GB1289858A - - Google Patents

Abstract

1289858 Diaphragms; moving coil instruments J J BERTAGNI 29 Dec 1969 [30 Dec 1968] 63105/69 Heading H4J A substantially rigid diaphragm of planar form for a loud-speaker, microphone or other electroacoustic transducer comprises a platelike member (Fig. 3) made of cellular plastics material such as polystyrene or polurethane. The member has a flat front face, a rear face and side faces; and is prestressed in a direction substantially perpendicular to the plane of the front face and in a direction substantially parallel to said plane, the magnitude of the stresses gradually decreasing from the front face to the rear face. Preferably a homogeneous cellular flat plate is first produced (Fig. 1, not shown) and is bent so that the front face becomes concave (Fig. 2, not shown). The granules of the front layer are thereby elongated and the spaces between them are reduced in size. Once bending has been completed, heat may be applied to release any moisture in the diaphragm. Sealing film may be applied to the various surfaces and a sheet of paper or fabric 32 applied to the front face. The sealing film may be adhesive, or minute spikes forming part of the wheel 32 may attach the sheet 32 to the front face. Instead of bending the plate, the sheet 32 may be made of fabric which soaks in part of the sealing film, thereby shrinking the fabric and bending the plate. The bent plate is next urged into the planar condition (Fig. 3), the granules being further flattened. The bending and straightening steps provide the required stresses. The margins 33 of the sheet 32 are adhered to the side faces of the plate. If large enough, the margins 33 may also be turned over on to the rear face. To assist in adhering the margins 33, the plate sides may be irregular (Fig. 14, not shown). Preferably the plate has an ear-shaped part 44 (Figs. 6, 9) within a margin portion 43, part 44 is cap-shaped and has a stiffer portion 45 located away from the geometrical centre. A voice coil is connected to the stiffer portion 45. The margin portion 43 is damped by a spray of material which hardens, or by a rigid plate (47), Fig. 8 (not shown), mounted on the rear face of the margin portion 43. The diaphragm is mounted in a frame 50. A non-magnetic cross-bar 53 supports a magnet assembly 54 for the voice coil. The frame 50 is closed by a rear cover (56), Fig. 8 (not shown), of the same material as the diaphragm and having frusto-conical damping projections (57). An air hole 58 is provided. If desired, a second voice coil 63 (Fig. 11) co-operates with a second magnet assembly and an insert 64 of aluminium or brass plate in the diaphragm provides "tweeter" action. In another diaphragm, the plate (Fig. 13) which is to be bent and straightened to give the required prestressing, is made from granules partially pre-expanded to be of different sizes. It has different zones, each zone having granules of the same size. The zone with the smallest granules provides a region for subsequent attachment of a voice coil.