Electrostatic ultrasonic transducers and their utilization with foil electrets

Electrostatic ultrasonic transducers are very attractive when considered from the point of view of simplicity. They are constituted by a condenser, the ultrasonic wave being directly excited on the electrodes. These transducers are currently used at low frequencies (microphones) and sometimes at higher frequencies (up to a few megahertz). At higher frequencies the bias voltage applied across the condenser has to be quite large and electrification of the central dielectric layer can appear. This paper describes such effects and presents the experimental conditions allowing the transducer to operate. The electrification of the dielectric layer is studied and the problems related to the conservation of the deposited charges are considered for Mylar and polypropylene foils of about 10-μ thickness. In the present work the ultrasonic waves generated or received by these transducers have frequencies ranging from 10 to 200 MHz.