Scalloped vs parabolic braces?
 

As far as I can tell, scalloping seems to be the most common way to shape top braces, but some builders use "parabolic" or convex braces. Is there science or lore behind each shape?

Yes, there is. The science probably isn't much better than the lore.
Not sure what you mean by parabolic, some builders carve the top or back radius into the braces and glue them in a radius dish. Other builders leave the braces flat and glue them in a radius dish, relying on the glue joint to maintain the radius. Some scallop, some taper, some use triangular, or any combination that provides the stiffness distribution for which they are looking.

Sorry, I meant the top shape of the brace (the side you see, not the glue side).

By scalloped, I mean the ones that look like the Golden Gate bridge.

By "parabolic" I mean the ones that look like a flat-ish hill.

Broadly speaking, there are three types of brace height profile:
'scalloped', 'straight', and 'tapered'. 'Parabolic' bracing is a variation on 'tapered' bracing. Each one has some effect on the tone of the guitar.

'Straight' bracing is pretty much the default in terms of production; you make all of the braces level along the top surface (away from the glue face), except for where they taper down at the very end. They can vary in cross section; anywhere from the sharp (or ragged) edged rectangles common in low end instruments to rounded over, to triangular. The glue surface may be cut to a radius to force the top into a dome or cylinder shape, or it may be flat, at least a built. Most guitars can be thought of as having started out with straight bracing, and have had wood removed to lower the brace height in some sort of pattern to affect the tone. Usually any such subsequent profiling is done in the lower bout, for the most part. 'Scalloped' bracing is cut down in height near the center of the lower bout under the bridge. This leaves peaks further out where the edge taper starts. 'Tapered' bracing, by contrast, is left tall in the center and under the bridge, and is cut away more as you go outward from there.

In contrast to straight bracing, a scalloped top is easier to move in the center. This favors motion of the top in the middle, and tends to move more air at lower frequencies. Scalloped braced guitars tend to gain in bass and have a sharper attack and shorter decay of the sound. When well done a scalloped top can also have very good highs. There is some risk to this: scalloping removes wood from the braces just where the stress from bridge torque is highest, and it's easy to go too far. It can take a while for the sound to really develop when the scalloping is done on a completed guitar. If you stop when it sounds 'right', it can end up that you've taken too much wood off and weakened the top.

Tapered bracing leaves more wood where there's more stress, so it's a safer option in that respect. Because the bridge area is stiffer, and perhaps somewhat heavier, it's harder to move the top there, and particularly at lower frequencies. Thus tapered bracing can give a more 'treble balanced' sound. It also tends to have a softer attack and longer sustain than the same mass of bracing in a 'straight' or 'scalloped' profile would.

Keep in mind that the brace profile is only one aspect of the whole construction of the guitar. It's an important one, of course, but in many cases other things can be manipulated to bring up aspects of the tone that the bracing may not favor. There are lots of ways to make a good guitar.

Thanks, Alan, that's great information.

What I was trying to reconcile in my head is the overwhelming popularity of scalloped bracing, which means it works extremely well, with my uninformed analysis that the peaks would cause "dead" spots, since they have the most mass and thus discontinuities in the top's ability to vibrate.

I'll probably do my first ones tapered because it looks a lot easier to do (and less risky, per your description) and try scalloped with #2.

"Parabolic" with regard to bracing is a useless term coined a few years back in order to make something that is convex sound more techie and mathematical. No one claiming to use it has ever plotted a parabola from a quadratic equation and cut his or her brace to that shape, much less produced any science-based reason why anyone should want to do so. In a word, it is hyperbole.

That's why I put it in scare quotes.

Regardless of the name, anything interesting about that shape v. scalloped that Alan hasn't covered?

The claim with 'parabolic' bracing is that it would give you similar stiffness with less weight than other types of profiling. That may be true in theory. "In theory there is no difference between theory and practice, but in practice there is".

Aside from the issue that Howard mentioned, that in practice the bracing was not really 'parabolic', there are two others I can think of. One is that a 'perfect' parabola may or may not put the stiffness where you want or need it for a particular top. Tops are made of wood that varies in stiffness from one point to another, and you need to accommodate by varying the brace profile to get the top to work 'right' (whatever that is!). The other issue is that, for an average top, the bracing only makes up about 25%-30% of the total weight, not counting the bridge (which, in some cases, can weigh almost as much as all of the bracing). If 'parabolic' braces have, say, a 10% higher ratio of stiffness to weight than the next best, then that's going to save you 10% of (at most) 30% of the weight of the top, or about 3%. If you're looking for a light top you'd do better to just find a light piece of wood to make it out of in the first place. Besides you're not always looking for the lightest top; if you were we'd all be using Western red cedar, or balsa. As Howard said, it was pretty much all marketing, of which we see a certain amount in this business..... ;)

:)

The more I learn about this, the more it looks like black magic.

