GB1082128A - Transducer - Google Patents

Abstract

1,082,128. Capacitor transducers. NORTHROP CORPORATION. April 27, 1965 [Sept. 11, 1964], No. 17598/65. Heading H4J. [Also in Division H2] A loud-speaker, hydrophone, microphone (e.g. for blast wave studies) or other transducer comprises opposed elements of electrically conductive material joined by an intermediary of compressible cellular foamed electrically non- conductive material, of uniform thickness, intimately bonded to the confronted surfaces of the elements. The bond is free of voids and the opposed elements are initially unstressed. The foamed material may have open cells, but closed cells permit good response at low frequencies. Suitable foams are of natural rubber, silicone rubber or neoprene. A capacitor microphone 1 (Fig. 1) comprises aluminium plates 2, 3a, 3b maintained in a parallel array by sheets 5, 6 of foamed material. The foamed material is homogeneous and is devoid of air bubbles, wrinkles or gaps at the interfaces. Epoxy resin adhesives, transfer papers, double-sided pressure-sensitive adhesive tape or, for neoprene, a neoprene solution may be used for the bond. Alternatively the foamed material may be intrinsically adhesive. The outer plates 3a, 3b may be larger than the inner plate 2 and the sheets 5, 6 may be coextensive with, or extend beyond, the inner plate 2. The plates 3a, 3b are preferably bonded together by an edge member (3c), Fig. 2 (not shown), of aluminium, copper or conductive epoxy resin. Member (3c) may extend along more than one edge and may frame the plate 2 completely, provided the plates 3a, 3b are able to move parallel to the plate 2 in response to pressure differences. An insulated conductor (7) connected to the plate 2 is brought out through an aperture in the member (3c) and a coaxial shielding conductor (8) is connected to the outer plates 3a, 3b. The transducer may be formed of an inner plate 2 with a single outer plate (3), Fig. 4 (not shown), wrapped around it and joined at the ends to complete the shielding. Foamed material (9) is wrapped around the plate 2 or formed in situ. For omnidirectional response the plates 2, 3 and foamed sheet may be in the form of concentric spheres (Fig. 5, not shown). For greater directionality, e.g. to cover a stage, the plates may be curved, Fig. 6 (not shown). Flat, circular transducers, non-directional below 10 c/s., and directional transducers comprising flat plates 1000 feet by 1 foot are envisaged. Large transducers may be made of small transducers electrically, but not mechanically, connected in parallel. Local circuit.-When used as a microphone, the grid voltage of a triode T is controlled by the capacitance of the transducer 1. A polarizing voltage is applied at terminal B1. The circuit components are selected for optimum performance in the frequency range ¢ c/s. to 5 c/s. Alternatively the transducer is excited by a varying voltage applied across the plates to produce an acoustic output.