It’s important to hold the sample as close to the center as possible, between thumb and middle finger, halfway from the end, and reaching in so that the palm of your hand almost touches the edge. Tap with a fingernail to hear the high frequency response. For bass response, hold about one-forth of the way from one end and tap with the pad of your finger.

I test all my top and back woods for stiffness, density, and Q before deciding whether to build with them. The stiffness and density tests take about fifteen minutes per sample. Density is just weight divided by volume. On rough stock I take a number of thickness readings around the perimeter with a cheap digital caliper and average them. This is important if you are to get accurate stiffness numbers, as stiffness varies by the cube of the thickness. Multiply the thickness by the length and width of the piece, and divide that number into the weight. I work in English units, and end up with pounds per cubic foot. If you want specific gravity, divide by 62.6.

I test stiffness, both long grain and cross grain, by supporting the wood on metal rods and measuring the deflection when weights are placed in the center. If you’re interested in more information, I can mail you a copy of the handout from my talks at the Healdsburg Guitar Festivals. It’s not yet a “how-to” manual, but it has more detailed descriptions of the setups and procedures. (See contact info on Authors’ page.)

If the wood has a relatively high stiffness-to-density ratio, I go on to a Q test using electronic equipment. This is much more precise than the traditional tap-tone test, but is a bit spendy, and depending on the method you choose, can take up to a half hour per sample.

One thing that can be said for sure about tonewoods is that they vary — a lot. All of the qualities I test for are largely independent of each other, and can vary by as much as a factor of five from one sample to the next.

I’m trying to sort out the excellent wood from the good, and my experience is, that for nylon string instruments, it’s really worth the trouble. The jury is still out, but evidence is mounting that wood with high stiffness-to-density ratios and high Q numbers makes superior instruments. Wood that doesn’t test well goes back to the supplier, who, with some exceptions, only grades the wood by appearance.

I strongly urge builders to test their woods, at least for stiffness and density, which is quick and cheap. Knowing the qualities of the wood that went into an instrument that turned out well can really help you to repeat the performance. Additional information can be found on my website: www.lessonsinlutherie.com.