GB1518285A - Acoustic reproducers - Google Patents

Abstract

1518285 Cone diaphragms J S AKROYD 18 Nov 1975 [19 Nov 1974] 50005/74 Heading H4J A cone for a bass loudspeaker or other acoustic reproducer comprises two laminated layers 10, 11 (Fig. 1), the two layers forming at least a substantial part of the wall of the cone. One layer 11 is a reinforcing layer and the second layer 20 is a softer more pliable layer. Both layers are impervious to air. The two layers may be shaped separately in a vacuum forming machine and then adhered together using air pressure to stretch the layer 10 and press it against the layer 11 or the second layer 10 may be applied to the reinforcing layer 11 by a vacuum forming technique. The two layers 10,11 are preferably of plastics material. The reinforcing layer 11 is relatively strong and rigid and may be formed from polycarbonate film, polyester film, cellulose acetate butyrate film, polyethylene terephthalate, or plasticized polystyrene. Alternatively it may be made from, or incorporate, carbon fibre. The second layer 10 is soft and pliable and may be formed from plasticized P.V.C. film or soft polyamide film. As described, a flat sheet of polycarbonate plastics is softened and sucked down on to a conical former to provide a sheet 11 of the shape of Fig. 1. The outer rim of the plastics cone thus formed is cut to size. The neck of the cone comprises a cylindrical portion 12 and the cone has a domed portion 13. When the cone has hardened small holes are pierced in the angle between portions 12, 13. The layer 11 is then placed in a female tool corresponding to that required for layer 10. A rigid gasket 14, coated with adhesive on one face is placed on the outer-edge of the female tool, adhesive coated side uppermost, and a blank of plasticized P.V.C. film treated with adhesive is clamped in a vacuum forming machine where it is infra-red heated and then sucked down over layer 11 tool and gasket 14 to give the shape of Fig. 1. The adhesive employed is heat reactivated water based P.V.A. glue. While the two layers 10, 11 are still under vacuum a reinforcing ring 15 is fitted into the neck of the cone to lie in the angle between portions 12 and 13. Ring 15 is of spring phosphor bronze, is split radially and urges layer 10 against layer 11. It reduces the tendency for the layers to peel apart. Layer 10 forms a U-section support surround 16 for the cone. In a modification (not shown) the reinforcing layer does not extend all the way to the rib 16. The rib 16 and gasket 14 may be formed as a separate component for subsequent attachment. A voice coil former 18 surrounds and is secured to the smaller end of the cone. A spider 19 is secured to the former 18. The thickness of layer 11 varies from 3À5 thou at its outer end to 4 thou at the neck of the cone. The thickness of layer 10 varies from 10 thou at the outer end to 2 thou at the neck. The ring 15 may be replaced by an aluminium clamping ring (22) Fig. 4 (not shown) of U-shaped cross-section, the outer profile of the U conforming to the profile of the acute angle between portions 12 and 13 of the cone. In another arrangement (not shown) the ring 15 is replaced by a small fillet of adhesive which sets to a rigid solid. The ring 15 may be replaced or assisted by a dust cap shaped to mate with the layer 10 and the dust cap and layer 10 may be adhered together over their entire contact area. A temporary clamping ring may be inserted until the adhesive has set or cured sufficiently. A polyester two-part adhesive may be used. Cryanoacrylate adhesive used to bond the coil former 18 in position may be used to fill the holes pierced in the neck of layer 11. Other bonding agents include solvent based glue, film glues or heat reactivated adhesive film. In an alternative neck construction (Fig. 2, not shown) the two layers 10, 11 are vacuum shaped to provide an open neck into which the former 18 is fitted (Fig. 2, not shown). The neck end of the inner layer 10 protrudes beyond and is formed around the neck end of the outer layer 11 and the protruding portion is clamped between the former 18 and spider 19 and reinforcing layer 11 by adhesive (21). Alternatively, an open ended end of the voice coil former 18 is sandwiched between the ends of the layer 10, 11 (Fig. 3, not shown). A dust cap (23), subsequently inserted, presses the rim of the layer 10 against the rim of the former 18. The dust cap (23), which may be adhered in place, acts as a holding device.