the topic... It's important to remember that the "cube rule" is just a guiding principle, not a set in stone method of designing the strongest/lightest braces. There are a multitude of forces acting on a guitar brace or top. If you optimize for stiffness in one direction, it could be very weak in the other direction, or could be weak if a twisting force was applied to it.

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______________________________
Jonathan Kendall, Siloam Springs AR

I think a lot of you blokes spend far too much time looking for the short cut that doesn't exist.

Your time would be far more wisely spent caressing a chisel rather than a keyboard.

_________________

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Bob Connor
Geelong, Australia

hoorah for Bob ...

Larrivee once said - I'm not a numbers guy, I'm a sawdust guy. Very wise words indeed.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

David Hurd's "Left Brain Lutherie" more than adequately describes the relevant math and furnishes photos of a simple and inexpensive setup to measure deflection.

Type of wood, thickness measured during the deflection tests, span between the supports, weight applied and point at which measured would all have to be identical amongst all those testing to build a data family that was relevant and meaningful if all we wanted to consider was "deflection."

The real term to be measured and understood is "MOE" (Moment of Elasticity) and I'm surprised that the other engineering types here haven't mentioned it yet.

When measuring deflection and then calculating MOE, then identical setups aren't the defining criteria, the resultant deflection from the weight applied and the distance from the end supports (hopefully symmetrical) become the basis for the MOE calculation. To share meaningful data we need to share the MOE for the top that resulted as one of the system variables in making a "good sounding" guitar.   The MOE calculation normalizes all of the deflection setup variables and lets us compare our results with one meaningful resultant.

As Al Carruth reminds us constantly, the system of all the variables is what combines to make a good, better or great sounding guitar.

Incidentally, the MOE for a good, stiff Spruce top is usually in the range of 1,750,000 - 2,000,000 PSI (varies by species) Anyone wanting more explanation or data can PM me and I'll be glad to help.

Peter

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Peter

Look even if you have two pieces of wood with the same amount of deflection they are not deflecting exactly the same way. In fact they may bend in quite different ways for many reasons. Science can help a lot help but in the end you have to feel it. Brace stock can vary in stiffness slightly from one end to the other.

Learn ways to learn to visualize and feel things.

Take 180 pounds of weight make a simple pulley to feel what that string tension is.
If you are building an archtop figure out your downforce at the bridge is and see what that is on a bathroom scale before seeing how your top reacts to your bending and taping and pushing.
Take a piece of brace and put a string through it tie the string off somwhere and pull it tight, pluck the string, hear and feel how the vibrations travel through the wood.

Learn from kiln operators. Handle green wood and dry wood to learn that you can feel the difference. The list goes on and on and on and on (whap). Oops sorry got stuck.

Listen to the many voices of experience here and the do some heavy thinking. Then do things in a way that suits you best.

Jeez, I talk to much.

              Kirby

P.S. there really are Tone Faerie

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"It's a Tone Faerie thing"
"Da goal is to sharpen ur wit as well as ye Sgian Dubh"
"Sippin Loch Dhu @Black lake" ,Kirby O...

I only test longitudinally. I don't know enough about testing the other way

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Andy Z.
http://www.lazydogguitars.com

I am asking this as an ex-engineer rather than an experienced builder (since I am about to start my first acoustic guitar , only having built electrics before.

-Should we not be looking at the brace as a T beam with an effective top web (comprised of the Guitar top plate)extending say 1/2" each side of the brace edge?

- If we really want a minimum weight for a particular stiffness, should we not start with a minimum brace width and use a plain rectangular section, rather than starting wider and cutting away just at the bottom of the brace (for a triangular or parabolic shape)

Not picking at anyone, just wondering whether Brace shape is also influenced by aesthetic expectations

Mario is right: use all the data you can get. I do an awful lot of flexing and tapping, as well as a fair amount of measuring. Either one by itself can get you someplace, but put them together and you've got a lot more data to go on than either alone can provide.

It is certainly true that the hand can be quite accurate at making stiffness judgements. There are folks who have been tested in objective ways who can judge the absolute stiffness of a small piece of wood within about 3%. I'm not sure my test rig will get me any closer. OTOH, a test done of a number of builders at a recent instrument maker's meeting found that most of them aren't anywhere near as good as they think they are. If all you do is flex and tap, how do you know whether you're one of the ones who's good at it or not? Consistent results would be one way, but then, again, the only way to know is to build a lot of instruments.

jeffhigh:
Good questions; I wish I had good answers. The problem is that, as is often the case, it's more complicated than it 'ought' to be.

In thinking about the aspect ratio (height/width) of a brace you have to take the gluing surface into consideration. Lutes use braces that are about 5 times as tall as they are wide, and that's about the highest aspect ratio I've seen on an instrument. There are stresses in the glue line, from humidity changes, if nothing else, and if the brace is too stiff those can be too localized and cause failures, I would think.

