Ultrasonic cleaning

 the lower frequency, fewer larger bubbles, rougher action, more cavitation
 the higher frequency, more smaller bubbles, smaller pitch between them, gentler action, better in-crevice penetration
 frequency sweeping used to disperse modes, move bubbles around

Ultrasonic cleaning frequencies

 15 to 400kHz used, with 40kHz most common, then 25kHz
 25 and 40 kHz can be substituted for each other with adjustment to time, temperature, bath chemistry

20-25 kHz

 large masses of metal where cavitation action on surface is not harmful
 high energy per bubble
 good for stainless and cast iron, may damage softer metal surfaces (aluminium, brass)
 transducers on ebay
 25 kHz, 60W
 28 kHz, 50W
 28 kHz, 100W

35-45 kHz

 most common
 compromise between roughess and precision
 used in routine cleaning, in lab and sanitary sector
 45kHz for light metal alloys
 transducers on ebay
 40 kHz, 35W, bare plate, 50mm dia, 2.6mm thick, very common
 43 kHz, 35W, less common
 41.5 kHz, 35x15x5mm rings, 1800pF, less info
 40 kHz, 60W, horn, whole module

130 kHz

 for precision optics, circuitboards, highly sensitive surfaces
 transducers on ebay
 120 kHz, 60W

Ultrasonic cleaning bath chemistries

 hydrocarbons
 chlorinated hydrocarbons
 aqueous
 neutral
 alkaline

finest cleaning with higher cleaner concentration, higher bath temperature, lower ultrasound energy

Ultrasonic mist generation

 the higher frequency the finer the mist
 locate transducer so the cavitation happens just underneath the liquid surface

Ultrasonic mist generation frequencies

108 kHz

113 kHz

 most common in mist generators and humidifiers
 common on ebay, including driver circuits
 diameters: 16mm, 20mm

142 kHz

 less common

1.7 MHz

 common on ebay
 diameters: 20mm, 25mm

2.4 MHz

 very fine mist
 common on ebay, harder to find drivers
 16mm diameter

Medical, energy delivery

 1 MHz probes for Ultrason 101 on ebay
 other 1 MHz transducers available

Imaging

 2 MHz
 3.0 MHz
 3.5 MHz, appears to be common as curved probe arrays for medical imaging
 5 MHz, common for medical imaging
 7.5 MHz
 2.5-8 MHz