In measuring sound waves, 1 Hz is equal to one wave per second.

Humans can hear sound from around 20 Hz to 20,000 Hz.
If a sound frequency gets high enough, it goes beyond the ability of the human ear to hear it.
This is known as ultrasonic sound.

Many ultrasonic cleaners use 40 kilohertz [40,000hz] sound waves.

When you pass ultrasonic sound waves through liquid, the process forms microscopic bubbles that grow to a certain size and then collapse, causing a tiny shockwave.

This shockwave helps to dislodge particles of dirt and grime on the surface of anything that is submerged into the liquid.

The size of the bubbles depends on the ultrasonic frequency.
Higher frequencies produce smaller bubbles.

The liquid can be heated and detergent added to make the cleaner more effective.

Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid.