An ultrasonic cleaner is simply a metal tank [stainless steel] that has pizeo ceramic transducers bonded to the bottom or side. These transducers have a unique property of changing size almost instantly when excited by an electrical signal. When excited the transducer increases in size and causes the tank bottom or side to move. This creates a compression wave in the liquid of the tank.
By using an electrical generator that puts out a high frequency signal [20 to 250 kHz] the transducer rapidly induces compression and rarefaction waves in the liquid. During the rarefaction cycle the liquid is torn apart. This creates a vacuum cavity within the liquid. These cavities will grow larger and smaller as the compression waves are continued. When the cavity reaches a certain size [based on the frequency and the wattage of the signal] the cavity can no longer retain its shape. The cavity collapses violently and creates a temperature of 5,000 degrees centigrade and a jet of plasma that impacts against whatever object is in the tank. There are millions of these bubbles created and collapsing every second in an ultrasonic tank.