Electrostatically charged cable transducer

Abstract

The sensitivity of a buried electrostatically charged cable transducer, such as an electret cable, to mechanical disturbances caused by men, vehicles and the like moving on the surface of the ground is greatly improved by disposing the cable loosely in a coextensive rigid hollow member such as a tube. The inside dimension of the tube is larger than the outside diameter of the cable, the cable being supported on the inner surface of the tube at longitudinally randomly spaced points. The cable is thus free to move radially relative to the tube in response to pressure waves or vibrations propagating through the ground and incident on the exterior of the tube so as to bend, twist, compress and/or otherwise stress the cable and generate corresponding electrical signals between the inner and outer conductors of the cable. The enclosing tube forms no part of the electrical circuit and accordingly may be and preferably is made of low cost nonconductive plastic material; with such a non-conductive tube, the outer jacket or covering of the cable may be omitted so as to further increase energy coupling efficiency between the tube and cable and thus increase the transducer sensitivity. The plastic tube additionally constitutes an effective moisture-proof corrosion-resistant seal for the cable. The cable may be the coaxial type or the twisted line type with the dielectric between the conductors being electrostatically charged.