GB724113A - Electrodynamic sound transmitting and receiving units - Google Patents

Abstract

724,113. Receivers. CANADIAN BEN. AUDI CO., Ltd. March 24, 1952 [March 29, 1951], No. 7469/52. Class 40 (4). In an electro-dynamic transducer the voicecoil has an impedance of at least 300 ohms and the magnetic field in which it works is at least 6500 gauss. The magnetic circuit of the receiver, Fig. 2, comprises a central magnet 22 extended at one end by a pole piece 27 housed within a cylindrical soft iron shell 23 wherein it is centrally mounted on a base member 24. An annular plate 25 forms the outer pole piece and a non-metallic locating ring 30 is fixed to its underside to locate the core extension. The locating, ring 30 is provided with air channels to control the damping and a groove 32 in plate 25 assists in concentrating the flux in the gap. The magnetic unit is held in a recess in a chassis member 19 of non-hygroscopic material which also provides a seat for the rim of the diaphragm 35 and housing for terminals 18. Grooves in the upper surface of the chassis 19 permit free movement of the coil ends 42. The receiver is claimed to reproduce frequencies from 60 to 12,000 cycles per second and a maximum figure for the D.C. resistance is given as 400 ohms. A moisture-impervious cloth 43 protects the diaphragm from the ingress of moisture and in a modification (Fig. 5, not shown) a second layer is stretched across the diaphragm to enhance the reproduction by providing amplitude compensation. Suitable housings for holding the moving-coil cartridge are described with reference to Figs. 1A and 1B (not shown), and Fig. 3 shows an earpiece, adapted by a flange 44A to fit around the ear, in which sound apertures 52 pass through a domed centre portion. In this embodiment a rubber plate 45 is inserted between the capsule and the outer case. Specifications 440,626 and 467,990 are referred to.