Acoustic radiation pressure

(acoustics) A unidirectional, steady-state pressure exerted upon a surface exposed to a sound wave.

The net pressure exerted on a surface or interface by an acoustic wave. One might presume that the back-and-forth oscillation of fluid caused by the passage of an acoustic wave will not exert any net force on an object, and this is true for sound waves normally encountered. Intense sound waves, however, can exert net forces in one direction of sufficient magnitude (proportional to the sound intensity) to balance gravitational forces and thus levitate an object in air.

Forces due to acoustic radiation pressure have been used to calibrate acoustic transmitters, to deform and break up liquids, to collect like objects or to separate particles (including biological cells) based on mechanical properties, and to position objects in a sound field, sometimes levitating the sample so that independent studies of the object's properties can be performed. Single bubble sonoluminescence phenomena depend on acoustic radiation forces to maintain a bubble in a zone while its substantial radial oscillations take place. See also Acoustic levitation; Sound; Sonochemistry; Ultrasonics.