by Alan Carruth » Mon Jul 29, 2013 11:33 am
When I'm sorting out my wood, I put all the stuff with the lowest density into the 'Classical' pile. Usually WRC is lower in density than the spruces, with those going from Engelmann to Euro to Sitka to Red, in order from the lowest to highest density. Those are averages! There's so much variation within any species that the only way to know for sure is to measure. Fortunately, if you're starting with rectangular pieces, it's pretty easy to find the density; you just take some measurements to get the volume and find the weight. Since I work in metric for this stuff I use kg/meter^3. The usual range is roughly from 300 to 550, and I like to use stuff lower than, say, 400, for Classicals.
It happens that the Young's modulus along the grain for softwoods pretty well tracks the density. Young's modulus (E) is a measure of potential stiffness: any two pieces of wood with the same E value along the grain will have the same stiffness at a given thickness. Since stiffness goes as the cube of thickness, you can take a low density piece of wood with a relatively low E value, and leave it a little thicker to get the stiffness you want, and the top will end up lighter in weight. That's why I save the low density ones for Classicals: there's less energy in the strings, and you need to keep the wight down.
As I say, you can't just go by species with this: I've gotten Red spruce that was below 330 kg/m^3, and Engelmann that was in the 550 range. That particular Red spruce top made a really nice little Classical.