"Inaudible vibration with a frequency exceeding 20,000. Ultrasonic vibrations are generally produced by piezoelectric quartz crystals, the oscillations of which are started and maintained by electric vibrations of the same frequency.
Ultrasonic cleaning.
Ultrasonic machining. A tool subjected to the action of ultrasonic vibrations disintegrates the material and acts as a drill or punch. This process enables materials such as stone, ruby, glass, hardened steel, etc., to be machined." (Berner)