GB511034A - Improvements in and relating to electric acoustic devices - Google Patents

Abstract

511,034. Microphones. HOULGATE, H. J., WHEELER, G. C., and FOUNTAIN, Ltd., G. R. Dec. 31, 1937, No. 36182. [Class 40 (iv)] In an electrodynamic microphone comprising a diaphragm 1 carrying a coil located in a magnetic field provided by an annular polepiece 8 and a centre polepiece 7 energized by a magnet 16 forming a closed chamber 13 behind the annular polepiece, a plurality of openings 9, 10 provided in the polepieces 7, 8 respectively communicate with the closed chamber through a quantity of acoustic resistance material which covers the inner ends of the openings. The resistance material is preferably in the form of rings 14, 15 of felt, and the chamber 13 may be wholly or partially filled with cotton wool or similar material. The diaphragm which is formed with a domed central portion and a number of annular corrugations 2 is preferably made of cellulose acetate. The central portion may be made of thicker material than the annular portion. The output of the microphone may be applied to an electric filter arranged to provide additional damping of the output at the resonant frequency of the vibrating system and the associated air chamber, and to this end the output transformer is preferably provided with additional windings having a predetermined leakage inductance with regard to the main winding and a tuning capacity across the additional winding.