F.A.Q. #3: More on Flexibility

November 3, 2011

Q: Do you use the same X amount of flexibility for all your guitar tops? Is there any reason to have a different, Z, level of flexibility when you use woods of different species? 

A: I certainly try to for the same level of stiffness in every guitar top I make, regardless of species of wood used, for reasons of consistency of sound and musical responsiveness.

However, it’s not quite a simple yes-no. The thing is, if you’re going to build a guitar that’s slightly bigger or smaller than the last one you made, then you’ll need to factor some accommodations into your measurements.

A bigger guitar top is weaker than a small one of the same absolute mechanical stiffness (i.e., the same mechanical stiffness is asked to cover a larger span or area), and will have to be left thicker to compensate for that weakening. And vice-versa. For example, imagine standing on a plank that serves as a bridge to cross a 5-foot wide creek, and a longer but otherwise identical plank spanning a 10-foot wide creek. The latter will sag more when you stand on it. Your weight is the same, just as the guitar’s string tensions are the same. The resistance over the span needs to be adjusted, however, if you want the sag to be the same amount.

That “sag”, in the guitar, goes to vibrating-plate motion, which has everything to do with sound. You probably don’t care how much sag there is in a simple footbridge, but in the guitar the ‘sag amount’ corresponds to how much or how little the guitar face can move and flex in order to produce sound. There’s a direct correlation, as sound is nothing but excited air molecules. Finally, we’re (you’re?) trying to build guitars that are optimally permeable and receptive to the strings’ energy level and budget. Assuming the use of standard strings of a standard scale — which goes to the energy budget — this implies the same (or at least comparable) optimal amount of structure.