GB527100A - Improvements in or relating to electro-acoustic devices - Google Patents

Abstract

527,100. Telephone transmitters and receivers. STANDARD TELEPHONES & CABLES, Ltd., and ROBERTSON, J. S. P. March 31, 1939, No. 10113. [Class 40 (iv)] The diaphragm is held in position only by the attraction of the magnet system which comprises a single pole of one polarity beneath the centre of the diaphragm and one or more poles of opposite polarity and of material of high coercive force disposed symmetrically around the central pole, thus producing a magnetic flux more or less radial. The magnet system may comprise a U-shaped or cup-shaped permanent magnet 22, Fig. 2, or Fig. 6 (not shown), of high coercive force with a central pole 23 of nickel iron alloy having 35 to 45 per cent. nickel. The diaphragm 25 rests on an annular seating 29, a membrane 28 of mica or waterproof material and a cover 27 with sound apertures 39 being arranged as shown. A plate 30 having apertures corresponding to the pole-pieces is arranged airtight behind the diaphragm, or the space behind the diaphragm may be filled in with a sealing compound, in order to leave only a small volume of air behind the diaphragm. The connections to the coil 24 are made through disc terminals 33, 35 separated by insulation. The casing 21 is provided with three pins 42, Fig. 4 (not shown), which can be fitted into a bayonet holder in an airman's face mask. The central polepiece and coil may be of substantially rectangular section, Fig. 7 (not shown). In an airman's transmitter, the resonant frequency of the diaphragm is made about 2000 c/s by adding acoustic stiffness behind the diaphragm, e.g. by the plate 30. This may be modified by a silk-covered aperture arranged in the flange of the casing, Figs. 8 and 9 (not shown). The stiffness of the air between the diaphragm and the membrane 28, or between the membrane and the cover, raises the response up to 2700 c/s. This may be arranged to compensate for the effect of the ring pads by which a receiver is held against the head of the user, the frequency characteristic being flat up to 3000 c/s. Specifications 481,351, 481,740 and 513,529 are referred to.