Optical methods in stringed instrument testing

Stringed instruments present an experimental challenge for several reasons. Radiated sound is a strong function of dynamic response, so particularly accurate measurements of structural and acoustic resonant frequencies are desirable. However, the structures tend to be very light and sensitive to the additional mass of contacting sensors. Thus, optical methods are attractive. The ready availability of lasers, inexpensive digital cameras, and powerful, inexpensive computers has made optical testing practical. As a result, a range of methods has been applied to musical instruments. There are many ways to organize the methods, but an attractive one is whether they require coherent light for interference effects. A hierarchy of methods is presented along with representative results. The simplest methods simply track motion of a reflected beam across an optical sensor. Coherent methods start with laser vibrometry and move through holography and speckle pattern interferometry. We pay particular attention to an particular application of laser vibrometry and to a low cost speckle pattern interferometer.