Official Luthiers Forum!

I realize we taper back braces to conform to the dome of the back, but I don't think I've ever seen the top surface tapered. Seems to me that full height (less the dome allowance) is not needed. all the way across the brace.

So, why?

Thanks!

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Peter Havriluk

1930's Martin 00

I tend to make back braces level across the top surface, with a scoop at the ends. The makes them taller in the center than at the point just before the scoop, but not a whole lot. I do know folks who will actually scallop back braces, making the lower in the center, to lower the pitch of selected back resonant modes. Much depends on how you understand the guitar to work (there are competing models)and what you're trying to do.

Thanks, folks. Nice to see the notion-supporting picture. Not much weight-saving is involved, but this might be a place to remove non-performing material. Scalloping I'll leave to folks who know what they're doing.

Competing design models? What on earth for?

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Peter Havriluk

Tapering the tops of back braces may have been considered an unnecessary refinement by manufacturers. Making the back "lighter" may not add anything to the guitar (and in some cases may be detrimental). Why did Martin give the size 1 five back braces?
I am working on some "romantic" guitars and you can see the influence of the violin makers who often made them - simple brace cross sections, linings that are barely larger than those used on fiddles, and bindings that are only as deep as the plates (so if you need to remove the back you can remove it and the binding together).
I think general practice often comes from previous experience and simplified construction methods. Most makers probably didn't feel the added work of tapering the back braces was worth it.

I'm trying to pay attention to construction nuances that manufacturers avoid as not worth their trouble. I hold the opinion that mass-produced guitars are not models to be copied by us basement builders, and we can have a positive influence on the results by paying attention to small stuff, like tapering back braces. And the 5-back-brace Martin I suspect was replaced by a 4-back-brace design because the four-brace design worked okay and doing five braces had an extra and generally unneeded step for a manufacturer. I think manufacturers like keeping the parts count as low as can be sold.

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Peter Havriluk

I've always felt the back should be 'tuned' similar to the top. I am still working at what values to use but in general I try for the braced/carved back to tap at 2-4 semi-tones below the braced/carved top.

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Joe Beaver
Maker of Sawdust

Back tuning for flat tops has proved to be something of a dark art. With archtops it's pretty easy, once you know how to tune the plates, but flat tops vary too much. If you can settle on a brace scheme, stick with it for a while, and keep good records you can eventually figure out what modes to look at and how to place the frequency relationships, more or less. It will vary for different sizes and shapes, though. At this point, having worked my way through four or five different back braces schemes in the hope of finding one that would more or less mimic the modes you see in a top, I've gotten down to using four cross braces, with the upper two tall and narrow, and the lower two low and wide. CFM1 knew what he was about. Fortunately, the back has a lot less of an effect on the tone than the top, so you can get away with more.