There is disclosed an apparatus and method for detecting cavitation in fluid machines, for example pumps (100). In one embodiment a piezoelectric gasket (102) is used as a sensor to sense cavitation. In some embodiments highpass filters (302), (501) are used to detect ultrasonic acoustic signals in about the MHz range. If the energy in the MHz range is excessive then cavitation is deemed to be occurring and the speed of a motor (110) may be reduced in proportion to the degree of cavitation deemed to be occurring. In another embodiment (FIG. 5) the energy in the MHz range is normalized against the energy in the kHz range. Other sensors (600, 701) are also disclosed.