Patent GB-386478-A (GB)
Title: Improvements in or relating to microphones
IPC: H04R1/22
Filed: 1932-03-18
Abstract: 386,478. Microphones. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London.-(Assignees of Olson, H. F.; 153, East 24th Street, New York, U.S.A.) March 18, 1932, No. 8158. Convention date, March 31, 1931. [Class 40 (iv).] Relates to microphones comprising a light substantially straight vibratory ribbon, adapted to be subjected to incident sound waves on both sides, and associated with which is a baffle so arranged and dimensioned that the relatively unimpeded path for the sound waves between opposite sides of the ribbon is approximately half the wave length of the highest frequency sound wave to be converted into electrical variations. According to the invention, the parts are so arranged that symmetrical short paths of the length specified exist round both edges of the ribbon, and the forces on the ribbon due to the magnetic system are also symmetrical about the centre line of the ribbon. In the construction shown in Fig. 1, a ribbon-like conductor 1 made of thin flexible material of low elasticity (such as crimped aluminium foil) is supported at its ends so as to be located in and almost completely to fill an elongated air gap between two magnet poles 6, 7. These are secured to arms 8, 9 of a magnet, which may be a permanent magnet or may have an energizing winding 11 as shown. Leads 4 are connected to the ends of the ribbon, and the connections may be made through the supporting devices 2, when each of these comprises an insulated conducting member. The ribbon 1 has a very low natural frequency, and its mechanical reactance is small compared with the impedance of the air. When sound waves impinge on the ribbon, causing varying differences of pressure on the opposite sides thereof, the vibrations so produced set up a varying E.M.F. in the ribbon, which is utilized in the ordinary manner. The polepieces and the arms 8, 9 form a baffle which is dimensioned so as to provide symmetrical sound wave paths round both edges of the ribbon, of the length specified above, e.g. approximately equal to half the wave length corresponding to a frequency of 6,000 cycles per second. In the range of frequencies being dealt with, the difference in pressure acting upon the two sides of the vibrating member is therefore proportional to the frequency, and the velocity of the member and the generating voltage is independent of the frequency. Modifications in the construction of the ribbonsupporting devices are described. Fig. 5 shows a construction in which the polepieces 26, 27 are spaced from the magnet arms 28, 29 but connected thereto by ferromagnetic rods 33. These are separated from each other, leaving openings 34 which form sound wave paths between the opposite sides of the vibratory ribbon 21. The polepieces have converging sides to increase the flux density, and the arms 28, 29 are out away towards their lower ends, as shown. This apparatus may be designed to respond well to a higher range of frequencies than that shown in Fig. 1, e.g. up to 10000 cycles per second. A similar construction is described, Fig. 9 (not shown), in which the magnet arms are not cut away, and the magnet has a single energizing winding. The theoretical principles underlying the invention are discussed in the Specification, and a chart is given showing the response characteristics when baffles of different sizes are used. It is stated that with the microphones described, highly directional effects are obtained, which do not vary appreciably with frequency variations. Polar diagrams are given to illustrate these properties. The Specification as open to inspection under Sect. 91 describes also a microphone in which a ribbon-like body is not used, but which is of the moving-coil type. As shown in Fig. 15 (Cancelled), the coil 102 is attached to a diaphragm 101 of double conical formation, the diaphragm being preferably of heavy grade paper, aluminium, or the like, and being flexibly supported at 103 by cloth, skin, &c. The magnet has a central polepiece 108 with an enlarged head 104, and an annular pole 105 supported by four strips 110 secured to a disc 111. A washer 112 is so arranged as to leave a considerable space 114 behind the polepiece 104, which has a number of perforations 109. The dimensions of the parts and the size and number of the holes are so chosen that the requisite length of the sound wave path between the front and rear sides of the conical portion 106 of the diaphragm is provided. A similar path is formed as regards the frusto-conical portion 107 by reason of the arrangement of clamping rings 116 on spaced brackets 117. This subject-matter does not appear in the Specification as accepted.