GB513160A - Improvements in or relating to microphones - Google Patents

Abstract

513,160. Microphones. McMILLAN, D. March 1,1938, No. 6394. [Class 40 (iv)] A variable-resistance microphone or transmitter of the kind in which carbon or other conducting granules forming one or more current paths between electrodes are arranged to be agitated in response to applied sound waves by a mechanical modulating system which may comprise an electrode, is constructed and dimensioned in such a way that in the normal position of use of the transmitter or microphone and in so far as the granules contained in a hypothetical cylinder coaxial with the axis of the electrodes and of a diameter equal to the maximum diameter of the modulating member are concerned, the static horizontal force acting on an individual granule is not greater than SP 1 /0.7N where S is the effective area of the-diaphragm in sq. cms P 1 is the lower limit of sound pressure in dynes per sq. cm. for which the transmitter is designed to give an adequate and level response over the useful frequency range and N is the number of granules in contact with the surface of the moving electrode. The electrodes are shaped so that their greatest projection into the granule chamber is along the common axis of the electrodes and so that the distance between the opposing surfaces of the electrodes increases with the distance from the common axis in the direction of increasing static force. As shown in Fig. 1 a granule chamber 1 having a filling of granules occupying not more than about 80 per cent. of the available volume of chamber has a fixed electrode 7 and a hemispherical electrode 5 of graphite mounted in a cup 6 attached to to the diaphragm 3. The granule chamber is closed by silk washers 9 and a sealing disc 13 of paper retained by a spring ring 10. A conical member 11 located behind the diaphragm and formed with holes 12, which may be covered with Swiss silk, damps the oscillations of the diaphragm 3 at its resonant frequency. The length of the granule chamber is preferably between 0.5 and 1.5 times its diameter and the diameter of the electrodes is not greater than 0.7 times the diameter of the granule chamber. The electrodes may be conical with rounded apex portions or the moving electrode may be conical with the fixed electrode cylindrical and having a plane end surface. The angle at the apex of the cone is preferably about twice the angle of repose of the granules used in the chamber.