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After seeing Hesh's request for what we want to see on the forums, it looks like voicing is at the top of the list for lots of us. I thought I'd post up this so we can have a discussion on the why's of brace shape. After watching the Everett DVD and seeing his approach there's a big difference in what's working for him, and what I see as the "traditional" scalloped braces.

Do we really need all that time carving the scalloped braces to nice pointy peaks? Pros and cons??

Anybody wanna' take a crack at the benefits of scalloped as opposed to parabolic/triangular bracing?

Feel free to take this thread wherever you see fit, as long as it's in the realm of voicing and bracing.

Let's see what we get

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Rob

Rob,
I just finished voicing my most recent top with the help of Edward Dick, a local long time guitar builder. We discussed his views in length. He is one that believes the shapes of the braces, and the locations of the peaks on scalloped braces is what gives a guitar a unique sound. He thinks small things like this contribute to the overall sound of the finished guitar although he doesn't have any statistical data to back that up. But having built over 368 instruments, you really have to believe in his intuition. I love the sound of his guitars, so I believe in what he says. He says you don't have to do, and if you don't you will still get a great sounding guitar, but there are some small overtones and sounds that happen when the braces are shaped like this. I've done all kinds of bracing, including parabolic and lattice braces, and I can say they all sound good, but it really is the details that make great guitars. So I do believe there is some merit in shaping the braces. This is the one I just finished:

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Tracy
http://www.luthiersuppliers.com

The benefit of scalloping is reducing the braces section modulus in specifically desired regions. That was the historic way to get the job done. in more modern parabolic bracing lighter bracing is commonly used and typically the section modulus is adjusted by plane-ing off at the sides of the brace and blending into the parabolic shape.

Actually you should not take scallops to truly pointed peaks, In other words sharp peaks as that induce a stress riser location. You reduce the stress riser by rounding over the tip.

I'm going to have to agree with Al on this one.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

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Formerly JJH

I learn more from my mistakes than my successes

I have a few questions as I am a completely new to building.

The first is how to go about making braces without a radius dish. As buying lots of tools like that is very expensive for a first time thing.

Also how different radius' do different things. I have always been under the assumption a radius top/back is to prevent it collapsing but I would be interested in hearing how different radius' affect tone, structural stability etc.

Actually, I use my brace maker from Tracy. I love it. You can find it by clicking on the Luthier Suppliers link in his signature.

Right-on Rob! Great thread and I'll be back when I catch up on other things.

What are the most common radius' on guitar tops and backs? Do they vary from model size to model size or is it personal preference? I plan on buying a brace maker from Tracy and wondered if there are two radius I can use for all sizes from parlours to dreads.

Coke (or should I call you Zero? <smile>),

I think each mfg comes up with their own standards. On tops the radius is usually larger (meaning the radius is a longer distance so the arch is less). Also, sometimes the top is glued up at a particular radius then "sprung in" when glueing to the top at a different radius (or no radius at all).......so even measureing a finished instrument may not tell the entire story. some folks select a top radius so that the the top has the proper fall-away for a good matchup with the neck angle. Some folks build the top perfectly flat so no radius at all. A 25ft - 28ft radius for a top is fairly typical.

On backs, there seems to be more variation. I've read that Martin uses a 15ft radius for backs on 000/OM models and that they use a 20ft radius on dreadnaught sized instruments. Some folks standardize on a single radius for all their models to keep things simple.

How does the radius affect tone? That's a good question. I've read where more top arch produces a "tighter", more focused sound (maybe favoring trebles) and that the flatter you go the more open, or "loose" the sound probably lowering the resonant frequency helping the bass response. I don't know this by experience so take it with a grain of salt, but it makes sense that the resonant frequency of the sound board drops since the arch stiffens things.

How 'bout the back arch?.......I've read conflicting opinions. I've read posts by some very well respicted luthiers with big names have totally opposite opinions on the affect of changing the back arch.

One thing I think I can state with a little confidence is that arching the top and back plates provides a little insurance against splits in low humidity conditions.

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Formerly known as Adaboy.......

There's an interesting article in the lastest GAL quarterly called; The Guitar as a Structure and Some Practical Information on Bracing by Jim Blilie

An excellent read!

