MICROMACHINED ULTRASONIC CAPACITANCE TRANSDUCERS FOR IMMERSION APPLICATIONS

Investigations into the characteristics of water-coupled ultrasonic ca pacitance transducers have been undertaken for a range of transducer c onfigurations. The radiated fields have been scanned in water using a miniature hydrophone detector, and the results compared to theory base d on a plane piston approach. Micromachined back-plates in conjunction with thin Mylar and mica membranes have been investigated, together w ith aperture modifications such as an annulus and Fresnel zone plate. The measured results agree well with theory, thus demonstrating that w ideband predictable performance (> 8 MHz) is obtainable with such tran sducers. Additionally, pulse-echo C-scans of a Plexiglas plate contain ing an artificial defect have been undertaken with the capacitance tra nsducer, in order to demonstrate that the capacitance transducer has s ufficient sensitivity to allow routine nondestructive testing within i mmersion applications.