Abstract: Apparatus and processes are disclosed for treating materials by exposure thereof to sonic or ultrasonic oscillations produced by oscillating plates which form part of a processing chamber and which are each activated by a plurality of transducers adjacent thereto. The transducers are excited by an electronic circuit capable of driving each transducer at selectable frequencies, phase relationships and amplitudes. The invention also comprises processes for the treatment of materials by exposure thereof to oscillating plates excited at predetermined combinations of frequencies, phases and amplitudes.