GB646164A - Improvements in or relating to electroacoustic transducers - Google Patents

Abstract

646,164. Sound producing or receiving devices. WESTERN ELECTRIC CO., Inc. Oct. 16, 1947, No. 27761. Convention date, Oct. 19, 1946. [Class 40 (iv)] In an electro-acoustic transducer having a ring armature 14 carrying a non-magnetic diaphragm 15 at its centre, the top of the annular magnet 12 is co-planar with the pole face of an annular inner pole-piece 10 and the working air gap is determined by a magnetic spacer 13 between the top of the magnet and the outer edge of the annular armature. The permanent magnet 12 is soldered to a flange 11 on the cylindrical pole-piece 10 and the spacer 13 supports the armature 14 and two additional spacers 16 and 18 which in turn support an auxiliary radially magnetized magnet 17. Spacer 18 is of magnetic material, and with the magnet 17, which is weaker than the main magnet, gives increased efficiency for a given air gap whilst spacer 16 which is non-magnetic is spaced from the armature 14 so that restraint of vibration of the latter is minimised and since it extends over a substantial portion of the armature it prevents adherence of the armature to the magnet 17. Operating coils 22 and 24 are connected in series to terminals 26 and 27 in the base-plate 21 which is shaped to create a damping chamber at the back of the diaphragm. In a further embodiment, Fig. 4 (not shown), equally-spaced polepieces carrying individual coils replace the single polepiece 10 and the baseplate has apertures covered with acoustic resistance material which serve, together with the chamber referred to above, to give an improved response throughout the working range. Radial slots from the outer periphery of the armature (Fig. 7, not shown) and armatures of varying radial thickness for securing a desired resonance frequency are also mentioned. The dish-shaped diaphragm 15, which is formed of a synthetic fibre cloth impregnated with a phenol-aldehyde condensation product giving it a low mass and a high modulus of elasticity, is secured to the inner marginal portion of the armature by a suitable cement and is protected by a moisture-proof membrane 19 which overlies it. In a further embodiment (not shown) the outer rim is stiffened by fluting.