Letter: Technical qualm with Jim Blilie’s article in AL #100

by Alan Carruth

Originally published in American Lutherie #101, 2010



Tim —

I really enjoyed AL#100. It’s a nice mix of “technoid,” “art,” and “craft” articles. Lots of good info, but I did see a couple of things I wanted to respond to.

First, in Jim Blilie’s article, which was excellent overall, I have a disagreement that rises above the level of minor. He says, on p. 31: “The fact that the relationship between stiffness (Young’s modulus) and density is inherently linear shows that just changing wood species doesn’t affect the stiffness-to-weight ratio very much.”

The problem with that is, while the lengthwise Young’s modulus (E) values for both hardwoods and softwoods tend to fall on straight lines, they are different lines, owing to differences in basic structure in the woods. I’ve been measuring the properties of wood samples for several years. I’m enclosing a graph of long grain E vs. density for most of the pieces I’ve measured so far (147 samples), with eyeballed “average” lines drawn in. The softwoods include all of the usual-suspect top woods, as well as white pine and Mediterranean cypress. As you can see, the points fall very close to a straight line toward the left side of the chart. The hardwoods include a lot of lutherie woods (most of the samples are Indian rosewood), and some others, with balsa and blackwood being the end points. As you can see, the scatter of the points is greater, but they do at least suggest a line. One could, of course, draw a single line that took in all of the data points, but at the cost of accepting quite a lot more scatter in the softwood data. Given the relative homogeneity of softwood structure, this seems unwarranted. Besides, the resulting line would not approach the origin at all closely, which would be illogical. As is, the hardwood line is in no way an extension of the softwood line. Balsa, and yew, ’way down on the left, overlap the softwood area, as do a few of the softer hardwoods, such as butternut. Still, compared with the softwoods, hardwoods tend to have higher density for the equivalent E value.