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So my top, sides, and neck are all assembled and in the solera. I braced the back yesterday and it taps at just about the same tone as the top. If I shortened the length of the back braces by lengthening the amount of scallop at the ends, would that raise the tap tone?

I believe that would lower the frequency.
Think of it like a drum head. As you tighten the tuners around the edge and tighten it... it goes up... release the tension and it becomes more bouncy and lowers.


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I thought that with braces, the shorter in length and taller the brace is, the higher the tone, and the longer in length and lower height the brace is more bass?

Interesting. In general I'm with Snow, loose is lower. But as you say Wendy, what happens when the brace peaks are farther apart? More Bass? Closer together, match the higher frequencies better and more Treble?

It seems logical but I don't know. I'll be waiting for one of the tone guys to jump in

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Joe Beaver
Maker of Sawdust

Yes, but you're not dealing with a brace, you're dealing with a structure: the brace glued to the back panel. Removing material from a braced structure generally takes the material from the the part of the brace doing the most work, i.e. the exposed surface of the brace, so in just about all cases by removing material you loose stiffness faster than you loose mass, so the panel frequency (proportional to SQRT(stiffness/mass)) reduces. To raise the panel frequency, you will need to add more stiffness than mass, which usually means making the braces taller or the panel thicker, because stiffness rises as the cube of thickness and height, but mass rises only in direct proportion to thickness or height.

Usually it is better to have the back pitched at a higher frequency than the top on the boxed guitar. But right now you are comparing a free plate (the back) with maybe the top on the rims, or maybe you are remembering the top free plate pitch. Either way, free plate pitches are poor predictors of boxed pitches, due to all the coupling effects. However, the top will have a bridge attached, which usually (but not always) adds more mass than stiffness, which will bring the top frequency down. So you may be OK. Or not.

Building to dimensions on a plan predicts very little about the quality of the finished guitar, because most plans omit (at least) two critical pieces of information - the properties of the materials to use (stiffness, density, etc.) and the target mode frequencies to aim for. Cutting a very long story very short, the material properties tend to influence the volume you can get from an instrument, whilst the modal frequencies have a significant effect on the tone. That's why these things are critical information.

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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 better clarify here. No peaks. This is a classical and I am talking about the braces on the back, not the soundboard. Only the ends are scalloped. Maybe scalloped is not the right word in this case. I’m referring to shortening the length of the brace, hoping that will raise the pitch. The braces are fairly tall and stiff.

Sorry Trevor, typed and submitted before I saw your post. I am referring to the tone of the top before it was joined to the sides and neck. You are right, the bridge will change that. I just got worried when the back tapped at the same pitch that the top had tapped at. I guess I should just proceed.

That is the opposite of helpful. The height of the brace ends where they notch into the lining has the strongest effect on pitch (as well as amplitude of vibration). Tall brace ends prevent the perimeter from flexing, pushing the node line inward, almost like shrinking the size of the back. Although the effect isn't audible on the free plate.

Spool clamping the back onto the guitar seems to be a relatively good predictor for the top/back pitch relation (the mass of the clamps makes both much lower, but the semitone relation doesn't change much after gluing and removing the clamps).

In general, you can always make the pitch lower, but not higher. I've had to chisel all the braces off and try again a few times, due to getting unexpectedly low pitches.

FWIW, I usually use 4-6mm notch height on the back braces. But I'm not sure if that's ideal in any way. Might be better to use 2-3mm notch height and increase the center stiffness further, to get higher amplitude at the same pitch. My guess is that would reduce volume and increase sustain (more energy absorbed by the back and then returned later). Or maybe I have too much amplitude already and it would be better to increase the notch height. Who knows...

well, -regardless, who says the back pitch needs to be higher? what law will you break? obviously, you don't want the back to sound like a useless wet board, but down a whole tone or more isn't a disaster as long as it doesn't steal energy and it adds musicality to the overall sound and the whole guitar works as a unit.

you didn't mention whether or not your binding is on.

It's fairly easy to lower the back tap tone once the box is assembled, so why not go ahead and assemble it?

Generally speaking, you'll drop the tap tone of a plate faster by removing material from the center of the braces than the ends, and the same holds true for thinning the plate: the center has more effect than the edges. You could actually wait until you've got the back on and shave one or more of the back braces by reaching in through the sound hole. It's not as easy as 'free' plate tuning, but more likely to get you the pitch you want. This will tend to lower the 'main air' resonant pitch as well, which is usually seen as a good thing. Of course, if you're using F-holes or some other configuration that makes access through the hole difficult you're out of luck, unless you can train some termites to read blueprints.

The opportunity for interior access (to tune top or back braces or to verify well-seated string balls) seems like the best argument for putting the sound hole in its traditional, but brace-unfriendly, position.

To raise the tap tone the structure must be made stiffer and/or lighter.
Adding wood to the braces will make them stiffer, so that in spite of more mass the stiffness will raise the tap tone.
Removing wood from the braces will lower the tone even though you are removing mass.

I agree with other posters that it may be best to assemble the guitar and compare it to guitars having the sound you want before tinkering with the braces. Sometimes a guitar with theoretical negatives sounds so great that you don't want to change it. Sometimes a guitar where you are sure everything is ideal needs some tinkering when it is done.