A simple neutrally buoyant sensor for direct measurement of particle velocity and intensity in water

Acoustic particle velocity is commonly inferred from measurement of pressure or pressure gradient; however, in water, direct measurement is simple. A moving-coil sensor embedded in a neutrally buoyant package produces a voltage directly proportional to the particle velocity in the surrounding fluid for frequencies above the mass-spring resonance. Leslie et al. [J. Acoust. Soc. Am. 28, 711715 (1956)] built such a sensor by mounting a moving-coil element inside a hollow brass sphere. The sensor described in this paper is identical in principle but is considerably easier to fabricate. Useful from tens of hertz to several kilohertz, this sensor consists of a glass-microballoon-and-epoxy composite cast around a small, commercial geophone. The sensor is inexpensive, rugged, and has good immunity to interference. In conjunction with a pressure hydrophone, acoustic intensity can be measured without the errors associated with subtraction of nearly equal signals (as in the two-hydrophone method).