Patent GB-417718-A (GB)
Title: Improvements in or relating to vibratory devices such, for example, as are used in the electrical recording or reproduction of sound
IPC: H04R9/12
Filed: 1933-03-07
Abstract: 417,718. Indicating - apparatus. ELECTRIC & MUSICAL INDUSTRIES, Ltd., Blyth Road, Hayes, Middlesex, and BLUMLEIN, A. D., 32, Woodville Road, Ealing, London. March 7, 1933, No. 6951. [Class 106 (iv).] A vibratory device such as a phonographic recorder or reproducer, an oscillograph, a vibration galvanometer or an optical sound recorder comprises a vibratory member supported wholly or in part upon a fixed member by means of a resilient member which, by virtue of its torsional compliance, permits rotary motion of the vibratory member about an axis, the free length of the resilient member in the direction of the axis being less than the dimensions thereof in a direction normal to the axis, and the torsional compliance of the resilient member being greater than the compliance thereof to motion in a direction normal to the axis, both said compliances being expressed in C.G.S. units. According to a further feature of the invention, a sound recording or reproducing device is provided with a vibratory member constituted by or adapted to carry a stylus and supported wholly or in part by a resilient member having substantial internal mechanical damping which permits motion of the vibratory member about an axis by virtue of its torsional compliance and also permits motion thereof at right angles to the axis, the arrangement being such that, for a predetermined force applied to the driving or driven point, the displacement of said point due to the torsional compliance of the resilient member is much greater than the displacement of said point due to the compliance of the resilient member to motion in a direction at right angles to the axis. Fig. 1 shows a construction of moving coil phonographic recorder of the kind described in Specification 350,998, [Group XXXIX], in which a, pivotally mounted coil 4 constituted by a single, closed rectangular turn of aluminium acts as the secondary winding of a transformer. In accordance with the invention a mild steel peg 5 is inserted in the central portion 3 of the transformer core, a small frusto-conical depression being formed in each end surface of the peg to receive tapered rubber pivots 7, 7a for the coil, each of the pivots 7, 7b being arranged and dimensioned as set out above. The upper pivot 7 is held in position by a short steel ram 8 which is driven into an aperture in a central boss 4<a> on the coil by a torsion wire nut 9 in a central cavity in which is soldered the enlarged end of a steel torsional control wire 10. The remote end of this wire is considerably enlarged and is held in a clamp 11. The lower pivot 7<a> is held in position by a steel ram 8<a> and a grub screw 12 threaded into an aperture in the base 4b of a fork formed on the lower side of the coil 4, a stylus arm 13 carrying a cutting tool being mounted between the limbs of the fork with interposed rubber sheets 15. A mass in the form of a split ring 16 sprung over rubber pads 17 around the boss 4<a> may be provided in order to damp any resonance effects due to the masses of the upper and lower sides of the coil resonating with the compliance of the sides which are located in the air gap. Fig. 4 shows a construction of gramophone pick-up employing a stylus 18 operating both as armature and needle which is mounted within a brass block 19 on rubber pivots 21 engaging in diametrically opposed frusto-conical depressions formed in the enlarged central portion of the stylus. Each pivot is forced into its corresponding depression by a ram 22 and grub screw 20. The upper end of the stylus is inserted in a rubber damping pad 23 mounted between the opposed tips of a pair of flanged pole pieces 24, 25 secured to the block 19 and fitted between the limbs of a U-shaped permanent magnet (not shown), a coil 26 surrounding the upper half of the stylus being disposed between the upper surface of block 19 and the flanges on the pole pieces. When applied to oscillographs, vibration galvanometers and optical sound recorders, the cutter of the arrangement shown in Fig. 1 may be omitted and a mirror attached to the vibratory member.