Patent GB-361988-A (GB)
Title: Improvements in or relating to condenser microphones
IPC: H04R19/04
Filed: 1930-09-03
Abstract: 361,988. Microphones. GENERAL ELECTRIC CO., Ltd., Magnet House, Kings. way, London, and OLIVER, D. A., of General Electric Co., Wembley, Middlesex. Sept. 3 1930, No. 26272. [Class 40 (iv).] Relates to condenser microphones in which one electrode comprises a vibratory diaphragm which is held in a suitable clamping ring and has its tension adjusted by means of a ring pressed against the back thereof and consists in the provision of one or more shims between the body of the microphone and the tensioning ring, the shim or shims deterniining the gap between the diaphragm and the other electrode. The arrangement is preferably such that after the tension of the diaphragm has been adjusted, the diaphragm and tensioning ring can be removed together without disturbing the adjustment. The microphone shown in Fig. 1 comprises a body member 1, electrode 2 insulatingly mounted therein, a diaphragm 3 clamped in ring 4 and tensioned by ring 5 and an adjusting ring 13. The electrode 2 comprises a disc 6 on one end of a spindle 7, the other end of which is threaded at 8 to receive a lock nut 9. The spindle 7 passes through quartz washers 11 located in recesses in the body 1, a metal ring 12 being interposed between the rear washer and nut 9. The tensioning ring 5 is formed at its outer end with a projecting face 14, is provided with four equally spaced longitudinal grooves 15 on its outer surface and is formed with an annular recess at its inner end to accommodate the adjusting ring 13. The ring 5 is secured to the body member 1 by screws and the surfaces of disc 6 and face 14 are then accurately ground co-planar and finally lapped dead level. One or more shims 20 are then interposed between ring 5 and body member 1, the shim or shims determining the gap between the disc 6 and diaphragm 3. The latter is held between ring 4 and a steel ring 17 bv a clamping ring 16 screwing into ring 4, ring 17 being prevented from rotating by grub screws 18. The inner surface of ring 16 is formed with four equally spaced longitudinal grooves which correspond with the grooves 15 in ring 5 to form cylindrical passages in which rods 19 are fitted so that ring 16, together with diaphragm 3 and ring 4, can slide longitudinally relatively to ring 5 without rotating, the ring 16 engaging an internal thread in the adjusting ring 13. Rotation of ring 13 draws diaphragm 3 into engagement with face 14, further rotation increasing the tension of the diaphragm. The ring 13 is finally clamped in position by grub screws 21 and a ring 22 is screwed into ring 4 to clamp the diaphragm against the face 14. The static pressure on either side of the diaphragm is equalized by connecting the air in the gap between the diaphragm and electrode 2 with an air space 25, enclosed by a flexible oiled silk diaphragm 26, by means of ducts 23, 24. Small finely drilled plugs 29 in these ducts prevent acoustical resonance effects from affecting the normal motion of the diaphragm. The back of the microphone is closed by a cover 27 provided with an air duct 28 and connectors 30, which may be insulated from member 1 by bushes 31, are provided for bringing out shielded lead connections. Fig. 2 shows a simplified construction in which the body member 1 consists of a metallic disc having a central aperture provided with a small flange 35. A quartz washer 11 fits in this aperture and is clamped against flange 35 by ring 36, the stem of electrode 2 passing through washer 11 and being fastened in place by nut 9. The tensioning ring 5, which is of L-section and is secured on the front of member 1, has its projecting face finished accurately flat and level with the face of electrode 2, a thin shim 20 being then inserted between the ring and the body member so that the shim determines the distance between electrode 2 and diaphragm 3. The diaphragm, which consists of a thin gold-plated disc made of the material known under the registered Trade Mark Duralumin, is clamped between rings 4, 16, recessed at 38 so that only their outer and inner edges actually nip the diaphragm. These rings are held together by screws between which are provided longer tensioning screws 39 which enter tapped holes in the face of member 1. A ring 40 closes the cavity 41 which may be filled with cottonwool to prevent resonance. A single-core shielded cable is passed through an ebonite bush 44, the shield being clamped by a strap 45 which is insulated from the body of the microphone but electrically connected by a lead (not shown) to a terminal 42, the core of the cable being connected to a lug 43 on electrode 2. The terminal 42 is insulated from the casing but connected thereto by a calibrating resistance.