Abstract

The separation of different types of particulate matter in a carrier liquid is obtained by using an ultrasonic standing wave and relying on the different acoustic responses of the different particle types. By varying the acoustic energy propagation cyclically a more effective separation rate can be obtained, with a more readily attracted particle type being subjected to a further discrimination step in each cycle. The cyclical energy variation may be in the intensity of the standing wave, e.g. using suppression means, and/or the velocity of the standing wave relative to the liquid medium, e.g. using phase control means.