GB332996A - Improvements in or relating to electric soundboxes - Google Patents

Abstract

332,996. Voigt, P. G. A. H. June 4, 1929. Gramophone pick-ups.-An electrostatic pickup comprises a non-plane diaphragm the displacement of which causes a capacity change. Fig. 1 shows a sound box combined with an electrostatic microphone of the kind described in Specification 263,300. A sheet of foil J is insulated from the back plate A by a thin air cushion, and a sheet of insulating material R. A screw H makes contact with the foil and engages insulating material G plugging the usual sound opening. Variations of a.ir pressure produced by movements of the conical diaphragm B produce corresponding movements of the foil J and vary the capacity of the condenser comprising the plate A, dielectric K and air cushion and the foil J. In a modification, a diaphragm Q, Fig. 3, of the shape of the cap of a sphere, forms the moving electrode of the con. denser. An internal electrode M is insulated from the back plate A by material N, and the diaphragm is driven by spokes R secured to a hub S. As the thin air cushion causes the diaphragm to appear stiff, the stylus D may be made springy by cutting it down as at P to avoid excessive record wear. Damping is introduced by a flat spring V fixed to the needle holder Wand pressing on a fibre plate U fixed to the end of the stylus. In another modification, Fig. 4, the internal electrode M may be cut away to provide an air chamber X communicating with the space behind the diaphragm by small holes Y which serve as damping means. The damping holes may open into the air. A foil electrode Z may be electrically connected to the internal electrode M. Specification 332,995 also is referred to.