The Physics
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Frequency Used in Ultrasonic Cleaning

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Cutnell, John & Johnson, Kenneth. Physics: 3rd Edition. New York: Wiley, 1995. "Attached to the bottom of the tank is an ultrasonic transmitter that typically produces 40-kHz sound waves." 40 kHz
"Ultrasonics." Encyclopedia Americana, Vol. 27. Connecticut: Grolier, 2003. "Piezoelectric and magnetostrictive tranducers with large radiating areas are used in cleaning, with frequencies ranging from a barely audible 15 kilocycles per second to 2 megacycles per second." 15โ€“2000 kHz
Ultrasonic Cleaning Systems. Clean Tech. "Cleaning applications typically fall in the frequency range of 40-400 kHz. Frequencies between 72 and 104 kHz are most often employed, in conjunction with a subsequent distilled water rinse, to minimize cavitation erosion that would occur at other frequencies" 40โ€“400 kHz
Gooberman, G.L. Ultrasonics: Theory and Application. New York: Hart., 1968. "This frequency, however, should not be below about 20 kc/s since this frequency is approaching the upper frequency of audibility of the human ear." 20 kHz
"Megasonic Cleaning." Industrial Physicist. Vol. 3, Issue 4 (Dec. 1997): 19. "Conventional ultrasonic cleaning typically operates at 20 to 350 kHz with continuous power input, which produces random, and occasionally violent, cavitation." 20โ€“350 kHz

Ultrasonic cleaning is a method mainly used industrially to remove dirt, grease, oil, baked-on carbon, paint, and rust from materials. Unlike other cleaning processes, the ultrasonic process will not damage fragile or lightweight components.

Normally, the object is placed in a tank filled with an aqueous chemical cleaner. This technique works by producing sound waves in liquids by a transducer that converts electricity into ultrasound vibrations. The waves are made of both high-pressure and low-pressure fronts. The low-pressure fronts cause bubbles to form while the high-pressure fronts trigger the bubbles to collapse. The expanding and collapsing bubbles loosen the impurities on the surface of the object, and the chemical cleaner then dissolves the free impurities.

Ultrasonic cleaning utilizes frequencies that vary from 15 kHz to 400 kHz. The lower range of frequencies is typically used in automotive cleaning. However, the computer industry employs cleaning systems with much higher frequencies to remove tiny particles.

The ultrasonic process can be friendly to the environment when aqueous cleaners are used instead of chlorinated hydrocarbons. Many of the widely used aqueous cleaners have a near neutral pH chemistries, which reduce the health risks of workers and clean well at the same time. The finest cleaning results when higher temperatures and concentrations of the cleaner are used, but there is less electrical energy converted into ultrasound vibrations.

Chan Maung -- 2004