GB290235A - Improvements in and relating to diaphragms for reproducing sound - Google Patents

Abstract

290,235. Electrical Research Products, Inc., (Assignees of Schlenker, V. A.). May 11, 1927, [Convention date]. Diaphragms; loud. speakers. - A directacting sound-radiator consists of a thin flat diaphragm 10, Fig. 2, tensioned to such a degree that the velocity of transverse waves therein is greater than one-fourth, and may equal the velocity of sound in air. For instance, a. circular sheet of aluminium-manganese alloy 29 inches in diameter and 0.002 metres thick may be clamped peripherally under a tension of 40 lb. per inch. The electrodynamic driving-unit 16 comprises a coil-wound ring 19, Figs. 4 and 5, cemented along the edges of a circular strip 30 of fabric to the face of the diaphragm. The ring vibrates between the annular pole-faces 20, 21, Fig. 5, of a permanent or electromagnet 22 having the tapered cross-section shown in Figs. 3 and 5. The driving unit 16 is fixed eccentrically on the diaphragm, and in order to damp out undesired vibrations reflected from the edge of the diaphragm an absorbing unit is fixed symmetrically on the other side of the central point to the unit 16, where the reflected waves come to a focus. The absorber unit may be identical with the unit 16, except that the ring windings are closed, or shunted by a resistance, so as to dissipate the induced currents. Or a porous or other buffer, or a suitable dash-pot, may be arranged to damp-out the undesired reflections. The optimum proportions between the dimensions of the diaphragm, and the mass and size of the driving coil 19, for various frequencyranges are set out in the Specification. The diaphragm as a whole is preferably inclined to the vertical when mounted in a cabinet, so that the walls of the latter project both back and front from the plane and act as a horn or baffle. Instead of being circular in shape the outline of the diaphragm may be elliptical or it may be formed by portions of four ellipses having a common focal point. In such cases the driving unit may be located at one focus, and the dissipating or absorbing unit for the reflected waves located at the conjugate focus.