Official Luthiers Forum!

Usually, for guitar tops, most of the mass is in the top plate and bridge whilst most of the stiffness is in the bracing. For structural integrity we're mostly interested in stiffness and creep resistance, so that deformations don't get out of control. For the acoustics (tone, if you like) we're interested in the ratio of stiffness to mass, because that's what governs the modal resonant frequencies. It's those resonances that are responsible for the sound you hear. A guitar top is, of course, the total structure, panel plus bracing and how its mounted, and it is the ratio of stiffness to mass of the whole structure that matters. However, if you want to lose mass, you'd thin the top rather than attack the bracing. If you want to lose stiffness, you'd go at the bracing before going at the top.

The full story is somewhat more nuanced than the short form here, of course.

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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

Colin North wrote:
"I've read estimates that 80% of a top's stiffness comes from the bracing in a martin type X braced guitar."

That sounds about right as a 'seat of the pants' guess. It also points up the reason why it's just about impossible to say how thick you should make this top. When you give that number for a Martin top, is that a scalloped braced top, or a straight braced one? Dread or 0 size? Six or 12 strings? Do you suppose Gibsons or Taylors come out what that same percentage, or does it vary a little? Some guitars vary a lot from that sort of proportion. On a typical steel string the top plate itself might account for 70% of the mass of the total system, minus the bridge, with the rest coming from the bracing. On a Smallman 'lattice' braced top I suspect the proportions are nearly reversed, with the top, which is only about .8mm thick (.032"), most likely accounting for only about 30% of the total, if not less. Different bracing schemes achieve the needed stiffness in different ways, with different amounts of wood, and they all can work well enough. In the end, every maker has to work out the system that gives the result they want, and the top thickness will be part of that system. Most builders do tend to stay pretty close to 'standard' models, and the designs are robust enough to work pretty well with a variety of thicknesses and wood properties, within reason. That's why you can make an acceptable guitar most of the time by sticking with the plans. If you're looking for better than just acceptable, or want to deviate from the standard models, you'll have to dig a bit deeper, and that's a lot of work. Trevor's books, and others like them, can help, but you will still need to work it out for your way of doing things.