Patent GB-447777-A (GB)
Title: Improvements in or relating to microphones
IPC: H04R21/02
Filed: 1934-11-27
Abstract: 447,777. Microphones. SIEMENS BROS. & CO., Ltd., Caxton House, Tothill Street, Westminster, and SUTTON, G. W., 25, Cranleigh Road, Merton Park, London. Nov. 27, 1934, No. 34066. [Class 40 (iv)] A differential microphone or transmitter comprising two fixed electrodes and an intermediate moving element is constructed so that the active face of each of the fixed electrodes stands away from the end of the granule chamber on which the electrode is supported and so that a space for the accommodation of granules exists around the body of the electrode and /or around a member supporting the electrode and projecting into the granule chamber, this space being of such dimensions as to ensure that with a proper filling of granules in the chamber a substantial head of granules acts on the active face of the electrode for all positions of the microphone or transmitter. The electrode 1 is moved between fixed electrodes 2, 3 by a short rod 11 secured to the apex of a diaphragm 4 which is clamped between a brass ring 8 and a casing 7 and is protected by an apertured guard plate 9. A brass casing 17 which carries the electrodes and forms the granule chamber is secured to, but insulated from, the casing 7 by a ring of insulating material 18 formed with rivet members 19 engaging holes in the casing and three equallyspaced projections 20 entering holes in the brass wall of the casing 17. The front electrode 1 is mounted on the wall 21 of the casing provided with a hole larger than the stem 11 and closed by a felt washer 14, an insulating washer 15 of a synthetic resin and a spring ring 15. The rear electrode 3 is mounted in a recess in a brass member 23 which is formed with a hollow stub 26 carried in an insulating member 24 forming the rear end of the casing. Between the members 23, 24 is a felt ring 12 pervious to air and co-operating with air passages 29, 30. The hollow stub is closed by a screw 28. A spring ring 33 forces the transmitter against the mouthpiece part of a holder in which it is inserted. The casing 7 is formed with three projections 35 equally spaced around its circumference and a fourth projection midway between two of the projections 35. The ring 33 has three equally spaced cuts which extend over a distance greater than half that between the equally-spaced projections 35 so that it can be slipped over the four projections and rotated to be locked in position. In a modification of Fig. 1, Fig. 2 (not shown), the granule chamber is divided into two parts by a silk diaphragm which is clamped at its centre between the separate portions of the moving electrode and at the rim is held between two parts of the casing 17. Alternatively, a flexible disc, clamped at the rim, is arranged to leave a small space surrounding the moving electrode. In a form of this modification, Fig. 3 (not shown), the insulating material 18 of Fig. 1 is extended within the casing as an annular web leaving a small clearance around the moving electrode. In a further modification, Fig. 4, the granule chamber is divided by a diaphragm 55 of mica or of thin metal, covered with an insulating varnish or enamel, which is clamped at its rim between two screwed members 38, 39 forming the cylindrical wall of the chamber. At the centre the diaphragm is threaded on an extension of the member 11 and is clamped between carbon members 57, 58 which are channelled to decrease the weight of the moving part. All surfaces except the active faces of the electrodes are coated with an insulating varnish or enamel. The diaphragm 64 is clamped at its rim between packs of paper washers 65, and electrical contact between the diaphragm and the casing 7 is facilitated by a flexible metal spider 82. In a further modification, Fig. 5, the third electrode constituted by two parts 72, 73 is fixed and co-operates with the fixed electrodes 2, 3 and a movable actuating element 67 coupled to the diaphragm by an aluminium rod 11. The terminals of the microphone are in this case constituted by the casing 7 or a spring ring attached thereto, the casing 75 or the clamping ring 80, and a metal cap 79 secured to the brass member 23.