"Ultrasonic"

Ultrasonics, when mentioned in a metal finishing context, refers to the use of ultrasonic transducers in cleaning solutions and other immersion applications. The transducers produce a high frequency vibration which results in rapidly imposing a compressive force on the solution and then releasing/retracting that pressure wave.

As the pressure which was imposed on the solution is very rapidly removed at the high vibration rate, it creates tiny areas of vacuum, "cavitation", where the solution in that area vaporizes; in so doing, tiny scrubbing bubbles are created which can assist in agitation, cleaning, etc.