Abstract: Apparatus for the treatment of materials by the application thereto of ultrasonic longitudinal pressure oscillations, for example, for dispersing or emulsifying or dissolving or mixing or deagglomerating consists of an enclosure having an interior with two closely-spaced walls at least one of which is made to oscillate at ultrasonic frequencies, or otherwise impart such oscillations to the interior, for example, by ultrasonic transducers mounted on the wall exterior. The spacing between the walls is such that the longitudinal pressure oscillations produced at the oscillating wall are reflected by the other wall, preferably several times, before they are attenuated to a negligible value. Preferably, the other wall also vibrates to produce doppler effects and avoid standing waves. The distance between the inner surfaces of the two walls is normally not more than about 1 inch (2.5 cm).