Patent GB-366962-A (GB)
Title: Improvements in and relating to electro-magnetic sound reproducing devices
IPC: H04R31/00
Filed: 1930-09-09
Abstract: 366,962. Receivers ; diaphragms. GOLDSCHMIDT, R., 45, Linden Allee, Charlottenburg, Berlin. Sept. 9, 1930, No. 26922. [Class 40 (iv).] In an electromagnetic sound-reproducing instrument, such as a telephone receiver, of the kind in which the diaphragm acts as an armature and co-operates with one or more magnet poles, the magnetic and elastic characteristics of the magnet and diaphragm, as determined respectively by curves correlating the distance of the diaphragm from the poles with the magnetic attractive force between the diaphragm and poles on the one hand and with the mechanical restoring force due to the elasticity of the diaphragm on the other hand, are adjusted so as to bring the diaphragm to an approximately pseudo-astatic condition, such as is referred to in Specification 321,395, [Class 40 (iv), Telephones &c], by subjecting the diaphragm, after assembly with the magnet, to stresses which give it the desired stiffness. The object is to improve the response of the device at frequencies below the principal resonance and the response characteristic of the diaphragm so stressed shows a pronounced peak at a frequency equal to about one-half that of the principal resonance. The theoretical considerations involved are stated to be that in the usual form of telephone receiver &c. when the sound intensity has its maximum value, corresponding to the principal resonance, the diaphragm M vibrates with nodal points at U as illustrated in Fig. 6, but when, according to the invention, the elastic restoring force and the magnetic attractive force approximately balance over a substantial range of displacement of the diaphragm, the latter is almost astatic and vibrates with a movement corresponding to the fundamental resonance as illustrated in Fig. 5. In order to obtain a suitable form of characteristic curve, either the polepieces of the magnet P may be stepped as shown at K, L, Fig. 3, so that the attractive force on the diaphragm as the air gap diminishes does not increase so rapidly as with unstepped polepieces, or the distance apart of the polepieces may be variable so that they can be adjusted to positions opposite the points U, Fig. 6, thereby tending to prevent the formation of nodes. In order to match the elastic characteristic with the magnetic, the diaphragm M is preferably initially shaped as shown in Fig. 12, that is, with a central dished portion surrounded by a frusto-conical annulus i and is then deformed, when in a position associated with magnet poles, until it displays the resultant stiffness desired. The apparatus for producing the deformation comprises a motor s, a platform b that supports the diaphragm and is vertically reciprocated, a pressure plate g that is simultaneously fed forward by a fine screw e, and an automatic cut-out for the motor which comes into operation when the required deformation is produced, e.g. after the diaphragm has been forced into contact with the polepieces P<1>, P<2>. According to an alternative arrangement, Fig. 16, for deforming the diaphragm, the rim of the diaphragm M when assembled projects over an annular recess 3 in the casing 2 of the telephone receiver, and the screw cap 1 has a circular ridge 4 projecting internally and adapted to depress the diaphragm rim to the desired extent into the recess 3, the ridge 4 being disposed nearer to the outer shoulder of the recess than to the inner shoulder.