Patent GB-599790-A (GB)
Title: Improvements in electro-acoustic transducers
IPC: H04R9/10
Filed: 1943-11-11
Abstract: 599,790. Moving coil telephone instruments. PERMOFLUX CORPORATION. Nov. 11, 1943, No. 18807. Convention date, Nov. 25, 1942. [Class 40 (iv)] [Also in Group XXXV] A telephone transmitter or receiver of the moving coil type has a cup-shaped housing formed of magnetic material. The form shown in Fig. 2 is intended for use as a receiver or transmitter and comprises a housing 11, of steel for example, a central permanent magnet 20 having a soft iron polepiece 21 secured to the base of the housing by screw 27, and an outer pole-plate 19 pressed into a groove 18 near the open end of the housing and cooperating with the inner polepiece to define an annular air gap. Plate 19 carries a non- magnetic locating ring 35, e.g. of brass, for the polepiece 21. Diaphragm 25 is stuck at its periphery to rim 14 of the housing, and coil former 31 is stuck to the diaphragm about a central opening therein which is closed by a cap 65. A flexible suspension member 44 is connected between the diaphragm 25 and plate 19 to damp movement of the diaphragm. The space between the diaphragm and the plate 19 is vented into the interior of the housing by springs 67 and to the open air by vents 43 in the housing, the edges of the latter being swaged to lock the plate 19 in position. Openings 67 may be covered with foraminous material to afford additional damping. The lead wires 56 to the speech coil are attached by rivets 55 to the diaphragm and pass through vents 43 adjacent a terminal block 50 mounted on the outside of housing 11 and having terminals 49, the wires being threaded through holes in an insulating sheet 57 riveted to the housing, one of the wires being extended to a rivet 58 for earthing purposes. Fig. 10 shows a headset receiver wherein the air gap is formed between magnet 97 and pole-plate 79 riveted to flange 74 at the open end of the housing. The magnet is held by a screw 98 engaging washer 101 on the outer end of the magnet and cooperating with nut 100 welded to base 72 of the housing. Diaphragm 82 and cap-plate 83 are clamped over the plate 79 and flange 74 by a ring 93, which may include a packing ring 145, Fig. 16, if required. The cup-plate is formed of insulating material moulded on a perforated sheet of magnetic material 85 affording a magnetic screen. Locking projections 88 on plate 83 pass through diaphragm 82 into depressions punched into the edge of plate 89, the reverse side of the depressions forming projections 86 for locking plate 79. The coil form 96 is moulded integrally with the diaphragm and has an annular shoulder 107 for locating the cap 108. The leads 110, Fig. 12, from the speech coil are threaded out and in through the diaphragm and are attached to the ends of clips 112 secured adjacent venting openings 111 on the rear face of plate 79. This plate supports an impedance-matching transformer 125, Fig. 10, which is mounted on a bracket 126 and has its primary leads 128, Fig. 12, connected to the clips 112. The secondary leads 129, Fig. 10, of the transformer are connected to clips 123 on a terminal block 115 having a socket portion 116 passing through a hole in the housing. To accommodate headband clips of differing sizes the housing 71 is formed with flats in its sides and trunnion sockets 130a and 131a, Fig. 16, are punched in the flat and round portions of the sides. In a further form, not shown, the thickness of housing 71 is reduced and a low inductance path for magnetic flux over the base of the housing is provided by the nut for securing screw 98. In Fig. 16, the housing 71a and plate 85a serve as a screen for the magnetic circuit constituted by the soft-iron yoke 140 and magnet 97. The yoke is carried on a locating plate 79a, also of magnetic material, and carries locating ring 105a for the magnet. Cap plate 85a is plane and is separate from the plate 85a which completes the screen. Plate 85a has a perforated and dished central portion covered by a foraminous screen 148, which improves the frequency response. For use in aeroplanes, diaphragm 82 may have pin holes in it to permit pressure equalization between the two sides of the diaphragm, but if the instrument is required to remain waterproof, embossed panel 72b at the back of the housing may be apertured and sealed by a rubber membrane covering the cavity within the panel. The housing may be wholly or partly filled with fibrous material to afford damping, the damping material being inserted loosely or as preformed elements and restrained from the diaphragm by screens covering the venting apertures in plates 19 or 79. The diaphragm 82 is provided with an annular corrugation 82b which improves the frequency response.