Patent GB-551995-A (GB)
Title: Improvements in methods of making elastic diaphragms
IPC: H04R31/00
Filed: 1941-09-19
Abstract: 551,995. Films etc.; diaphragms ; telephone instruments ; air-velocity meters. BRITISH THOMSON-HOUSTON CO., Ltd. Sept. 19, 1941, No. 12045. Convention date, Sept. 21, 1940. [Classes 2 (ii) and 2 (iii)] [Also in Groups XXIX, XXXIX and XL] Thin membranes are obtained by immersing a solid base member in a solution in a volatile solvent of a cellulose derivative or a synthetic resin, withdrawing the base member from the solution to cause formation of a layer of filmforming material thereon, immersing the base member with adherent layer in a non-solvent liquid, removing the base member and layer after an immersion of a few seconds, and thereafter lowering the base member through the liquid surface at a slow rate in order to separate the layer from the base member and to float the freed layer on the surface of the liquid as an independent membrane. Separation of the membrane from the base member by the penetration of the non-solvent liquid may be assisted by lifting a corner of the membrane from the surface of the base member with forceps during the second immersion, or by cutting through the layer at its edges before the first immersion. Where water is used as the non-solvent liquid, the base member may be rendered hydrophylic, e.g. by thorough cleaning to remove grease, prior to immersion in the film-forming solution. A mounting framework, e.g. a metal ring may be brought into contact with the floating membrane and caused to adhere to it, if desired with the aid of cement or heat. Diaphragms capable of substantial elastic deformation and of a thickness of about 100 to less than about 20,000 Angstrom units may thus be formed. They may be metallized if desired. They may be used in the construction of microphones, phonograph sound pick-up mechanism, and air-velocity measuring apparatus. The membranes may be of cellulose acetate, nitrate, or other ester, or of polystyrene, polyvinyl chloride, polyvinyl formal, acetal, butal, or other reaction product of an aldehyde with a hydrolyzed polymerized vinyl ester. The membrane substance may be plasticized. Two wedge-shaped film layers may be formed on the two major surfaces of the base member particularly if placed in a vertical plane during evaporation of the solvent. These layers may be brought together with the thinner portion of one in contact with the thicker portion of the other. The base member may be slowly rotated during the film setting step. A plurality of membranes may be laminated. A layer of a polyvinyl resin may be coated with a layer of a silicone resin such as a chlorphenyl silicone or a a methyl silicone. The base member may be dipped in a succession of different film-forming solutions and then in the non-solvent liquid in order to remove the composite membrane. As shown in Fig. 3, the former 24 carrying two film-layers 26, 27, and already soaked in non- solvent liquid is again slowly immersed in liquid 23. The layers are detached and they float on the surface. As shown in Fig. 1, the thin cellulose derivative or synthetic resin membrane 14, metallized by deposition of aluminium or other metal is positioned in microphone casing 10, provided with screen 12. Sound waves cause vibration of the diaphragm and variations in capacitance affecting an external circuit. As shown in Fig. 4, the membrane 31, metallized to form a mirror surface, is used in an air-velocity meter. Duct 34 is placed in an air stream and a pressure proportional to the velocity is set up in cup 32. Variations in the pressure produce variations in the bulging of the diaphragm and variations in the light reflected from the diaphragm through a V-slot in screen 42 on to a photosensitive cell. Specification 542,655 is referred to.