Abstract: Apparatus providing ultrasonic control of a fluid medium to provide selective motion and cavitation comprising a transducer and an associated chamber providing a controllable acoustic travelling wave therein. The chamber includes an inlet, an outlet and a tapered passage through which the liquid medium flows and/or cavitation is induced and/or controlled. Embodiments of the present invention provide the production of light by controlled sonoluminescence and the generation of electrical energy by the flow of a medium through a magnetic field.