If you're not a memebr of the Guild of American Luthiers I would highly recommend that you become one.

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Michael Lloyd

“I was born to ignorance, yes, and lesser poverties ...
I was born to privilege that I did not see ... I didn’t know it, but my way was paved” – John Gorka

you can make a tool to shape your braces at any radii you wish easily, with a tape measure , jig saw , a spindle sander helps but the sanding could be done by hand. you need an area the lenghth of your desired radius, about 4 feet wide . use the tape as a large compass, hook the ' zero " end on a nail or have someone hold it , then extend the tape to the desired radius (EXAMPLE 28' ), using a pencil , scribe a line by rotating the tape , side to side while keeping it taught . do this with 4 pieces of 3/4 inch plywood, glue them together , then finish sanding to your pencil lines, apply some adhesive sand papaer (ie 220 grit) , this will allow you to radius your braces all the same . Jody

I use a hand plane, and stick a shim on the sole with super glue. With a .05" shim about 110mm back from the front of the throat opening I get a 25' radius, and moving the shim up to 85mm gives a 15' radius. This will depend a lot on things like how heavy a cut you take, and you have to work out the distances for your own setup. Once you get them you can scribe them on the sole of the plane, though. Plane from the center of the brace stock toward the end, and switch ends often. I find it easiest to do three or four at a time, by clamping them together with small c-clamps and using a stop block on the bench.

I like using this method because it's simple, quick, and accurate. One object in using a jig or fixture is, as Brian Burns says, that you can have an argument with your wife and still go out to the shop and do your best work. This still calls for some tool skill: you have to keep the plane from rolling off to one side or the other, but most people get that pretty quickly. For those who can't, you can always clamp the braces to something that will hold them off the bench top on thier sides, and 'shoot' the surface.

BTW, that freshly planed surface will tend to glue far better than a sanded one.

Whenever I bring this up Mario chimes in to say that he does it freehand. That's a good way too, if you've got the tool chops.

I think alot depends on what type of glue you are using , with alphatic resin , sanded edges ie 220 grit works just fine . of course with hide glue the joined surfaces must be planed . jody

I am not expecting a beginner to have the chops to plane braces at a radii. Jody

I certainly don't consider myself a person with tool chops, but I started doing mine freehand on my second guitar, and have not looked back. It takes about 2 or 3 minutes to plane a 25' radius freehand, and I can do 3 braces at a time, which is all I have to do for one guitar.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

On the question about super duper high/pointy scallops vs low scallops...

In my mind, the "Cube Rule" answered the question for me...

A brace section that is 25% taller is 2x stronger.... If your scallop "Peak" is 5x taller than the "Valley" (Say 0.750" vs 0.150") --the location at that peak is 125x stiffer! What does that really get you besides a heavy, stiff, lump... especially if the Stiffness of the "Valley" is actually sufficient (Which 0.150" may not be...) Wouldn't 2x or even 10x stronger be sufficiently inflexible to do the job? That really, the body of the brace should really be significantly shorter...

That isn't to say that I don't believe in scalloping... I feel like it has a perfectly valid place... I just feel like the scallops can be a little bit more subtle and still do the job just fine... Say peaks at 0.500" thick and valley at 0.400" or even at 0.300" thick or some such hypothetical arrangement (Of course, all of this is my mental model, supported only by the slightest actual experience with such things...)

Thanks

John

Now that's what we're looking for John

I get how the peak of a scalloped brace makes that area stiffer, but why do we need to make that one spot stiffer?? Most people's v-brace peaks fall pretty close to the back edge of the bridge plate. Seems to me that the "less stiff" area in the valley being at the same area as all the force applied by the bridge would be what we don't want. Wouldn't we want to make the peak more in the middle of the bridge plate, therefore spreading the bridges load across the stiffer area of the brace??

Or, does the inclusion of the peaks simply make the structure of the whole brace stiffer? Does this give us more strength to weight ratio than parabolic/triangular braces? Any engineers out there wanna educate us??

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Rob

My question about the lows and peaks is this -what is the purpose for leaving the hights in the scalloups just at the location we often leave it ? is there a reason to have it just at that location ? If doing lattice beneath the main X there are about 4 x points where the mass is thicker and in that scenario the peak is then in 4 places mostly in the middle. So if looking at it in this was I guess that to have no peak at all is the best ? Larrivee bracing pattern have his braces leaking towards the middle ! .

