As is true in many other applications for piezo¬electric materials, an assembly of multiple ceramic elements offers considerable performance and production advantages in ultrasonic cleaning transducers, relative to a single ceramic element. In order to provide the most efficient operation, simplify manufacturing, and reduce costs, more complex transducers intended for ultrasonic power applications usually are a composite of a piezoelectric ceramic center (multiple thin rings or disks of ceramic, for example), encompassed by metallic end or top and bottom parts. Under no liquid load, the mechanical quality factor, Qm , for a well-designed composite transducer will be greater than the correspond¬ing value for an equivalent single piece ceramic transducer, and efficient heat conduction by the metallic portions will ensure a lower operating temperature in the ceramic portion of the transducer. The coupling factor, k, will approach that for a single-piece ceramic transducer.

Pressure Transducers

The metallic portions of a composite transducer should have the same acoustic properties and cross-sectional area as the ceramic portion. Both metallic parts can be constructed from the same material or combination of materials, or the two parts can be made from materials with divergent properties. Potential construction materials include steel, aluminum, titanium, magnesium, bronze, and brass. Often, only one of the metallic parts is intended for high intensity output.

For maximum energy transfer from the transducer to the solvent in the ultrasonic cleaning tank, a composite ultrasonic transducer usually is a half-wavelength transducer with a resonance frequency of 20 kHz or 40 kHz. The electroacoustic efficiency of a composite ultrasonic transducer has an inverse relationship with the electromechanical coupling factor and the various quality factors of the components.