The acoustic radiation force

The physical origin of the radiation force exerted by an ultrasonic beam on an absorbing target is explained. It is shown that the force may have two sources—a nonzero time-averaged sound pressure in the ultrasonic beam, and the momentum transported by the beam—but that there is a movement of material out of the ultrasonic beam to prevent a nonzero time-averaged sound pressure from being established. Consequently, the transfer of wave momentum is the sole cause of the force. The assumptions made in evolving this explanation are discussed, and the resulting change in mean density within the ultrasonic beam is considered. It is pointed out that the acoustic radiation force is but one example of a universal phenomenon associated with all forms of wave motion.