Patent GB-378459-A (GB)
Title: Improvements in or relating to acoustic devices
IPC: H04R7/16
Filed: 1931-05-12
Abstract: 378,459. Loud-speakers. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.- (Assignees of Harrison, H. C.; 94, Bayview Avenue, Port Washington, New York, U.S.A.) May 12, 1931, No. 13999. Convention date, May 14, 1930. [Class 40 (iv).] A loud-speaker comprises in combination an elongated direct-acting diaphragm tensioned to a high degree in one direction and a currentconducting member in the form of a strip secured to the diaphragm transversely to its longitudinal length and positioned in the air gap of a magnetic system. The loud-speaker shown comprises a frame on which is supported a diaphragm 61 adapted to be driven by a transverse conducting strip 40 which is located in the gap between the polepieces of a magnet structure 16 also carried by the frame. The frame comprises vertical iron pipe supports 1 united at their extremities by channel members 11 and is strengthened by angle-irons 3, 4 and bracing members 2 spaced from supports 1 by members 7, 8. The diaphragm 61 comprises a plurality of closely-spaced tensioned wires 13 of the material known under the registered Trade Mark " Duralumin," the wires being covered by thin pamper 12. The length of the diaphragm is preferably greater than twice its width, and also preferably greater than onehalf the wave-length of the lowest frequency to be radiated. The diaphragm is preferably tensioned in the direction of its length to such a degree that the sound vibrations are propagated therein at a velocity greater than onequarter of that of sound in air. A slight lateral tension may be introduced, if desired. Fig. 5 shows means for securing the wires 13 in spaced relation. Such means comprises a member 49 bolted to the channel member 11 and formed with parallel saw cuts 52 and at rightangles thereto a series of saw cuts in which the ends of the wires 13 are positioned and clamped by forcing the portions 53 between the vertical saw cuts over the wires. Such an arrangement is employed at each end of the frame, the wires being initially clamped at one end, then tensioned to the desired degree and finally clamped at the other end. An elongated member 46 protects the wire securing member 49. According to a modification, Fig. 5A (not shown), the cross cuts 52 may be omitted and the wires clamped in continuous parallel vertical slots. A leather strip 51 is secured as shown in Fig. 5 between the support 1 and the diaphragm edge, such a strip being employed along each lateral edge of the diaphragm. Alternative forms cf diaphragm described comprise a uniformly tensioned sheet diaphragm, a transversely corrugated sheet diaphragm, or one made up of a plurality of elongated uniformly tensioned strips. The diaphragm is driven by a current-conducting strip 40, a portion of which is. shown in Fig. 3, the strip extending across the diaphragm intermediate its ends and being preferably provided with feet 54 each of which is secured to one of the wires 13. The strip is of a material having a high ratio of conductivity to mass and is formed with flexible end portions, either by slotting it as shown at 55 or by making the ends of thin foil. Each end of the strip is secured to a terminal 19 mounted on but insulated from the tubes 1, each terminal being connected by a low impedance conductor 18 to the secondary of a transformer 14 mounted on one of the tubes 1. Conductors 18 are secured to but conductively insulated from a portion of the magnet structure. The impedance of strip 40 is substantially equal to the combined impedances of the secondary of the transformer 14 and the conductors 18. If desired, a plurality of conducting strips forming a portion of the winding of an elongated coil could be employed. The magnet structure 16 which is secured at its opposite ends to the supports 1 and supported by members 5, 6 is shown more clearly in Figs. 6 and 10. This structure comprises an elongated magnetic plate 24 which is secured to an angularlydisposed plate 27 which forms one of the polepieces and has a tapered edge portion 65 converging to form a long narrow edge 66. Triangular members 23 of magnetic material are secured to the under surface of member 24 .which supports magnetic cores 31 surrounded by energizing coils 17, each core being secured to one of the members 23. The other polepiece comprises triangular members 22 and V-shaped members 21, each member 22 having an opening 69 into which one of the cores 31 project. A screw 37 passing through member 21 into core 31 and screws 38 passing through member 22 into the non-magnetic portions 33 of blocks 32 on member 27 hold the members 21, 22 firmly in place. The portion 70 of member 22 projecting out beyond the blocks 32 terminates in an elongated surface 71 which forms with the surface 66 on polepiece 27 an elongated gap in which the strip 40 is located. The width of the air gap may be varied by using different thicknesses of washers 36 between the under surface of member 22 and the portions 33 of blocks 32. An elongated permanent magnet with oppositely-disposed pole portions may be employed, if desired.