Patent GB-458287-A (GB)
Title: Improvements in or relating to loud speakers
IPC: H04R23/00
Filed: 1935-11-04
Abstract: 458,287. Loud-speakers. VOGT, H., 53, Birkbuschstrasse, Steglitz, Berlin. Nov. 4, 1935, Nos. 30402 and 30403. Convention dates, Nov. 3, 1934 and Oct. 1, 1935. [Class 40 (iv)] A loud-speaker has a magnet system comprising a rotating magnetic core provided with a coil carrying the signal currents, a magnetic yoke mechanically coupled to rotate with the core and magnetically attracted to it, and a thin magnetic disc arranged between the core and the yoke so that in response to the signal currents a variable frictional torque is exerted on the disc which is operatively connected to a diaphragm. A counteracting force is applied to the disc to return it to its normal position of rest. The shape and dimensions of the magnetic system, the degree of magnetization and the material of the magnetic circuit are such that the effective frictional surface-magnetic resistance is greater than 2.10<3> cm<3>. In the embodiment shown in Fig. 1a, the speech coil 4 is located in a magnet 1 rotated by a shaft 8 which drives a magnetic yoke 5 by means of a pin 10. The thin disc 6 is lightly mounted on the shaft 8 between the magnet 1 and the core 5, and is provided with an arm 16 attached to a diaphragm 15. The pin 10 engages in an oblong slot in a sleeve 3 which is biassed to its normal position by a spring 9. The disc is of a hard material and is brightly polished. Hardened metal inserts may be provided in the members 1, 5 on opposite sides of the disc. The speech or signal currents are fed to a contact ring 13 engaged by a contact pin 11 and to the disc 16 which is slidably engaged by a pin 12. The magnet may be E-shaped with the coil mounted on the centre limb. As shown in Fig. 1d, the magnetic system comprises four E-shaped magnets 58 with their coils 56 mounted on a disc provided with metallic studs 57 to increase the frictional effect. To avoid frequency doubling without premagnetization of the magnet system, a second magnet system controlled by current derived from the negative half-wave of the signal current may be employed to effect the return movement of the disc. The signal current may be fed to a transformer connected to two valves biassed so that they are rendered alternately conducting by the positive and negative half-waves respectively. An arrangement embodying the positive return of the diaphragm movement is shown in Fig. 3b in which members 20, 21 connected to magnetic discs rotated in the same sense, are positively moved in opposite directions during the positive and negative portions of a cycle. The diaphragm 80 is connected to the junction of the members 20, 21 by a rod 27 and flexible springs 81, 82. In a modification Fig. 4a (not shown), the magnet systems are rotated in opposite directions about the same axis by a roller which engages flanges on the magnet members, and the associated discs have extensions connected to a rod attached to the apex of a diaphragm. The loud-speaker may be utilized solely to reproduce sounds of low frequency, an electromagnetic or electrodynamic unit being employed for the middle and high frequencies.