ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

3aEA10. Theoretical prediction of the natural frequency of electrostatic ultrasonic transducers.

Li-Feng Ge

L. Finkelstein

Dept. of Elec., Electron. and Information Eng., City Univ., London EC1, England

Electrostatic airborne ultrasonic transducers with an air-gap structure are widely used in automatic ranging, acoustic imaging, NDE, etc., so that there is an increasing need to develop a reliable model for their optimal design. A Helmholtz resonator model and a plate-under-tension model were presented successively in recent years, but both are still not very satisfactory. In this paper, an electrostatic transducer is treated as a thin plate supported by an air spring, in other words, its diaphragm is viewed as a plate situated on an air cushion. An interesting point revealed by this model is that its fundamental frequency stems from the air-gap spring, and the higher resonant frequencies are caused by the combined contributions of the bending stiffness of the diaphragm, in-plane tensile forces applied to the diaphragm, and the air-gap stiffness. The model is applied to predict the natural frequency of a typical V-grooved transucer with a 8-(mu)m-thick Mylar diaphragm and a 0.5-mm pitch air gap. The first resonant frequency predicted by this model is 50.8988 kHz, and its nominal measured value is 51 and 52 kHz, respectively, as reported by some researchers. [Work supported by the UK Royal Society.]