Real Time Acoustical Imaging with a 256 × 256 Matrix of Electrostatic Transducers

Abstract

Electrostatic transducers for ultrasonic applications are a natural extension of the classical condenser microphone. For this latter device, numerous studies have shown the pos— sibility of obtaining an internal polarization due to the electret effect in a dielectric sheet. In the ultrasonic version also, electrostatic transducers may be polarized alter-natively by an external circuit or by using the electret effect. Their interest in the acoustical imaging field is that it is very simple to build complex arrays with printed circuits. The difficulty lies rather in the correct interrogation of the array. A first solution is to build a square array of N² independant transducers and to interrogate them with parallel circuits or with a suitable electronic commutation [1] . A much simpler solution has been proposed which requires 2 N circuits only. In this case, although it is still possible to use electret transducers according to NIGAM, the operation with external polarization is preferable. This technique was presented in Vol.4 of this series [2]. New refinements and progress in experimental results are shown here.