Determining Top Plate Profiles in Assembled Guitars via Measurements of Magnetic Field

The thickness of wooden top plates is of considerable importance to luthiers. However, the geometry of an assembled instrument makes thickness measurements difficult using standard techniques. Here we describe a device that measures the thickness of a top plate by measuring the magnetic field produced by a small rare-earth magnet located on the inside of the instrument. The magnetic field is measured using a Hall device in conjunction with an analogue-to-digital converter. The instrument is based around a micro-controller that provides several advantages including the following: (i) a simple calibration procedure for different magnets and sensors; (ii) an LCD display that can show sample thickness in microns or 'thou'; (iii) automatic determination of thickness at the magnet location when the probe is swept over the surface; and (iv) the recording of thickness at multiple locations for subsequent analysis.