GB1317924A - Capacitive transducers - Google Patents

Abstract

1317924 Capacitor transducers MARCONI CO Ltd 28 July 1970 [4 Aug 1969] 36381/70 Heading H4J A capacitor transducer has at least one capacitor plate (e.g. the back plate) in the form of a spiral winding of metal strip susceptible to anodic oxidation. Asperities (i.e. upstanding areas) are provided on one edge and a layer of oxide is formed on the edge by anodic oxidation. The oxide layer may be an electret. Preferred metals are aluminium and tantalum. A rod or tube of the same metal may be used as a winding form. The aperities may be formed in a regular pattern by rolling or punching. The main body of the plate may be commercial grade metal whilst that part which is to be anodically oxidized is of high purity metal provided by evaporation after winding. Alternatively commercial grade metal strip may have an edging of very pure metal rolled or coldwelded on to the edge prior to or coincident with the formation of the asperities. The wound plate may be retained by a band or by cementing of the outer layer. Further asperities may be provided on one surface of the strip to hold apart successive layers of the wound strip. In this way acoustic holes are provided through the plates. Blind holes may be provided by closing off the rear ends with lacquer or wax, or by cementing a thin synthetic film to the rear of the plate. The wound plate may be held in position thereby. In a noise cancelling arrangement a diaphragm moves differentially between fixed front and back plates both as described above.