Patent GB-795962-A (GB)
Title: Transducers
IPC: H04R19/00
Filed: 1955-04-26
Abstract: 795,962. Loud-speakers. PHILCO CORPORATION. April 26, 1955 [April 26, 1954], No. 12005/55. Class 40 (4). In an electrostatic loud-speaker the metal backing-plate is perforated and is of trough shape; the diaphragm, which is in the form of a sleeve, is spaced from the concave side of the backing-plate and closely follows the contour of the convex side; and the diaphragm is tensioned by means mounted on the backing-plate. Preferably the backing-plate 10 (Fig. 1) is of aluminium sheet of thickness 0.016 inch and the large number of perforations 11 render it substantially acoustically transparent to minimize power losses due to the acoustic resistance of air between it and the diaphragm 16. The plate 10 is embossed to provide elongated ridges 15 (Fig. 4) which space the diaphragm 16 from the plate 10 by about 0.004 inch. The diaphragm 16 consists of a sleeve, the base element of which is a very thin (25 x 10<SP>-5</SP> inch) sheet 17 of elastic dielectric material, preferably of the Type " A " polyester sold under the Registered Trade Mark " Mylar," upon which is deposited an electrically conductive coating 18 (Fig. 3), of about 1 or 2 x 10-<SP>6</SP> inch thickness, of aluminium, gold or silver. If, as is preferred, the coating 18 is of aluminium, it is covered by a thin varnishlike layer 19, preferably of vinyl plastic material. The diaphragm is tensioned by a U-shaped channel 21 (Figs. 1, 2), the web 22 of which bears against the protective layer 19 and urges the diaphragm towards the backing-plate and into a concave shape generally conforming to the concavity of the backing-plate. The diaphragm is formed by folding a flat sheet of material and securing adjacent edges together in the area beneath the web 22. The layer 19 being of thermoplastic material may be bonded to the base sheet 17 (Fig. 3) by the application of heat and pressure. The channel 21 is mounted on the brackets 13 secured to each end of the plate. 10, each bracket 13 having a projecting lug 23 through which passes a screw 24 which engages one end of the channel 21. A non- conductive grommet 25 insulates the screw 24 from the lug 23, electrical contact to the screw and hence to the channel 21 being provided by terminal 24a (Fig. 3). Each screw 24 has a shoulder 26 against which a coil-spring 27 reacts to effect the tensioning of the diaphragm. The web 22 is provided with apertures 28 (Figs. 1, 3) and a metal paint, e.g. silver paint, which includes as a solvent acetone or methyl ethyl ketone, is applied to the web 22, the walls of the apertures 28 and the protective layer 19, as at 29, 29a (Fig. 3). The portions of the layer 19 which are painted are dissolved by the solvent and a conductive path is established between the web 22 and the layer 18, the residual metal paint protecting the portions of the layer 18 from which the protective layer 19 has been removed. The diaphragm is spaced from the lateral edges of the backing-plate by a thin strip 30 (Figs. 1, 4) of flexible dielectric material, and arcing between the edges of the diaphragm conductive coating 18 and the backing plate is prevented by a tape 31 (Fig. 1) of dielectric material.