GB1376335A - Electrostatic transducers for converting electrical signals into ultrasonic signals and vice versa - Google Patents

Abstract

1376335 Capacitor transducers AGENCE NATIONALE DE VALORISATION DE LA RECHERCHE 22 Nov 1971 [20 Nov 1970] 54192/71 Heading H4J A capacitor transducer for ultrasonic use comprises two vibratile plates 1, 2 between which is an electret film 3. The plates 1, 2 are either conductive or have conductive faces. In use an external D.C. voltage 6 is applied between the plates 1, 2 in the reverse sense to that used for permanently prepolarizing the electret film 3 so that the two electrostatic fields created by permanent charge of the film 3 and this external D.C. polarization are additive. The external D.C. voltage is as high as possible bearing in mind that the electrostatic field in film 3 must be less than the breakdown field and that air layers 4 must not ionize. The source 6 is protected from high frequency signals by a choke 10. The plates 1, 2 are connected by a D.C. blocking condenser to a transmitting circuit 8 and a receiving circuit 9. The film 3 is preferably between 3 and 70 microns thick and may be made of polypropylene, polystyrene, polycarbonate, mica, polyethylene terephthalate, phenol formaldehyde resin or "KAPTON" (Trade Mark). Permanent polarization of polypropylene film of 10 microns thickness is obtained by applying 1200 volts between its surfaces for 5 mins. The external polarization, in reverse sense, may be slightly below 400 volts. Transducers having operating frequencies in the range 100 KHz to 200 MHz are obtainable and having wide bandwidths. Applications include long range ultrasonic transmission of acoustic signals and frequency modulated ultrasonic submarine communication.