Abstract: Particulate material suspended in a fluid is separated and recycled by means of an ultrasonic resonance wave. In a preferred embodiment, the ultrasonic resonance field is generated within a multilayered composite resonator system including a transducer, the suspension and a mirror parallel to each other. Dimensions and frequencies resonant to the whole system but not exciting Eigen-frequencies of transducer and mirror itself are chosen so that thermal dissipation is minimized. Criteria for flow direction and flow rate are defined in order to maintain a high-quality factor of the composite resonator and to achieve a high-separation efficiency. Generally, the process is suitable for all kinds of particles (solid, liquid or gaseous disperse phases) especially for hydrosols (particles in water) and for separation of biological particles such as mammalian, bacterial and plant cells or aggregates.