Reader Question – Ultrasonics – “Near Field”

A reader has asked for a description of how “near field” ultrasonics works.  First of all, the term “near field” is one that is not well defined in the ultrasonic cleaning world.  For the purpose of this discussion regarding ultrasonic cleaning, “near field” means putting the ultrasonic source close to the work piece.  It may mean something different in other contexts including ultrasonic non-destructive testing but that’s not within the realm of ultrasonic cleaning.

It is well known, however, that there are some different things going on in the area immediately adjacent to an ultrasonic transducer in an ultrasonic cleaning tank.  These include the extreme effects present at the radiating surface of the transducer where sound energy is coupled to the cleaning liquid.  If the velocity of the transducer is high enough (due to high amplitude) the transducer may actually separate from the liquid causing a condition commonly known as “unloading” or “de-coupling.”  This effect, should it occur, is detremental to the ultrasonic cleaning process as all or most of the energy of the vibration is expended at the transducer-liquid interface in the form of “surface cavitation.”  Surface cavitation acts as a barrier to the transmission of ultrasonic energy thereby limiting or eliminating ultrasonic cavitation and implosion in the rest of the tank volume.

Assuming that unloading or de-coupling does not occur, the ultrasonic intensity near the transducer/liquid interface is extremely non-uniform with significant variations depending on positioning relative to the individual ultrasonic transducer elements.  As the distance from the transducer radiating surface is increased, the effects of the individual transducers combine and the ultrasonic energy field becomes more uniform as indicated in the illustration below.

 

In the ultrasonic cleaning world, the second zone in the illustration (Relatively High Intensity, Relatively Uniform Field) is usually what is considered the “near field” zone.  The zone immediately adjacent to the transducers gives non-uniform results and, because of the extremely high intensity, may also cause part damage.  Cleaning results in the “near field” zone can be impressive.  The removal of particularly stubborn contaminants requiring intense mechanical action, including buffing compound or pitch, for example, can be significantly improved by using “near field” techniques.

Because of the fact that there are still variations in intensity within the “near field” zone, it is common to move parts through the zone rather than place them stationary.  This is usually accomplished through the use of a conveyor or other material handling device.  “Near field” cleaning is most effective if parts are presented in a single layer.  “Shadowing” effects are enhanced due to the proximity to the ultrasonic source.

The inevitable question now is what distance from the transducer will provide the best near field cleaning?  Although these dimensions are not “etched in stone,” the general consensus is that the highly non-uniform zone shown in the illustration is usually about 1/4 to 1/2 wavelength in thickness.  The wavelength of sound in water at 25KHz is about 2.37 inches.  To avoid this zone, parts would need to be at least .6 to 1.2 inches from the transducer.  The extent of the “near field” zone is also not well defined but, usually, anything within 3 or 4 inches of the transducer output face is considered “near field.”  The uniform field zone extends nearly to the liquid surface, but not quite!  There are also some “funny” things going on near the liquid surface that we will discuss in an upcoming blog.

I hope this discussion of “near field” ultrasonic cleaning has been helpful.  Readers with additional questions or who have first hand experience that may be helpful to others are invited to leave a comment in the “comments” area below or by email directly to me at jfuchs@ctgclean.com.

–  FJF  –