GB325478A - Improvements in sound reproducers - Google Patents

Abstract

325,478. Western Electric Co., Ltd., (Bell Telephone Laboratories, In,c.). Nov. 14, 1928. Gramophone pick-ups. - A sound-reproducer comprises a movable armature 4 co-operating with a magnetic circuit and engaging a record surface through the medium of a long, light and rigid stylus 5 which is attached directly to the armature and adapted to oscillate about an axis in or closely adjacent to the armature. The ratio of the stiffness of the stylus to the effective mass of the moving system at the stylus point is so great that the natural period of oscillation of the assembly is near or above the upper limit of frequencies to be reproduced. The stylus 5 is fitted with a threaded base 11, Fig. 3, for engaging a threaded stud 7 projecting from the armature 4 as shown in Fig. 1, and the armature together with a pair of arms 8 extending at rightangles to the armature, is sweated or brazed to a diaphragm 2 constituting a flexible closure to a casing 1 which encloses a bi-polar reproducer and viscuous damping material 3. In one form the stylus 5 is a conical nickel shell having a length of 1 inch and effective mass of 0.0034 grams, and the effective mass of the armature and diaphragm is 0.0066 grams. The point of the stylus may be shaped to conform to the record groove and then plated with a hard metal such as chromium. If desired, the stylus may be formed by electro-deposition. The damping material 3 in the container 1 is arranged to offer a mechanical resistance to motion substantially equal to 2 multiplied by the resonant frequency of the movable-system multiplied by the effective mass of the moving system at the stylus point. The net restoring force acting on the system, which represents the difference between the stiffness of the diaphragm and the elastic force due to the magnetic, effect of the polarizing field, is reduced to a limit very little above that necessary normally to maintain the armature in a free central position. The reproducer is stated to be particularly applicable for the reproduction from soft wax records such as the original wax record upon which the sound is recorded. Various formulae and graphs of such formulae are given in the Specification to illustrate how the invention is to be applied.