| sensor that provides an electrical output signal in response to the acoustic particle velocity
Note 1 to entry: A particle velocity sensor can be constructed using a sensor which responds directly to velocity, and whose output is typically determined by the relative motion of a reference mass and the sensor casing (an example being a geophone).
Note 2 to entry: A particle velocity sensor can be constructed using two sensors which determine the acoustic pressure gradient, by measuring the difference of two pressure measurements made at different locations in space. In practice, this can be done with a pair of hydrophones with a fixed spacing, if the specific acoustic impedance of the medium is known.
Note 3 to entry: A particle velocity sensor can be constructed using a sensor that responds directly to acceleration, using an accelerometer as the sensing unit. The acceleration signal can be integrated with respect to the time to generate a velocity signal.
Note 4 to entry: Acoustic particle velocity is a vector quantity. In order to determine the direction of the particle velocity, three sensors are typically combined to measure the three orthogonal components of the field.
Note 5 to entry: Particle velocity sensors typically operate at frequencies below 2 kHz, and are sometimes used for measurement of the sound exposure of marine fauna that respond to particle motion rather than to sound pressure (for example, many fish and invertebrates).