Vector sensors and acoustic calibration procedures

The measurement of underwater sound for the purpose of environmental impact studies usually only involves the measurement of the sound pressure by hydrophones. Sea mammals are sensitive to sound pressure in water like land mammals are for sound pressure in air. However, some species of fish and invertebrates are also sensitive and responsive to sound particle motion (PM) in terms of the sound particle acceleration. Therefore there is a need for measuring, standardization, calibration and assessment of particle motion. Particle motion can be measured directly by a vector sensor that contains sound pressure and acceleration sensors. An inventory of commercially available vector sensors was made and one vector sensor with suitable specifications for bio-acoustic research has been purchased by the Acoustics and Sonar department of TNO. A calibration procedure for vector sensors has been developed. It comprises a dry part, on an electro-dynamic shaker, and a wet part in a semi-anechoic basin. This procedure has been applied on two different types of vector sensors, which differ in terms of geometrical dimensions, weight and suspension. The importance of buoyancy and suspension aspects and phase-match between pressure and particle acceleration in the calibration procedure is shown. With the calibrated vector sensor two field measurements have been conducted. The field test reflect two use cases for industrial activities at sea resulting in underwater sound: seismic surveys and marine pile driving in shallow water. In these experiments the method of deployment of a vector sensor was studied. Additionally, the conversion (assuming plane wave conditions) of sound pressure measured with a single hydrophone to particle motion is studied. This conversion is subject to large errors when a point source with frequencies below 1 kHz is used, but shows good potential for a line source like a marine pile driver. Also some examples of directivity aspects of vector sensors are shown. Finally the vector sensor is connected to a stand-alone 4-channel underwater sound recorder, which allows monitoring of sound particle motion and sound pressure close to the sediment over longer period of time. The viability of this system was tested in a field test and in the semi-anechoic basin. The vector sensor of TNO is operational now and can be used in bio-acoustic projects, in the field of the sensitivity of wildlife to acoustic particle motion generated by anthropogenic sound sources in the sea.