Patent GB-275567-A (GB)
Title: Electrodynamic devices for use in acoustic devices, such as loud-speakers
IPC: H04R9/02
Filed: 1927-06-07
Abstract: 275,567. Electrical Research Products, Inc., (Assignees of Thuras, A. L.). Aug. 4, 1926, [Convention date]. Loud-speakers; magnetic instruments; diaphragms.-An electrodynamic device for acoustic instruments, such as loud-speakers, comprises a coil 14 situated in a magnetic air gap and secured near the periphery of the stiff dished central portion 17 of a diaphragm, the flexible outer portion of which is formed with tangential corrugations 18 as in Specification 263,411, and with a flat portion 19, which is clamped between the housing 20 and the portion 15 of the core structure. The coil 14 is formed of an insulated ribbon conductor wound edgewise i.e., so that the plane of the ribbon is perpendicular to the coil axis. A process for making the coil is described. which consists in coating a length of aluminium ribbon having a thickness of about 0.005 cm. and a width of about 0.036 cm. with a thin solution of the substance known under the Registered Trade Mark bakelite dissolved in acetone. The coated conductor is then subjected to a temperature of about 350 C. for a short period and then passed through a second solution of bakelite, preferably slightly more viscous than the first, after which it is subjected to a temperature of about 250 C. The coated ribbon is then wound edgewise on a collapsible mandrel to form a coil about 5 cm. in diameter which is baked at a temperature of about 250 C. and gradually cooled to room temperature. The coil is then removed from the mandrel and trimmed down to about 50 turns, which gives it a height of about 0.267 cms. The inner surface of the coil is coated with a cementing material, such as bakelite or shellac, and one edge of a strip 30 of a stiffened fabric, such as oiled silk, is secured thereto by means of the cement. The coil is again placed on a mandrel, the diameter of which is less than that previously used by an amount equal to the thickness of the silk, and heated to about 150 C. after which the opposite edge of the strip 30 is secured to the diaphragm near the periphery of the stiffened portion 17 and the end portions 35, 36 of the conductor are insulated and secured along the troughs of the corrugations of the portion 18 of the diaphragm by a coating of shellac. The assembled structure is then heated to a temperature of approximately 150 C. The ends of the conductor are secured to screws 31, 32 electrically connected to the terminals 33, 34. The diaphragm employed is preferably of aluminium about 0.005 cm. thick, the portion 17 being about 5 cms. in diameter. The effective mass of the diaphragm is approximately equal to that of the coil and the centres of gravity of the diaphragm and coil are substantially coincident. The material of the coil is also preferably such that the product of its specific resistance in microhms per centimetre cube and its mass in grams per cubic centimetre is less than 9. The electromagnet comprises a hollow annular core 10, winding 11 and annular pole pieces 12, 13, the portion 15 being detachable to permit assembling of the winding 11 on the core structure. The space at the mid-portion of the core 10 is closed at each end by wire gauze members 27, 28 and sound absorbing material 29. A metallic plug 23 in the form of a spherical sector is secured to the housing 20 by projecting lugs 24 and screws 25, the radius of the plug being slightly smaller than that of the adjacent surface of the portion 17 of the diaphragm so that the area of the sound passage formed between these surfaces progressively increases from the centre of the diaphragm and from its peripheral portion toward the common sound passage formed by the housing 20 and plug 23. The cross sectional areas of these sound passages are such that the air displaced from each of the converging passages flows into the common sound passage at substantially the same velocity. The remainder of the sound passage is also preferably designed to increase progressively in area from the diaphragm to the mouth of the horn 26. Specification 275,571 also is referred to