The propagation in a liquid of an acoustic wave that is caused by a rapid change in fluid velocity. Such relatively sudden changes in the liquid velocity are due to events such as the operation of pumps or valves in pipelines, the collapse of vapor bubbles within the liquid, underwater explosions, or the impact of water following the rapid expulsion of air from a vent or a partially open valve. Alternative terms such as pressure transients, pressure surge, hydraulic transients, and hydraulic shock are often employed. Although the physics and mathematical characterization of water hammer and underwater acoustics (employed in sonar) are identical, underwater sound is always associated with very small pressure changes compared to the potential of moderate to very large pressure differences associated with water hammer. See also: Cavitation; Sound; Underwater sound