Scalloped vs parabolic braces?

[Alan Carruth]

The person who did the most to publicize the 'parabolic' notion advocated using that profile both in section and along the length of the braces. If memory serves, he was primarily re-working straight-braced tops, and may not have tried to meld the parabolic concept with scalloped bracing. I could be wrong: it's been known to happen...

[murrmac123]

Intuitively, it would seem to me that a catenary profile would be more efficient than a parabolic profile.

Please don't ask me to justify that notion mathematically ...

[rogthefrog]

Originally Posted by redir

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?

Interesting, I wasn't aware of the term being applied to the cross section. I was thinking of braces shaped like an open umbrella longitudinally. Convex. Regardless of their cross section shape.

Regardless, I couldn't care less whether "parabolic" is a mathematically correct descriptor. I'm just curious about convex vs Golden gate bridge profiles.

[Bruce Sexauer]

In cross section my standard brace shape, at least in my mind, is the one described as parabolic. Except when I make Gibson inspired guitars, when I purposely make them clunkier.

[rogthefrog]

It seems I've been using the wrong term(s).

What I'm inquiring about is the difference between this, which I've been calling "parabolic" (or "tapered"):



and this (lower bout braces), which I've been calling "scalloped" or "Golden gate bridge":



(ignore the holes, and the different brace layouts; I'm just asking about the shape of the braces)

I.e. (forgive the horrible Paint job):



The top version seems more "logical" to my untrained brain, as the brace's strength drops gradually as it is less needed (going away from the bridge). Especially compared to the bottom version, which alternates shorter and taller points along the brace and thus would seem to cause dead spots (at the peaks).

Now obviously I'm missing something, because as far as I know the peaks-and-valleys style (bottom) is very common; Yamaha, Martin and Taylor all offer it, so that's potentially a lot of guitars with that type of bracing, and clearly it's a good way to do things.

Hence my confusion.

[dekutree64]

Originally Posted by Bruce Sexauer

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.

If you don't mind sharing, I'd love to hear more of your thoughts on scalloping. Obviously works very well, but it's never made any logical sense to me. Why would you need the stiffness to increase as you move away from the bridge? To my mind, the bridge is where stress is highest. As you move away, that stress is spread over a larger area, so the stiffness can be lower, on and on until you reach the perimeter.

With scallops, the span between the bridge and the peak must be at least the minimum survivable stiffness, which means the peak is adding unnecessary stiffness above that. And that extra stiffness at the peak causes more stress to focus back onto the weak point closer to the bridge, so then that needs to be beefed up a bit as well.

But that's working from the mindset that minimum total stiffness is desirable, which may not actually be the case, especially for larger guitars. But then what criteria do you work to, which calls for higher stiffness in the area of the peaks? More like a system of rigid elements connected by hinges, rather than a continuously flexible surface? Or is it specifically related to the difference in long grain versus cross grain stiffness of the plate? That is, if you project everything onto a horizontal line, do the peaks come up just as the bridge wings go down, to carry stress horizontally to parts of the soundboard that would otherwise be taking up less than their fair share?

[rogthefrog]

Originally Posted by dekutree64

If you don't mind sharing, I'd love to hear more of your thoughts on scalloping. Obviously works very well, but it's never made any logical sense to me. Why would you need the stiffness to increase as you move away from the bridge? To my mind, the bridge is where stress is highest. As you move away, that stress is spread over a larger area, so the stiffness can be lower, on and on until you reach the perimeter.

With scallops, the span between the bridge and the peak must be at least the minimum survivable stiffness, which means the peak is adding unnecessary stiffness above that. And that extra stiffness at the peak causes more stress to focus back onto the weak point closer to the bridge, so then that needs to be beefed up a bit as well.

That's exactly what I'm wondering, expressed much better than I did.

[murrmac123]

Originally Posted by Alan Carruth

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

Actually, Bruce didn't write that ... there was no "about" in the sentence.

This is because he was referencing Senator Everett Dirksen's immortal quote , which he made on the Johnny Carson show ..."a billion here, a billion there, pretty soon you're talking real money".

The quote loses all its pithiness and impact if you say " ....talking about real money "

And now, back to your scheduled programming.

[Bruce Sexauer]

Originally Posted by murrmac123

Actually, Bruce didn't write that ... there was no "about" in the sentence.

This is because he was referencing Senator Everett Dirksen's immortal quote , which he made on the Johnny Carson show ..."a billion here, a billion there, pretty soon you're talking real money".

The quote loses all its pithiness and impact if you say " ....talking about real money "

And now, back to your scheduled programming.

Using the "quote function" here in the forum works well to avoid this kind of thing, and it's easy! I have found I get misquoted quite a bit in the forums, often obscuring my original intent far more than in this perfectly valid example.

The difference between an acceptable guitar and a great guitar really can come down to a few grams in (or not in) the right place IMO; this is a great example of the hand builders' advantage . . . Or disadvantage, depending on the hand builder.

I do not have time to do the complete run down on my concept of top plate bracing here and now, but I will add a couple of tidbits for the deep thinkers among you.

I view the top as a membrane. It is all about integrity of the membrane. The membrane has greater integrity fore and aft than it does side to side. The bridge footprint already has 99+% membrane integrity, and this integrity projects further fore and aft than in other directions. The area of the top with the least membrane integrity is halfway between the high integrity bridge and the high integrity rim in the cross grain direction. Assuming mission critical structure is handled, the big issue getting tone and balance in a guitar is the even distribution of structural integrity within the membrane. IMO, the vast majority of guitars I have ever seen are vastly overbuilt where mission critical structure is concerned.

I will probably say this better next time I try.

