GB363449A - Improvements in or relating to sound reproducers - Google Patents

Abstract

363,449. Gramophone pick-ups. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.- (Assignees of Keller, A. C. ; 197th Street, New York, U.S.A.) Oct. 16, 1930, No. 30996. Convention date, Oct. 24, 1929. [Class 40 (iv).] The moving coil 13 of an electrical sound reproducer for hill-and-dale records is rigidly attached to a substantially coaxial stylus 15 and is adapted to be moved axially thereby through a radial magnetic field. In one form, a U-shaped magnet 1 is mounted in a casing 2 and has polepieces 4, 5 so shaped and positioned that a detachable central piece 6 of the pole 5 projects into an opening in the polepiece 4 to form an air gap 8 in which the coil 13 is situated. The coil 13 may consist of about 40 turns of fine ribbon preferably aluminium wire wound on a hard rubber former 14 carrying the stylus 15. The ends of the coil 13 are connected to terminals 18. The moving system is elastically supported on two pieces of spring steel wire 19 which also serve to hold the stylus in contact with the record. The air gap may be of any desired shape and a brass bracket 9 maintains the alignment of the polepieces. The reproducer is so designed that the maximum mechanical impedance in dynes per centimetre-per-second measured at the stylus point. at any frequency within the range of frequencies to be reproduced multiplied by the maximum velocity in centimetres per second to be attained by the stylus point at this frequency is less than the stiffness in dynes per centimetre multiplied by the deflection or strain in centimetres of the elastic support when the stylus is resting upon the record groove normally but in the static condition; and the mass of the coil and stylus assembly and the stiffness of the elastic support are so mutually proportioned that these form a resonant mechanical system having a natural frequency at about the geometric mean of the highest and lowest frequencies to be reproduced. The Specification contains an analytical mathematical consideration of the manner in which the design may be varied to suit requirements and conditions. Specific values are given and claimed in the Specification for the dimensions and mass of the movable parts. The coil may be damped by using oil around the coil.