Ultrasonic cleaning utilizes the energy of collapsing bubbles to create an agitating action and clean objects on a microscopic level. Ultrasonic cleaners produce these bubbles when high frequency sound waves are transmitted through the liquid bath containing the object to be cleaned. The effect of temperature on the cleaning action was presented in a previous post. In this post, the effect of frequency has on the impact of industrial ultrasonic cleaners will be presented.
Sound waves are compressional waves. This means the molecules of whatever substance they are traveling through (air or water, for instance) are alternately squeezed together and spread apart as the wave passes. When the frequency of a wave increases, the number of compressions occurring at a point each second increases. Since the wave moves at a constant speed, this means the compressions are closer together in space as well as in time.
Imagine watching as a train passes by. Now imagine counting the number of cars on the train passing by each minute. If the number of cars passing by increases, but the speed of the train stays the same, the cars must be getting shorter in length. As the frequency of cars passing by increases, the distance between the cars must decrease if the speed stays the same.
The result of increasing the frequency in ultrasonic cleaning is to reduce the space between compressions, which reduces the size of the bubbles created by the sound waves. This also reduces the energy released when these bubbles collapse. Therefore, industrial ultrasonic cleaners operating at higher frequencies are good for cleaning fine particles or very small features. Lower frequencies are best suited for heavy cleaning and coarse contaminants.
Ultimately, the choice of frequency depends on the type of cleaning desired and the type of object being cleaned. In the final installment, we will discuss why the frequency must be varied throughout the ultrasonic cleaning process.