A simple sensor for direct measurement of particle velocity in water

Abstract

Normally, acoustic particle velocity is inferred from measurement of pressure or pressure gradient. In water, however, direct measurement of particle velocity is simple. A moving-coil sensor embedded in a neutrally buoyant package produces a voltage output directly proportional to the particle velocity in the surrounding fluid for frequencies above the resonance of the moving coil. Leslie etal. [J. Acoust. Soc. Am. 28, 711–715 (1956)] built such a sensor by mounting a moving-coil element inside a hollow brass sphere. They also presented the theory for its response with the assumption that the sphere was acoustically rigid. The device described in this work is considerably simpler to fabricate—the moving-coil element is cast into a small cylinder of syntactic foam to make the entire sensor neutrally buoyant—and all of the materials are readily available. In addition, the sensor is small and does not perturb the acoustic field as much as a rigid body would. This sensor can be used to measure acoustic particle velocity directly; it can be used in conjunction with a pressure hydrophone to distinguish between real and reactive power flows or to measure acoustic intensity; and, it can be used to demonstrate effectively the spatial variations of pressure and particle velocity in standing waves.