GB318279A - Improvements in moving coil microphones - Google Patents

Abstract

318,279. Lanchester, F. W. June 1, 1928. Magnetic instruments; diaphragms; loudspeakers; mouthpieces and attachments. - A moving coil microphone comprises a diaphragm disposed in a chamber interposed at a point intermediate the ends of a conduit open at both ends, the cross sectional area of the diaphragm chamber being greater than that of the conduit and the arrangement being such that the sound waves operating the diaphragm are adapted to pass around the periphery thereof, the direction of ripple propagation within the, diaphragm being the same as that of the sound waves traversing the sound inlet side of the diaphragm chamber. The areas of the cross sections of the conduit and diaphragm are preferably of the order of 1 : 12. Fig. 1 shows a construction in which a conical diaphragm H carries at its periphery a coil J located in the air gap G of a pot magnet B, the core A of which is bored with a passage D communicating with the extension pipe E. The diaphragm is made of paper, cork &c. and the angle of the cone is so adjusted that any disturbance initiated in the centre thereof travels outwards to the periphery with a velocity approximating to that of sound in air. The diaphragm is suspended by threads K carried upon pins L in the manner of piano tuning pegs. The outlet tube F is formed with a flanged portion N adapted to be fixed to the shell B, both the tubes E, F being provided with horns. The reception area of the inlet horn is preferably of elongated form, the shorter axis being arranged horizontally. The upper and lower walls of such a horn may consist of bent wood strips and the sides of parallel walls separated by a distance comparable to the shortest wave length to be received. The side walls are preferably made of a number of slots arranged not quite in contact. The distance through the microphone passage is approximately equal to the shortest distance from the inlet aperture to the outlet aperture in the surrounding space. In order to diminish resonance the outlet horn may be loosely plugged with a ball of cotton wool, cotton waste &c. In the modified construction, Fig. 2, the flange N is arranged to provide a larger chamber which is divided into two parts O, S by a baffle P supported by screws R. The form of the spaces O, S is preferably such that the area of the radially moving wave front is substantially constant.