A confused Lars.

I think the location for the peaks on scalloped bracing have to do with controlling the frequencies of the modes above the 'main top' mode. Basically, you're working with the stiffness of the top/bracing system in different areas, and the effect on the mode frequencies will depend on whether the brace is bending or not in that location for that mode.

The lowest four modes of the top do most of the work in producing sound, and particularly the 'main top' mode, which is thought by some to be the most important sound producer all the way up to 1000 Hz. That's the mode that works like a loudspeaker, with the whole lower bout moving in and out.

On loudspeakers they make the edges really loose, and try to get the center to move like a piston, with no bending. We can't do that: you need a certain amount of stiffness at the edges, and anyway, we're not trying for the sort of 'flat' frequency response the loudspeaker people are. One researcher, Richardson in Wales, has shown via computer models that a top that is loose in the center will actually move more air in the 'main top' mode than one that is loose around the edges, assuming both tops are stiff enough to take the loads. Scalloping reduces that central area stiffness, and thus accentuates the power of the 'main top' mode, as well as the 'main air' mode that it drives.

The next three modes up also contribute to the tone, even if they are not as effective at producing power as the main top mode.

From what I can see, the 'cross dipole', which is usually the next higher one in pitch, tends to be a 'loser': it's so much less effective than the 'main top' mode that you'd rather not have it stealing energy. You do this by making the areas of the top on either side of the centerline, about halfway to the edge, stiffer, to move the cross dipole pitch up away from that of the 'main top' mode. Meyers (iirc) showed that guitars that had a narrow 'main top' peak in the spectrum tended to sound 'harsh', and too low a cross dipole pitch contributes to that by stealing energy and cutting off the main top mode. So, having the X braces peaked outside of the bridge wings can raise the cross dipole pitch, and make the 'main top' spectral peak broader. I've noticed on the guitars that I've measured that this seems to contribute to a 'full' or 'solid' sound.

The 'long dipole' could be a 'waster' like the cross dipole, but it often couples strongly with one of the inside air resonant modes, and ends up producing a significant amount of sound; often around A on the high E string, where nothing much else is going on. Loosening the area in the center of the top behind the bridge with scalloped tone bars will drop the pitch of the top 'long dipole' mode.

The fourth mode, the 'cross tripole' may or may not show up on any given steel string guitar. It's a power producer, though; generally up around 500 Hz (C on the high E). The peaks on the lower end of the X ,and the tone bars, are in places that probably help to move that one upward in pitch.

The important thing to think about is that each system of shaping the bars tends to establish a certain relatinoship between the mode pitches, and thus to form the spectrum of the guitar's output. That's why scalloped, tapered and flat bracing tend to sound different.

Also, of course, there's no free lunch. Loosening up the top in the center is structurally risky, and also can, if not done right, give too much bass and less sustain. All of this is pretty complicated, and good, stable, balanced, and 'clear' sounding instruments can be built with all sorts of bracing. Still, knowing the strengths and weaknesses of each pattern, and how they tend to sound, can help you build what the customer wants, even if that's just you.

Alan, thanks for that great response. That makes a lot of sense. I'll let it sink in a while, then I'm sure I'll have questions. I really gotta' get into Chladni one of these days....

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Rob

Thanks alot Alan for taking your time to write this !! .

Sincerely Lars.

Another thing to think about, John, is that the scalloping has other effects than just adjusting the stiffness. It also creates an intentionally non-uniform distribution of mass. Of course the mass changes directly, while the stiffness does the cube thing, so my intuition says it's likely to be less significant than the stiffness, but I can also imagine that adding or subtracting mass would have to impact the way the whole system vibrated.

Mike

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Mike Lindstrom

I am sure that leaving little "lumps of mass" can enter into it too. That specific technique has been used in voicing before.. and it does work (Gluing weights to the underside of the soundboard) I have a feeling (Unproven.. just an impression) that once you get past some massive amount of stiffness variation... "Extra" material just acts like weight...