I'm not entirely sure that we want to maximize the stiffness/weight ratio of the braces. There could be things like overall compressive strength along the brace, and the degree of mismatch in impedance at high frequencies, that would favor lower and wider braces. There may well be a reason for those low braces in the loer bouts of Martin backs, for example.

Aesthetics may well have someting to do with it, but for the most part I think we tend to make them the way we do because they work pretty well that way, don't take too much time to make, and allow for a certain amount of flexibility. For example, you can probably knock out a plain rectangular brace with the proper arch for the top in ten minutes or less. An I beam might take a half hour, and if you drill it to really reduce the weight, allow ten minutes or more extra. For all that you've saved perhaps 30% of the weight of the brace. All of the bracing on the top amounts to about 30% of the total weight of the top, so by trippling the amount of work you've shaved 10% off the top weight. This is not a negligable saving, but you could have done better by finding a lighter piece of wood for the top. On top of that (so to speak), you can't do much to alter the stiffness of your I-beams: if the top doesn't work very well, you're stuck.

This is exactly the problem that some of the 'sandwich' and 'lattice' tops have. If you think of the top and bottom surfaces of a sandwich as the 'flanges' of an extended I-beam, and the 'filler' as the webs, you'll see why these tops are so light and stiff, but also why they can be hard to fine tune. The lattices bracing setups that use carbon fiber caps on balsa webs are similar, except that they bridge the gaps with a minimal membrane of wood that is not meant to contribute to the strength or stiffness in a meaningful way. Once that lattice is built, it either works or it doesn't; you can't do much to change the resonant behavior except to add mass here and there. Since the object is to get the least weight for the stiffness, that's counterproductive.

There's an old joke about the repairman who's sign reads:"Shop Time - $30/hr
       If you Help - $40/hr
       Engineers who Help - $60/hr"
I come from a long line of engineers and teachers, and most of my students are engineers. I like engineers. I know it's natural to question and analyse and wonder whether there might be a better way. There very well could be, but there are a lot of dimensions to 'better' in this business, and a lot of folks have worked hard for a long time to approach all of those 'betters'. I know it's hard, but try to stick pretty well to the 'standards' on your first one or three. You'll have a much better view of the territory then. :)   

Thanks for the reply Alan,

I guess the crux of my question is why use a triangular or parabolic shape rather than a plain rectangular brace?

I have seen comments to the effect of "removing material from the sides reduces the weight without reducing the strength much" to justify a triangular shape. yet doing this IS removing material from the area subjected to most stress.

So is a lot of brace shaping to do with aesthetics?

With the braces that you see below I am able to obtain the same
strength/stiffness of the wide low braces with around half the mass and
weight.



This is a back that I am working on tonight and I thought that it might
be helpful to some to see what I mean by triangular braces.  Mind you
this back has not been cleaned up yet so please forgive me for showing
a work in progress.





Thanks

I think there is more to brace shaping than just trying to make things as light and stiff as possible. In an instrument, a brace is more than just a static load bearing member. When a guitar is played, the brace moves back and forth through some range of deflection. The frequency of that oscillation varies depending on the frequency of sound being produced.  Sometimes a brace must oscillate at a low frequency, sometimes at a high frequency and most of the time at both at once.

Because of this I find it useful to think of the brace as a spring. Tall narrow braces are like springs with large spring constants. That is, the force required to deflect them increases rapidly as the brace is deflected. Wide, low braces have a lower spring constant. The force required to deflect them does not increase as quickly as the brace is deflected. The result is that it is easier to move the wide, low, brace through a larger range of motion.

It is possible to make a tall, thin, brace and a wide, low brace that require very similar amounts of force to deflect them a small amount (such as when responding to high frequency vibration) and require very different amounts of force to deflect them through a larger range of motion (such a when responding to lower frequency)

An example of how we might apply this principle can be illustrated by considering the lower bout back braces. Some believe that the back should be very stiff so that it will act as a reflector to high frequency vibrations created by the top. Others say that the back should be loose to allow it to pump low frequency air out of the sound hole. By making the back braces low and wide, they can "appear" very stiff to the high frequency, low range of motion, vibrations associated with treble response, and flexible to the the lower frequency, high range of motion, vibrations associated with bass response and air moving out of the sound hole.

In other words, there is more to tone than just reducing weight. Changing the proportions of a brace can be quite useful in manipulating how the various parts of the guitar react to the range of frequencies they are required to handle.

Mark

Wow!!!

What a great thread. Thanks for all the info. This has answered many of my questions.

In other words, there is more to tone than just reducing weight.

Amen to that! if mass(weight) was everything, we'd build guitars without braces. Laminated and/or pressed backs dn't require bracing. And my friend, Martin Brunkalla builds guitars, even dreads, without a single brace in their tops(and the tops aren't any thicker than yours or mine's) through some interesting engineering. yet they aren't louder or even more responsive than mine. The tone's quite different, though(and that's subjective, so we won't say better or worse than "conventional" guitars).

A highly domed top of regular thickness would also need little to no bracing, yet we know from past trials that this results a thin-sounding guitar that acts completely overbraced.

Nope, there's a ton more to it than simply reducing mass while maintaining structural integrity.

And while on the subject of brace shapes, consider also that everything inside the instrument also acts as a baffle, further complicating matters.....

So much fun!

Luckily fr everyone, the flat top guitar is among the toughest of all instruments to screw up. As long as it looks like a guitar and is even close to what it should be, it will sound rather decent.

[QUOTE=grumpy] .....So much fun! Luckily for everyone, the flat top guitar is among the toughest of all instruments to screw up. As long as it looks like a guitar and is even close to what it should be, it will sound rather decent.[/QUOTE]

So lots of GLO's qualify as guitars, just because they share so much in common it's unmistakable that they are intended to be a guitar. I've always been impressed by folks with innate ability to build. Seems like they were born to build. There very first instruments are wonderful.

Okay, I'm claiming that statement for myself and making it my trademark "Born to Build". Great thread, I really didn't know the cube stiffness rule. They don't teach that in Business classes or Carpentry classes or Electrical classes.

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http://www.dickeyguitars.com

[QUOTE=JohnAbercrombie] Andy-
Thanks for the info.
Next time you are out in your shop (no rush) perhaps you could measure the separation between those rods- no rush at all on this, but I would appreciate it.
Thanks.
John
[/QUOTE]

Mine is 18" between the tops of the supporting dowels.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

[QUOTE=grumpy]Nobody started out with years of experience. Nobody. We all started out with the same amount of experience: zero.[/QUOTE]



Hehehe...true that.  It's like the question asked of the town historian:



Q: "Were any famous people born in this town?"

A: "Nope...only babies."

Mario wrote:
"Luckily for everyone, the flat top guitar is among the toughest of all instruments to screw up. As long as it looks like a guitar and is even close to what it should be, it will sound rather decent."

Yup; the designs are really good; if you use decent material and build carefully, you'll end up with a pretty nice guitar. That said, I think the harp is the hardest instrument to build really badly. If you get the strings right (and that's a biggie!) and it doesn't self-destruct it'll sound fine no matter what you make it out of.

OTOH, classical guitars can be pretty tricky. I just finished my third archtop classical, and it sounds pretty good: I think I'm getting it! Almost no bracing on the top, and none at all on the back.

Ok - this thread has been incredibly interesting.

So, just to clarify - those people who just shape their braces (triangular or parabolic) without scallopping would then have braces which is stiffer in the centre of their length due to the radiussing alone then?

[QUOTE=Dave Bamber] Ok - this thread has been incredibly interesting.

So, just to clarify - those people who just shape their braces (triangular or parabolic) without scallopping would then have braces which is stiffer in the centre of their length due to the radiussing alone then?[/QUOTE]

Dave-
It sounds like you have the idea- if a brace were 'straight' across the 'top' but curved along the edge that was glued to the soundboard (or back), it would be stiffest where it was deepest.

Your use of the term 'parabolic brace' is not the same as many folks seem to use, and may cause some confusion. (It always seems to, for me!). Most of the time when you see 'parabolic bracing' referred to, people seem to mean braces which have smoothly curved upper surfaces, with the deepest part of the brace being somewhere in the central region. Other top profiles are 'scalloped' and 'straight' which you see from time to time on transverse braces and braces in factory guitars.

Cheers
John

[QUOTE=grumpy] <snip>
... As long as it looks like a guitar and is even close to what it should be, it will sound rather decent.[/QUOTE]

And if sometime in the past, a guitars that sounded like an Ovation had become elevated to a standard, we'd be pursuing our own visions of perfection using plastic rounded backs, playing with dome shape, thickness and materials.

Funny how that excellence which we pursue with such passion can -in the largest scheme of things - be relatively arbitrary.

Food for thought. or not.

If the Ovation example, which sold in huge numbers, would have proved to be a sound and "look" that people still clamored for today, for sure. But even the greatest ideas can't change human likes and preferences.

Standards became standards not by force or by advertising(Ovation did this very well, with nearly every visible performer using one in the 70's), but because the listeners a players made them the standards. When something proves to be popular, many will tout it as the new standard(this is for all things), but yet, only time can make a true standard. Those popular items that fall by the wayside are fads, or become the stuff of cult followings(meaning, while popular enough to be famous or successful to a degree, and remains so, it never gains widespread or long term acceptance or success).  Clothing
 comes to mind as examples we can all understand. Why is it that jeans and cotton shirts are never out of style? Yet plaid pattern polyester(RIP) came and went? <bg>


 Mario