High-frequency sound is pulsed through a tank of hot water. These are not to be confused with 'sonic' cleaners (which get upgraded to "ultrasonic cleaners" on many websites!) which are merely 'vibrating machines' - the equivalent to cleaning with a toothbrush under running water.
Although we sell these for cleaning jewellery, they are used for cleaning spectacle frames, lenses and lens assemblies, and even (with great care!) coins.
To read an article about choosing and using ultrasonic cleaners, click here.