For example...
Say a Scallop Peak is at 1" tall and the "Valley" is at 0.125" tall... In theory, the peak ends up at 512x the stiffness... but that may be meaningless at that point.. It probably isn't bearing any more or less load than when it was a measly 50x stiffer.... so my guess is that it might just end up acting like weight... This is kinda how I think about the center of a "Conventional" bridge -- it is so much stiffer than anything around it, that at some point, it acts more like Mass than like Stiffness...

But.. I am new at this so I could be wrong...

Thanks

John

Hello, all:

This is an interesting and subtle topic -- about scalloping the braces and where to leave exactly how much of a peak . . . Obviously, from all the input that has been cited in this short thread (from luminaries like Dick Boak and Al Carruth, for starters) this is a topic that could engage one for years without exhausting itself.

I have some different thoughts and ideas yet. I think it is of interest that the Martin company was making scalloped 'X' brace guitars at the time of the civil war. Of course, all the guitars it made in those days had gut strings, and the bracing on these guitars was minuscule compared to what it became after metal strings came into general use. (It was actually the Larson brothers, of Chicago, who first successfully designed "X" braced guitars that could hold up to the increased pull of these new strings, beefed up the bracing, etc. But that's another story.)

To the best of my knowledge, scalloped bracing came about as the result of a successful experiment. Historically, "X" bracing was invented at about the same time that Antonio Torres was inventing the modern Spanish guitar that introduced fan bracing to the world: the 1850s. So both of these architectures arose virtually simultaneously and independently. I think it's likely that, initially, no one could claim that they knew what they were doing, and that there was a period of intense experimentation and tweaking of both of these wonderful and potential-filled designs of the guitar.

One such tweak was that some unknown craftsman took a guitar that he wasn't all that happy with and stuck his arm into the soundhole and began to shave off wood. He found that the sound improved! And scalloped bracing was born.

Two things, here: First, I very much doubt that anyone was thinking in terms of Helmholtz resonances, monopoles, specific stiffness, the Cube Rule, or the acoustico-dynamic underpinnings of systematic variations in bracing mass/stiffness. These things hadn't been invented yet, and woodworking was a very hands-on, ears-on, and little b.s. business.

And second, the peaks in scalloped bracing have always coincided with the farthest point an adult's arm could reach into the soundbox through the soundhole: that's where he had to stop carving if he was going to have any control over what and where he removed wood from. That's how I think the peaks came to be there. Furthermore, once the 'success' of that design was noted, it would have been very easy for craftsmen to simply replicate the scalloping on the braces before the top was glued on. And, for that matter, why keep on carving (and possibly ruining something) when the scalloped design worked? 'Let's keep on doing it' would have been the natural thing to do.

One might argue that scalloping the braces was an act of inspired genius on someone's part, of course. And it might have been. Yet, historically, of all the musical instruments that have survived from way back whenever, and that have been studied -- instruments such as lutes, chitarrones, theorbos, mandolins, citterns, all the Spanish guitars which were the early contemporaries of the American 'X' braced guitar, bozoukis, the baroque guitars and chitarra batentes, ouds, vihuelas, bandurrias, violins and violas and cellos (with their soundposts and braces) etc. etc. etc. -- not a single one had bracing that was anything but a more or less straight line in profile (with, of course, the scalloping-down at the very ends where they met the linings. No one 'scalloped' anything prior to the ends of a brace. So I'm really inclined to believe that it was a retro-fit type thing, and not part of an initial design.

I am in full agreement that it makes little sense to have a brace that is 125 times stiffer in one part as it is in another part two inches away. So if you're going to have scalloping, let's be moderate about it. But if there's truth in my understanding of the birth of the scallop, then a really good question to ask might be: why does one need one at all?

I have to stop before embarking on a discussion of what is "good sound". We'll be here until Obama's kids are retired from office. Let's just agree that there are many kinds of 'good sound'. But, for us lutherie nerds who like to fuss over details, the question remains: what are the scallop peaks there for in any other way than vestigial reminders of a retro-carving operation that some smart craftsman thought to do many years ago? I found that I couldn't answer that question some time ago, so I gave up the scallop peaks. My guitars work (I refer you to the first three sentences of this paragraph).

Cheers, Ervin Somogyi

P.S.: I've written two books this past year in which I discuss a lot of this material at some length. I recommend them.