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Bubble Diameter, microns Figure 1. Diaphragm changes relative to application of current. changes dimension in one direction. As shown in Figure  1, that dimensional change can be used to cause a diaphragm to move though driven by a different factor. Most magnetostrictive materials are metal alloys of nickel or contain significant quantities of nickel compounds. Magnetostrictive transducers are not used at frequencies above 30 kHz. The main reason is that the difficulty and cost of controlling the motion of the relatively large mass of material (dense nickel) associated with magnetostrictive transducer elements becomes too severe at frequencies above that level. MAKING A CHOICE Some may inform managers that the choice is between the Figure 2. Bubble diameter relative to ultrasonic frequency. higher purchase price and longer maintenance life of magnetostrictive transducers vs. the opposite for piezoelectric transducers, or to achieve a lower level of operating noise. That's a false choice. The choice should be totally based on the character of the parts. Ultrasonic Frequency, kHz • No one, for example, would consider using magnetostrictive transducers for cleaning of disk drive components where piezoelectric transducers are commonly used. The components would "dance" in the water bath and be destroyed. • Nor would anyone consider using piezoelectric transducers for removal of scale prior to painting of small engine blocks for lawn mowers. Nothing would be removed. Ultrasonic TransducersÑ How to Choose the Frequency. Ultrasonic cleaning systems use two familiar types of ultrasonic transducers: piezoelectric and magnetostrictive. Both generate cavitation bubbles. This section will cover how to choose the right kind of cavitation bubbles, and the right type of transducer to generate them. A TWO TYPES OF FREQUENCY The two prefixes normally attached to the word sonic are ultra and mega. Ultra refers to frequencies above those identified by the human ear — i.e., those above ~18 kHz. Ultrasonic transducers are the type most commonly used, with frequencies above 18 kHz, and below ~ 250 kHz. A manager purchasing a ultrasonic system without specifying the frequency would probably receive one operating at 40 kHz. 71

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