[kkrell]

Would there be any objective measure of bracing effectiveness? By this I mean, strength testing at various points of the top, perhaps using some type of strain gauge. Push on the top at those points and determine resistance to movement (from either/both above/below), and either design, place or modify bracing profile, shape & thickness until a desired result is achieved.

[redir]

I'm probably wrong about my take on parabolic meaning 'in cross section'. I think I see what you are talking about now withe the analogy to umbrella shape. That would almost be the opposite of scalloped bracing really. It seems too stiff to me but I guess it depends on what you are after. The picture above of the fan bracing with that shape seems fine but I don't think I would want that shape for lower cross struts. It just seems too stiff in the center where I usually like to lighten it up a bit.

The way I think of scalloped bracing which does indeed look like a suspension bridge design is to just brace the guitar as if you were going to brace it with straight bracing. Then scallop out the middle portion to get the tap tone or deflection or chaladni or what ever you are going for.

Here is a shot of the tapered bracing that I like to do. This isn't finished mind you, I would finish it off by rounding the bracing tops to be so-called parabolic... But as you can see the cross struts start off full height and taper down to near zero at the ends.

[charles Tauber]

Originally Posted by kkrell

Would there be any objective measure of bracing effectiveness? By this I mean, strength testing at various points of the top, perhaps using some type of strain gauge. Push on the top at those points and determine resistance to movement (from either/both above/below), and either design, place or modify bracing profile, shape & thickness until a desired result is achieved.

I use my thumbs to do exactly what you describe. When it is like "this" it is "right" for what I want.

Others do as you describe and use quantified deflection testing. Others use response to vibrational testing (e.g. Chaladni plates). Probably more repeatable and more easily taught to others without first-hand experience.

My approach is along the lines of what Bruce describes: a membrane with "infinitely" stiff edges, a very stiff brace in the middle of the lower bout (i.e. bridge and plate) and a huge hole in it that adds a stress concentration while the whole membrane is trying to be folded end to end. Appropriate stiffness needs to be added between the very stiff middle and the very stiff edges.

The stiffness (resistance to bending) of a rectangular beam varies with the cube of its height. I want the height of the braces to diminish very quickly towards the very stiff edges: scalloping does that, while "parabolic" shaping does so much more slowly and less effectively for my purposes. Unlike Bruce, in addition I thin the edges of the top in the lower bout to aid in reducing the stiffness of the "membrane" towards the edges. Lots of ways to achieve one's desired result: that seems to work for the result I want.

[Alan Carruth]

My apologies to Bruce for the mis-quote. Somehow I've been having problems getting my computer to highlight things when I want to quote them, so I redacted the sentence myself. I haven't thought about Everett Dirksen and his eyebrows and basso voice in years.

rogtehfrog:
You got the terms right, so far as I can tell.

kkrell:
David Hurd, in his book 'Left Brain Lutherie' discusses objective testing of top stiffness and what he considers to be 'desirable'.

Guitars have been made with all sorts of brace profiles that sounded good to somebody. Each scheme has advantages and disadvantages. I've been using a 'tapered' profile because it makes sense to me and gives the sound I like. It makes sense because, as has been pointed out, it puts material where the stress is, and less material where it's not as needed. Scalloped bracing has the drawback that it can be weak if you go too far with the scalloping, but in the right hands it works well. There are far more scallop braced guitars in Bluegrass jams than taper braced ones; they sound different, and each matches better with a different style. Note that it may not always be desirable to make the lightest possible top, or the stiffest, or whatever. The trick is to match up the top with the music.

Looking at the top as a membrane, I try to get it as smooth as possible, with the 'right' stiffness and mass distribution for the music. Stick a wad of poster adhesive on a banjo top some time to get an idea of what 'lumpy' sound like. Of course, the peaks on a scalloped brace are 'lumps' of a sort, as, for that matter, are the peaks on a 'tapered' brace, or the bridge on any top. Again, each different scheme has it's place, and the objective as I see it is to get each top to work as well as it can within the limits of the desired style. Working 'well' is a moving target, since different people have different ideas of what 'good' tone is. "If everybody liked the same thing they'd all want my wife".

[Bruce Sexauer]

Originally Posted by redir

I'm probably wrong about my take on parabolic meaning 'in cross section'. I think I see what you are talking about now withe the analogy to umbrella shape. That would almost be the opposite of scalloped bracing really. It seems too stiff to me but I guess it depends on what you are after. The picture above of the fan bracing with that shape seems fine but I don't think I would want that shape for lower cross struts. It just seems too stiff in the center where I usually like to lighten it up a bit.

The way I think of scalloped bracing which does indeed look like a suspension bridge design is to just brace the guitar as if you were going to brace it with straight bracing. Then scallop out the middle portion to get the tap tone or deflection or chaladni or what ever you are going for.

Here is a shot of the tapered bracing that I like to do. This isn't finished mind you, I would finish it off by rounding the bracing tops to be so-called parabolic... But as you can see the cross struts start off full height and taper down to near zero at the ends.

IE: In the pictured top bracing I question the need for stiffness or mass in the area between the bridge and the tailblock, particularly in the node which is nearly under the bridge.

[redir]

Originally Posted by Bruce Sexauer

IE: In the pictured top bracing I question the need for stiffness or mass in the area between the bridge and the tailblock, particularly in the node which is nearly under the bridge.

What would you do differently? I see a lot of deformation behind the bridge and like to strengthen it there and weaken it out to the sides. But also that was not a finished pic. I admit I don't know what I'm doing and guess a lot and almost always take shavings off before closing the box. Which then renders some of my data useless like deflection and weighing all the parts. But I only started doing that recently and the 'gut feel' method of building is still in my blood. That guitar actually turned out good but it's natural resonance is right on E. Which is kind of cool in a way but also can get in the way.