Patent GB-1277162-A (GB)
Title: Improvements in or relating to microphones
Inventor: GAYFORD MICHAEL LAWRENCE
IPC: H04R1/38
Filed: 1970-09-24
Abstract: 1277162 Microphones STANDARD TELEPHONES & CABLES Ltd 24 Sept 1970 45512/70 Heading H4J A second order gradient moving coil microphone which gives high noise discrimination up to higher than usual frequencies, e.g. those produced by jet engines, comprises an annular diaphragm which lies within a magnet structure so that the magnetic flux path of the structure threads the central aperture of the diaphragm. In Fig. 2 a slightly bowed annular diaphragm 1 is clamped at its outer and inner peripheries between polepiece 5 and a radially magnetized ceramic ring magnet 6 and a central polepiece 7, respectively. A flexible corrugation 3 serves to centre the diaphragm which may be made of plastics material. The magnet is formed by four segments or incorporates four unmagnetized segments. Sound access to the top of the diaphragm is provided via two diametrically spaced port holes 4 while access to the bottom of the diaphragm is provided via two similar port holes located on a diameter at right-angles to the first. All the portholes may be covered with porous metal, pierced foil or woven material to provide acoustic resistance damping and to prevent ingress of dirt or moisture. Leads to the moving coil 2 are accommodated in recesses 16 formed between the polepiece 7 and an insulating non- magnetic back-plate 14. The inner edge of the diaphragm may be clamped in a closed-cell, flexible, porous plastics mass located around the polepiece 7 in which case the corrugation is dispensed with (Fig. 3, not shown). In Fig. 5 a more symmetrical arrangement is depicted. Here the ring magnet 20 is axially magnetized, and two annular gaps 26, 27 are provided for a pair of moving coils 30, 31 which are connected in electrical opposition (in view of the opposite direction of the flux in the two gaps). The diaphragm 28 is clamped between rings 32, 33 at its outer periphery and in a plastics mass 34 at its inner periphery. The centre polepiece is hollowed out so that leads 42 to the coils may be taken therethrough. A twopart casing 37, 38 is provided. In modifications of this embodiment, the central polepiece may be permanently magnetized and the ring magnet replaced by a soft iron ring (Fig. 6, not shown) and, the diaphragm may be corrugated or fluted or moulded so as to have varying thickness, to provide stiffness. It must be remembered that with a second order microphone of this type sound is incident thereon in the plane of the diaphragm. Fig. 8 illustrates this for a local sound source which generates spherical waves, E1 and E4 being the entry ports to the top of the diaphragm (c.f. 4 in Fig. 2 and 35 in Fig. 5) and E2, E3 those to the bottom thereof. Fig. 11 (not shown) depicts the corresponding plane wave emanating from a distant (noise) source and Figs. 9-13 (not shown) depict the polar diagrams corresponding to the incidence of low and high frequency waves from local and distant sound sources on the microphone. It is pointed out that due to the thinness (top to bottom) of the microphone and the close spacing of the port holes the improved results are obtained. An equivalent circuit (Fig. 14, not shown) is provided.