Questions: Thickness Measuring Tool

by Tom Nelligan and R.M. Mottola

Originally published in American Lutherie #92, 2007



Georges E. Vougioukalakis from Athens, Greece asks:

In my experimentation and research on double-top guitar construction I find a need for an instrument that can measure the thickness of the outermost skin of the top on a fully assembled instrument without damaging the top. Does anyone know of such a tool? I remember a citation of an instrument that was used to measure the thickness of the paint in autos.


Tom Nelligan from Olympus NDT/Panametrics-NDT Products in Waltham, Massachusetts replies:

Commercial ultrasonic thickness gauges of the sort made by my company (and our competitors) for measurement of industrial engineering materials like metals, plastics, and ceramics will not work on wood, because wood does not efficiently transmit sound waves at ultrasonic frequencies in the MHz range. Pulse/echo ultrasonic gauging is based on very precisely measuring the round-trip transit time of a short pulse of very high frequency sound waves through the test piece. But the high frequency/short wavelength wave front is scattered by the microstructure of the wood, so it is not possible to get the necessary clean echo from the inner wall.