The ultra-audio-frequency electric signal produced by the ultrasonic generator in a ultrasonic cleaning machine is conversed to the mechanical vibration with the same frequency through the piezoelectric inverse-transforming effect of the transducer, and then it will radiate forward intensively in the cleaning solution in the form of ultra-audio-frequency longitudinal wave(The ultra-audio frequency longitudinal wave spreads alternatively between zones of negative pressure and positive pressure), producing ten thousands of small bubbles to explode in turn. It these small bubbles which form and grow in the zone of negative pressure, and explode instantaneously in the zone of positive pressure, to strike with high pressure the surface, gap,and small local of a cavity to be washed, leading to peeling off of fouling quickly, thus the good cleaning and purifying effect can be reached