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A number of luthiers have explained how you thickness your top plate until it flexes/loses structural integrity, and then build it back up again with bracing. But what about the back? Do you take it to that same level of floppiness, or does the fact that you're working with hardwood make that impossible?

And if so, how do you know when your back is thin enough?

Thanks in advance

Steve

Some like a little thicker back for more projection and others like a little thinner back for a more enveloping response. I lean toward the latter but it varies with the type of wood I'm working and the sound I'm going for. Some woods like ebony are heavy and I thin them more so the guitar doesn't end up too heavy. Light woods that might get too flexible like some mahogany I leave thicker than what I normally do. I don't let the backs get floppy but I like some fairly easy flex before I brace. I want to hear a nice musical tone rather than a thud when I tap it. I think if I voiced the backs like I do my tops they might end up too thin.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

My back range from .095 to .105 . I find this a good range .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

.90 to .110 is what it usually ends up, but depends the piece and feel of the back. I leave a little after bracing for some tuning and sanding. I base some of it also on the type of wood. Coco isn't Koa, which isn't IRW etc. I have found that each species has a different feel and maybe a different thickness on an an average, not much and maybe only a few thou. But that may just be me, but workes so far.

I tap it like the top. Have no clue what listening for, but it looks good. Kind of like lining up a putt with the putter. But as you thin it and get to about .115 or so, tap it and feel it. This can depend some on your bracing, type of bridge plate, which I have changed back to a style used by a great builder I worked for a few years and therefore somewhat of a thinner back. But that still requires me to feel and hear the back and what it will do, or think it will do anyway.

Some of it is just build and find the thickness of each piece and species of wood your using. That takes some time. Might start a little thicker and go from there. Easy to take some off, but to thin, for sure can't put it back.

Steve;
Are you talking about dreadnaughts?
OM's or classicals?
I've measured many great sounding guitars back thickness and found them to be on the thick side.
.110 -.135
Since then I've made mine thicker & my clients & I love the sound.
The back is not robbing from the top by flexing to much.
Only so much air can move in & out of a guitar-I want the top to do
that moving.
Mike

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Mike Collins

Mike - were those measurements taken from steel strings, classicals or both? Thanks.

Hi Mike, in this case I'm talking specifically about OMs, but I'm curious about the larger principles. You thin the top till it "flops". You break it down and then build it back up.

Can you do the same with the back, or does the fact that the back is hardwood change everything? If you thin it till it flops, have you gotten too thin?

Thanks

Steve

Those with a technical inclination might be interested in this paper, which addresses some of the issues. Wood has very variable material properties, even within species, so thinning to a specific dimension can give pretty variable results.

http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PMARCW000012000001035001000001&idtype=cvips&prog=normal

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I used to go by feel mostly, and tap tone. Currently, I'm building a set of three guitars (EIR, curly Claro and quilted Sapele). When I thinned out the backs, I was going for particular tone when tapping. I think I was pretty close with all three. I don't have my notes with me, but I think I was from about .080 for the EIR, 0.90 for the Sapele and .100 for the Walnut.

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I'd like to be able to prove, just for once, that money wouldn't make me happy...