Improvements in or relating to electro-acoustic transducers

881,584. Capacitor microphones. STANDARD TELEPHONES & CABLES Ltd. Nov. 27, 1959 [Dec. 2, 1958], No. 38787/58. Addition to 832,276. Class 40 (4). A pressure-gradient capacitor microphone has a diaphragm open to incident sound on both sides but one of the sides being shielded by a rigid porous member of bonded insulating particles. As shown, in Fig. 4, a metal casing 1 with a perforated back wall 7 encloses a porous member 3. A plastic diaphragm 2 covers the front of the casing over which it is stretched and fixed by adhesive 8. Sputtered gold and sprayed colloidal graphite, respectively, coat the top surface of the member 3 and the outer surface of the diaphragm 2. The coating of sputtered gold is porous. Additional acoustic resistance may be provided by a silk layer 6 covering the back wall 7. As shown in Fig. 1, a diaphragm 10 of polyester, polypropylene, or polycarbonate film is trapped between metal rings 11 and 12 and fixed with adhesive. The rings may be injection-moulded on to the wall of the diaphragm which is formed into a cup-shape and held in the moulding tool. The outer surface of diaphragm 10, and the top surface of porous member 13, is coated with metal or graphite. A spacing washer 14 takes thrust from the member 13 to tension the diaphragm. The microphones may be round or rectangular and may be as much as ten feet long to make a line microphone. The porous members 3 or 13 may be composed of sintered ceramic secured in position by adhesive 9 or 20. Powdered glass, sand or ceramic may be used and be bonded with an epoxy resin or a foaming resin ; or powdered nylon, polycarbonate, or polymethyl methacrylate may be used and be bonded by heat softening and compression. Except for a sintered ceramic the members 3 or 13 may be moulded in the case 1 or within the ring 12.