Patent GB-301437-A (GB)
Title: Improvements in electrodynamic loudspeakers
IPC: H04R7/04
Filed: 1927-05-27
Abstract: 301,437. Sykes, A. F. May 27, 1927. Addition to 160,223 and 227,162. Loud-speakers ; magnetic instruments ; diaphragms. - Relates to electrodynamic loud. speakers comprising a tone-producing surface, the whole periphery of which is flexibly attached to the magnetic structure or housing by a continuous substantially airtight connection so that the periphery can vibrate relatively thereto, the tone-producing surface being connected with and overspanning a conductor or conducting winding arranged in a magnetic field substantially at right-angles to the direction of the intended vibration of the conductor. The invention consists in connecting the conductor to the toneproducing surface by a transmission member which contacts therewith only at portions thereof on each side of the geometrical trace of the conductor on the tone-producing surface. Fig. 1 shows a construction in which a coil 1 is wound on the cylindrical portion 451 of an annular transmission member of Y-shaped cross-section, the annular conical portion 450 of which is connected as shown with an annular tone-producing surface 29, 32. The surface 29, 32 is mounted on stretched rubber 49 sandwiched between cardboard washers 255, 256 and 454, 455. The transmission member may be made of a metal, such as aluminium, in which case the current induced therein introduces damping forces, or it may be made of celluloid, wood, paper stiffened with synthetic resin &c. When a non-metallic member is employed, electromagnetic damping is obtained by coupling the coil 1 to a transformer such that the resistance of the coil as transferred to the anode circuit is equal to or greater than the anode impedance. The conical portion 450 may be perforated for lightness and provided with prongs which penetrate the surface 29, 32 and are then bent over. Alternatively, it may be formed with perforated flanges adapted to be stitched to the surface. The interspace 456 may be filled with damping material, such as cotton wool. Fig. III. shows a further modification in which the transmission member is formed in two pieces with the coil 1 between them. Alternatively, the member can be formed in one piece and the winding dropped in and cemented in the internal channel. Different ways of dividing up the surface 29, 32 so as to obtain the lines on which the conical portion 450 is to be attached are described. The complete Specification describes the application of the invention to tone-producing surfaces in the form of domes and cones and annular domes and cones. When a conical diaphragm is employed it may be mounted either in a baffle plate or board as described in Specification 186,658 or in an aperture in the side of a box or a division between two rooms as described in Specification 160,223. Centring arrangements may be provided for the coil, as described in Specification 253,159. Fig. IV. shows a flat tone-producing surface 29, 32 impulsed by an annular cone 450 with further annular conical transmission members 460, whilst Fig. VI shows a transmission member formed from a cylindrical ring split at regular intervals and splayed out to form the members 461, 462.