Measuring system for the radiation field of a violin

Abstract

Vibration patterns of the violin body have been difficult to interpret because they vary greatly in radiation efficiency; knowledge of the actual radiation field would do much to clarify the situation. Both magnitude and phase must be measured to allow an interpretation in terms of the wave equation. In our system, the violin string is driven electrodynamically by a sinusoidal oscillator locked to the resonant frequency of the string by a phase-sensitive feedback loop. The violin is mounted in an anechoic chamber, and a pair of microphones sweeps the surrounding space so that the complex sound pressure (using the string vibration as a phase reference) is measured at many points on two concentric spheres. The data is digitally recorded in computer-readable form and then analyzed. Knowledge of the field on two concentric spheres allows a mathematical decomposition into outgoing and incoming waves, so that corrections can be made for the nonideal nature of the anechoic chamber. The apparatus is suitable for analysis of radiation fields of other instruments and of loudspeakers. [Work supported by NSF.]