The term, ultrasonic, is one area of sound wave and refers to having the frequency above 16KHz, and moreover the term is used to handle a mechanical wavelength for frequency in engineering.
"Sonic" generally means a normal sound wave which has a frequency within the audibility range of the human ear, on the other hand, "Ultrasonic" is a sound wave that can not be audible. In general, an ultrasonic cleaner is made with the frequency above 20KHz.

When an ultrasonic wave, a sort of sound wave that human can not be audible, is launched into a fluid, the waves create alternating compression and rarefaction by the molecular vibrations and then are transmitted in the fluids.
As the ultrasonic energy is increased much more, the cohesion of the liquid molecules is destroyed and small vacuum bubbles or voids more than tens of millions are generated. It is called a cavitation effect.
Microscopic bubbles are formed, and they implode violently causing the cavitation which create an intense scrubbing action on the surface from deep inside of the item being cleaned.
The bubbles are small enough to penetrate microscopic crevices, cleaning them thoroughly and consistently without damage in a short time.
Ultrasonic cleaning is the best way ever to clean a metal or non ferrous metal, returning them to almost "like new" condition.

Cleaning is an essential process in any area of industry.
The cleaning methods depend on the levels of contamination, the types of pollution, and materials or shapes of the item to be cleaned.

In addition to cleaning, the vibration energy of ultrasonic waves is utilized in a number of processes.