1. An instrument for measuring the velocity of sound in water; used to correct Doppler-sonar data for salinity and temperature variations. An acoustic pulse is transmitted over a fixed distance between transducers in the instrument, amplified, and used to generate the next pulse to be transmitted. The regeneration frequency depends on the traveltime and hence on the velocity of the acoustic wave. This technique is sometimes called sing around. 2. A device which measures fluid flow; a flowmeter (q.v.).