Patent GB-1500398-A (GB)
Title: Electro-acoustic transducer diaphragms
IPC: H04R31/00
Filed: 1975-07-14
Abstract: 1500398 Diaphragm assembly; electrostatic transducers SONY CORP 14 July 1975 [16 July 1974] 29501/75 Heading H4J A diaphragm assembly is made by assembling a metal frame 13, 14 (Fig. 1) in supporting relation relative to a diaphragm 12 and heating a layer of thermoplastic material (which may be part of the diaphragm 12) by heating the metal frame 13, 14, while the frame and diaphragm are pressed together, so that heat transmitted through the frame causes the thermoplastic material to melt at the engaged surfaces, and halting the heating so that the thermoplastic material hardens to bond the frame and diaphragm to each other. An electrostatic headphone or microphone (Fig. 2) has metal frames 13, 14 abutting opposite sides of a diaphragm 12. A casing 15 is made up of complementary members 16, 17 including interfitting rim portions 16a, 17a and walls 16b, 17b which define a cavity 18. Aluminium electrodes 19, 20 are vapour deposited on the inner surfaces of walls 16b, 17b. The walls 16b and 17b have perforations 16c and 17c. The outer surfaces of walls 16b, 17b have shielding metallic members 21, 22 connected with the adjacent electrodes 19, 20 respectively. The diaphragm 12 (Fig. 1) is of high polymer synthetic resin, e.g. fluorinated ethylene-propylene resin (F.E.P.) of thickness 12 to several hundred microns. The metal frames 13, 14 are of aluminium, stainless steel or brass and are located in recesses of metal members 25, 26 having heating elements 27 incorporated. In operation of the apparatus of Fig. 1, the heating elements are energized to heat frames 13, 14 to a temperature above the melting point of the diaphragm 12 (e.g. 350 to 380 C.) with the diaphragm in tension and air under pressure is admitted to cylinder 31 above piston 30 through conduit 33 to urge locating member 25 downwardly and press the frames 13, 14 against diaphragm 12 with a pressure of 2À5 to 4À0 kg/cm<SP>2</SP> for 2 to 3 seconds. Heating of elements 27 is then terminated and, when the diaphragm 12 has hardened, air under pressure is applied to conduit 34 for raising support 28. The diaphragm assembly is removed, thermoplastic material projecting beyond frames 13, 14 is trimmed and the diaphragm 12 is electrically charged by an electron beam or corona discharge to form an electret. Alternatively the electret diaphragm is formed of a lamination of thermoplastic film, e.g. F.E.P. for bonding to a frame and of a plastics film (e.g. P.T.F.E.) which is thermosetting and is capable of retaining electric charge for about 20 years. In another embodiment (Fig. 3, not shown) a single metal frame (14A) is assembled with the previously described diaphragm (12A). A lower locating member (26) again has a heating element and pressure is applied pneumatically using a heat insulating body (35) of silicone rubber. The resulting diaphragm assembly may be installed in the push-pull transducer of Fig. 2 or in a transducer having a single fixed electrode (not shown). In this case an aluminium or gold electrode may be vapour deposited on the side of F.E.P. layer and sandwiched between P.T.F.E. and F.E.P. layers, the F.E.P. layer being between the electrodes. Similarly the diaphragm assembly of Figs. 1 and 2 may have a single fixed electrode, a vapour deposited metal layer being applied to the surface of the diaphragm 12 which faces away from the fixed electrode. Instead of the diaphragm being formed of one or more layers of plastics material it may be of metal foil and secured to the metal frame or frames by a layer of thermoplastic resin between the or each metal frame and the surface of metal frame to be bonded thereto. In the various embodiments the or each metal frame alternatively may be heated by passage of an electric current therethrough during bonding.