ToddStock wrote:
"Al C. is the guy to address this question, and cover the conflicting requirements of strength and role as 'graphic equalizer' for the top."
....in my dreams.....
Premise #1: The 'usual' bracing schemes are standards because they work well (whatever 'well' means). A lot of people have been experimenting with them for a long time. If there were huge improvements to be made with standard materials, they'd have been made by now and _those_ would be the standard designs.
Premise #2: Factories have to build with the expectation that the weakest set of bracing is going to end up on the weakest top. They can't take the time to test their wood for strength, and must over brace as a matter of course to be safe. You can probably reduce the sizes of normal factory bracings a bit. A BIT!
Premise #3: You'll get the best results if there is some sort of rough balance between the bracing and the top. You could make a top that was thick enough to take the load with no bracing, but that would weigh a ton and have no sound. You could make a top of tissue thickness, and put on really heavy braces, but that would probably sound pretty uneven, and, I suspect, 'tight'.
My friend Mark Blanchard always says to remember that the top is the sound. The bracing is there to help out, and fine tune, but it's the top that either makes it or doesn't.
So: start with your top, and try to figure out how thick you need to make it so that it will work right. If you can test the wood properties, that's a real help. Otherwise, use any info you can get: tapping, feeling the flex, chicken entrails, whatever. Get some idea of how this top 'wants' to work. If it's light and soft you might want to leave it thicker. If it's dense and stiff (Red spruce!) then thin it out some more. If you're looking for a bassier sound you might want to graduate it thinner toward the tailblock; for more 'brightness' thin it toward the wings. Or look for a top with a different stiffness ratio.
Think about what you want the guitar to do. If you're making a Dread for Bluegrass, then you will want to at least think about scalloped bracing. This will give you the 'thump' you want. If it's a fingerstyle box you might want to go to some sort of 'tapered' bracing scheme.
If the top is floppy in the cross grain direction you might try opening out the brace angle a little, so that they run more 'across' the grain. If it's stiff, go the other way.
If the top is soft you might want to make the bracing wider for more glue surface, but generally the highest stiffness/weight ratio comes from tall, narrow bracing. OTOH, don't bother to get too carried away: all of the bracing typically only amounts to about 1/5 of the weight of the top without the bridge. Generally the bridge weighs about as much as _all_ of the bracing put together. Shaving a couple of percent off the braces is not going to help you a lot. In any event, the tallest, narrowest bracing, on lutes, is usually five times as tall as it is wide, so don't exceed that ratio.
Don't get too carried away, adding lots of braces, particularly if they're tall and narrow. Structurally al you're trying to do is control the amount of 'belly' and, even more important, 'sink' in the top. 'Control', not 'eliminate'.
Most steel strings use asymmetric bracing, and that gives the 'normal' sort of sound. Symmetric bracing tends to work a little better for more 'modern' or 'progressive' styles, like fingerstyle or jazz. Maybe.
When you're voicing, remember that the bracing at the head end is mostly there for strength, and doesn't effect the tone much (yeah, I know, the 'popsicle stick'. I don't use it). The bracing near the tail block doesn't have as much effect on the strength, but effects the tone a lot. The critical area for strength is between the bridge and the soundhole.
Cap the X crossing(s)!
All of the statements above reflect the opinions of the poster, and may have little to do with reality as you experience it. Subject to change without notice. Caveat emptor. De gustibus non disputandem. Cogito ergo sum. Veni, vidi, vici.
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(Guitar Shaped Object) A known plan is essential in this biz, to start anyway.