On the conversion between sound pressure and particle motion

Since some fish and invertebrates are sensitive to particle motion there is a potential need for direct measurements of the particle motion in addition to pressure. In the absence of dedicated vector sensors, particle motion could be estimated from sound pressure data, measured with hydrophones, using relationships for freely propagating plane waves. Experimental and modelled particle motion and pressure data are compared to determine to what extent sound pressure data can be used to estimate particle motion data. Particle motion measurement results of various measurement campaigns in a laboratory basin and field tests in shallow Dutch waters are reported. It is shown that the conversion between sound pressure and particle motion can be assessed using the concept of scaled impedance, which is the ratio of measured impedance over free field impedance. For a point source this ratio is shown to become small for low frequencies and to approach a value of 1.0 for higher frequencies. The transition frequency depends on the receiver distance from water surface and seabed. The results of the field tests illustrate that careful consideration is required when particle motion is derived from a single sound pressure measurement, especially below or near the cut-on frequency.