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The heart of any ultrasonic cleaning system is the transducer. At the present time, the two types of transducers offered are magnetostrictive, made of nickel or its alloys, and electrostrictive, made of lead zirconate titanate or other ceramics. Electrostrictive materials change their physical dimensions when placed in an electrical field of varying voltage. This is known as the "piezoelectric effect." Magnetostrictive transducers are made of materials which change dimensions in a varying magnetic field. Regardless of the type of transducer, the common, but primary factor, is the intensity of cavitation produced. Ultrasonic energy, like any sound wave, is a series of pressure points, or rather a series of compressions and rarefactions (see figure below). If the sound energy is of sufficient intensity, the liquid will actually be pulled apart at the rarefaction stage and small bubbles or cavities will be formed. With the following compression stages, the bubbles collapse or implode throughout the liquid, creating an extremely effective force which is uniquely suited to cleaning. This is the process known as cavitation.

Ultrasonic Primer: The Theory Behind Ultrasonic Cleaning (.pdf) - Cleaning and Surface Preparation