"The more I learn about this, the more it looks like black magic."

Oh yeah.

I finished building my first guitar last fall. I read about a dozen books (the best were the Gore/Gilet books) and many hundreds of pages of web blogs and forum posts and tried to be scrupulous about following best practices. The finished product has a few cosmetic glitches, but I hoped it would sound comparable to my store-bought guitars, a Larrivee L-03 and a Pono 00.

When I finally strung it up and played it, it sounded terrible. Dull, lifeless, and drab. But I built it, so I should play it. And I did. And after a few weeks, in the matter of a day or two, the sound evolved dramatically. It now sounds (at least to my ear) just excellent, loud with musical resonance and great sustain. I like it far better than my commercial guitars.

I'm now half-way through building my second guitar. I've dealt with the cosmetic issues and I hope (fervently) that it will sound as good as my first guitar. But I have no idea whether it will.

Black Magic.

Hyperbole, another shape I have to try!

Brilliant.

I always enjoy reading what you have to say.

This is the first I've heard of "parabolic" braces. It does occur to me that the visual effect would be generated by a straight brace if the plate had curvature in it, and the more curvature in the plate the more this effect would be noticeable.

There is another way to look at scalloped braces than that previously described. When I look at many Luthier's scalloping systems it is apparent to me that they do not have the same understanding of the purpose that I do, not that mine is "right" just because it works in my mind. The goal is to remove stiffness where it is not needed, and just as important, to remove weight where it would impair the response of the string's limited energy input. Where the peaks and valleys of the scalloped brace are located relative to the bridge, the rims, AND the long grain vs cross grain in the top are the major factors. IMO, of course.

A gram here and a gram there, pretty soon you're talking real weight.

Bruce wrote:
"A gram here and a gram there, pretty soon you're talking about real weight."

Of course, it is all about that gram here or there; the trick is to have the wood in the right places. Since tops differ from one another any scheme that uses a pre-set brace shape might work well in some cases, but is bound to be less than optimal sometimes as well.

My impression is that local stiffness is far more important than local mass. We use bracing because it gives stiffness with less mass, as compared with just leaving the top thicker. The trick, it seems to me, is to get that stiffness where it's needed for the particular top, and not to make it any stiffer than it needs to be the do the job you want.

In some cases very small changes can be significant. I use the 'Chladni pattern' method to look at the distribution of mass and stiffness in the top. I once made a Classical guitar that had an unusual shape for one of the patterns, and I put it together to see what difference it made. It ended up being extremely uneven, with the high E in particular sounding like it came off a completely different instrument. On the next couple of Classical builds I looked at what made that shape happen. I found that removing small amounts of wood at the lower ends of the outer fan braces could shift it to a more normal mode pattern, while taking a small amount of wood off the center fans above the bridge brought it back. I used a small finger plane to remove a couple of shavings of wood about 2" long from each of the four outer fans, and the guitar tamed right down. A few tenths of a gram of wood in the wrong place was all it took to make a basically decent instrument sound really bad.

My understanding is that the term is used to describe the cross section of the brace once it's finished. It has nothing to do with the arch of the top and really nothing to do with scalloped bracing too. One can carve parabolic shaped braces into scalloped braces if they want. It's just a figurative word. A word used to describe the shape of the brace in cross section to be like a parabola. I think it's a useful term to describe the bracing shape.

The best guitars I have built have been with tapered bracing. When I first started building I had this idea in my head that scalloped bracing was rather silly. Why loosen up the top where it's strength is needed the most? Why have this big huge mass of a brace so close to the rim that scallops back up to the bridge? The rims themselves are a sort of brace. That's the strongest area of the top and no doubt the rims stiffen a good portion of the top around it. So tapered braces are strong where you need them, the center and weakest part of the top, and then taper off and get weaker and weaker as they approach the stiff rim.

Whether or not that is accurate is beside the point, it's what I adopted and has worked well. I have built a few scalloped braced guitars and will continue to do so. I just built a tenor guitar with scalloped bracing thinking that such an instrument would benefit from having more bass response. But so far I really like the tapered bracing.

All the braces on the guitars I build will have a 'parabolic' shape to them. Is that really even a new thing?

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