245,796. British Thomson-Houston Co., Ltd., (Assignees of Rice, C. W., and Kellogg, E. W.). Jan. 9, 1925, [Convention date]. Loud-speakers; magnetic instruments; diaphragms.-A sound-reproducing apparatus, more particularly a loud-speaker, is formed with a rigid conical diaphragm having its larger end surrounded by a rigid or non-resonant baffle. The actuating coil is preferably secured, to a reinforcing cylindrical sleeve, at the inner end of the diaphragm. The diaphragm is supported by flexible devices which have so low a restoring force that the natural periodicity of the diaphragm is low down in the range of audible frequencies. The frustum shaped diaphragm 1 is secured at its outer edge to a supporting ring 2 by a ring 3 of flexible material, such as silk, rubber or thin leather. The inner edge of the diaphragm is formed as a cylinder 4 reinforced by outer and inner rings 5, 6, an actuating coil 7 being secured around the inner ring 6 and being disposed in the annular air gap 8 between the concentric pole pieces 9, 10. In order to prevent any tendency for lateral movement of the coil 7 in the gap 8, the pole piece 9 is formed with a projection 10a to which is secured, by a screw 12, a spider 11a having resilient arms 13 connected by radially extending links 14 to the inner edge of the diaphragm. The links 14 may be in the form of a continuous length of cord or fine wire threaded through holes in the ends of the arms 13 and the inner edge of the diaphragm. The top of the diaphragm is preferably left open so as to eliminate any enclosed air space between tho diaphragm and pole pieces. The ring 2 is mounted on the pole piece 10 by brackets 15 and in order to prevent circulation of air between the front and back of the diaphragm around the ring 2, the path between the front and back of the diaphragm is lengthened to a distance of about a, quarter wave length or more by surrounding the ring 2 by a baffle 18 of fairly massive material. The baffle 18 may conveniently take the form of an enclosing cabinet for the diaphragm and its actuating mechanism, the ring 2 being fitted in an opening in the front of the cabinet, the back of which is preferably open to prevent any resonant effects due to confined air. The pole piece 9 is surrounded by a coil 11 which may either be supplied with a steady direct current or with rectified alternating current, in which case the end 19 is made to constitute a short circuited turn of low resistance by making it of copper. Specifications 231,420 and 231,421 are referred to.
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