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To me it's much more a matter of what the Brits call 'horses for courses'. I've used all sorts of wood for tops, and what's important to me is matching the wood to the shape and intended use of the guitar.
If you were lucky enough to see Mark Blanchard's talk on plate tuning at the last H'burg do, he has a great way to find the 'correct' body shape for any particular top. What he does is to join the top, and thickness it a bit on the heavy side, before cutting it to the shape of his largest guitar pattern. He then looks at the Chladni paterns. There are three in particular that he looks at: one that is mostly bending crosswise, one that is mostly bending along the grain, and another that has some of each. Ideally the lengthwise bending mode he looks at actually is bending in both directions as well, and forms a closed 'ring' pattern in the lower bout. If this is the case, and this pattern is not too far from the frequency of the crosswise bending pattern, he will use the top for that shape. If the ring is not closed he will cut the top down to the next smaller size, check that, and so on until he gets the pattern he wants. The third mode he looks at also ideally has 'closed' rings, and is a further check.
What he's looking at is the ratio of lengthwise to crosswise stiffness. Larger guitars are typically wider in relation to their length, and the top wood needs to have high cross grain stiffness in order to close the ring. If the cross wise stiffness is not sufficient for a wide lower bout, then it might work with a narrower one, and cutting the top down successively to smaller sizes matches up the wood characteristics to the shape. Mark notes that it's pretty difficult to get a top to work right by changing the bracing: about all you can do is 'fine tune' it. If the modes won't close on the unbraced top it will be hard to get them to close on the braced one. I measure the lengthwise and crosswise stiffness of the wood directly, and I'm now in the process of figuring out how my data ties in with his.
One thing that seems to relate to the intended use of the guitar is the density of the wood. This has a pretty direct relationship with the lengthwise stiffness, but none to speak of with crosswise. Since the limiting factor in how thin you can make the top is the stiffness required to resist static bridge torque, you can use lengthwise Young's modulus data to settle on a safe thickness for a given piece of wood. Generally speaking the denser woods will end up being thinner, but also a little heavier, at a given stiffness. This seems to bear on two things: high frequency tone and 'headroom'.
A thick, low density top will tend to have a lower 'coincidence frequency'. That's the frequency where the wave length of a bending wave in the top matches the wave length of sound in air. As this frequency is approached the top becomes very efficient at putting out sound. Having a low coincidence frequency helps to give a solid high end sound, I think. I tend to save my lowest density tops for classical guitars, where the low all up weight makes them eaiser to drive with the low string tension, and the high frequency output adds clarity.
The heavier the top is the higher the impedance on the average, all else equal. This means that it's a little harder to drive, but also harder to over drive. This is great on a guitar that's going to be hammered. I use the denser tops on flat picking guitars and 12-strings, and on 'parlor' sized instruments, where the reduced span allows you to make the top thinner and still keep the stiffness up. In these cases there will be plenty of high-end energy in the strings anyway, and you can use a the dynamic range.
There are a lot of other considerations as well, but that's a good start. I've used some pretty open grained and off-quarter tops from time to time, when I thought I could get them to work in a particular case. The really skew cut ones have found a home as harp soundboards, where the resistance to cracking is useful and cross grain stiffness doesn't matter anyway.
To me the biggest problem is cosmetic; and it's not so much an issue for me as it is for the customers. They've been fed all of the legends about tight grain and so on for so long that it's impossible to argue with them. Besides, they're always right, right? I have no idea how many 'Master Grade' tops I've gotten that were so floppy in the crosswise direction as to be hard to find a use for, but they looked really good. OTOH, I've had some really funky looking Red spruce tops, with about eight lines to the inch, red stain marks, and 'racing stripes' of hard grain, that were acoustically killers. Maybe some day people will learn to listen with their ears rather than their eyes.
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