We in the ultrasonics industry have long been aware of an effect which is sometimes called “ultrasonic shadowing.” In general, this is what happens when parts being cleaned are positioned in such a way that parts cast an “ultrasonic shadow” which prevents parts in the shadow from being effectively cleaned. This phenomenon, although we know it exists is quite difficult to characterize and predict.
Ultrasonic shadowing does not produce a distinct shadow as the case when an opaque object casts a shadow from a distinct light source like the sun or a bare light bulb. The following illustration will help clarify this point.
On a bright, sunny day, an opaque object held between the sun and a surface such as a sidewalk will cast a distinctly outlined shadow of the object. On a cloudy day, however, where the sunlight is distributed and diffused by a layer of clouds (for example), the outline of the shadow becomes less distinct as the light from a distributed source is able to illuminate what would be the shadow from a more concentrated light source such as the sun. Even in the cloudy day scenario, however, if the object casting the shadow is large enough, there will be a dark area where even the diffused light can not reach. The ultrasonic shadow is more like that produced on a cloudy day. This is because the ultrasonic energy source is distributed like the light on a cloudy day and not a point source like the sun. Even if the ultrasonic energy was delivered from a single point, there would likely be a distribution due to reflections and general diffusion of the energy produced by the transmitting liquid.
The other effect that tends to “temper” the sharpness of ultrasonic shadowing is the fact that, for the most part, items being cleaned are at least somewhat transparent to ultrasonic energy. It can be easily demonstrated that a thin plate of stainless steel held between an ultrasonic source and an object being ultrasonically cleaned has very little effect on the transmission of ultrasonic waves. That is not to say that this effect will go on indefinitely. As more thin plates are added, more and more ultrasonic energy will be lost to the slight attenuation in each plate and also to reflections between the plates. A similar effect can be seen if a light source is obscured by several sheets of glass that are separated from one another. Eventually, all of the light is reflected away.
The “take away” here is that although ultrasonic shadowing is real and does have an effect on the strength of the ultrasonic field, it is not a sharply defined phenomenon. Racking of multiple parts for ultrasonic cleaning is possible even if one part is partially “hidden” from the ultrasonic source. At some point, however, there will be a reduction of the ultrasonic field and subsequent cleaning effect as part density becomes more than the ultrasonic energy can overcome. The best way to determine if cleaning problems are related to ultrasonic shadowing either by the number of parts or as an effect of the fixture is to try cleaning a single part (hung on a wire, for example) in the tank. If it is possible to clean a single part but not a full load of racked parts then there is a possibility that ultrasonic shadowing may be the cause.
– FJF –