GB410778A - Improvements in or relating to diaphragms for acoustic devices - Google Patents

Abstract

410,778. Diaphragms. ELECTRIC & MUSICAL INDUSTRIES, Ltd., Blyth Road, Hayes, Middlesex, and DUTTON, G. F., 57, Redcliffe Road, South Kensington, London. Nov. 15, 1932, No. 32324. [Class 40 (iv).] A large acoustic diaphragm formed wholly or partly of a material having a higher ratio of Young's modulus to density than paper is provided with corrugations disposed in paths curved around the driving point or zone and merging into one another over at least a part of the surface of said material, the ratio of the pitch of the corrugations to the depth thereof being greater than 5 to 1. Fig. 1 shows a diaphragm 1 formed from a circular sheet of aluminium into a conical frustum having an apex angle of about 105‹, the radial edges 2, 3 being joined by a vinyl acetate polymer softened by heat. The frustum is then placed in a press and corrugated, the corrugations extending about two thirds of the distance from the periphery to the spigot 4. This spigot, which is adapted to have a coil former secured thereto by cellulose cement, is formed by a drawing process which puts the metal of the spigot into circumferential compression and the metal in the zone immediately adjoining the spigot into circumferential tension. The periphery of the diaphragm is mounted in an aperture in a box or baffle by means of a ring of velvet attached to the diaphragm by plasticised vinyl acetate polymer with which the ring itself may be impregnated. In order to damp circumferential vibrations, the diaphragm may be provided with a plurality of radial joints spaced at equal intervals and incorporating a damping substance such as a plasticised vinyl acetate polymer Figs. 2, 3 and 4, not shown). Modified methods described for forming the diaphragm consist of (a) spinning a conical blank from a sheet of a thickness equal to the maximum thickness of the finished diaphragm, the radially outer portion being drawn thinner by the spinning operation or (b) forming the conical blank from a flat sheet by a series of drawing operations interspersed with annealing operations. The inner one-third of the cone surface may be provided with a little more than one shallow corrugation. If desired, the pitch of the corrugations may become progressively smaller from the centre outwards and their depth increase progressively in the same direction. According to a further modification (Fig. 6, not shown), the diaphragm is formed with an inner portion of aluminium and an outer portion of paper, the inner portion being corrugated from its joint with the outer portion to about the mid point between the joint and the spigot. Various forms of resistance joint between the portions are described, plasticised vinyl acetate polymer being used as the adhesive and the joint being reinforced, if desired, e.g. by sewing with thread. According to the Provisional Specification, the base of the conical frustum may be inclined at an acute angle to the axis and the corrugations may be eccentric. A construction of flat corrugated diaphragm is also described.