Improvements in or relating to electro-acoustic transducers

Laminated material for use as the diaphragm of an electro-acoustic transducer comprises a sheet of heat-shrinkable thermo-plastic material having a thickness between 0.0002 and 0.003 inches pre-stretched uniformly under heat in at least two directions in the plane of the sheet and subsequently set without release of the strain, the sheet being then coated on one surface with a thin film of metal and then heat-treated so as to release at least some of the strain in the sheet whereby a pattern of corrugations and waves is imparted to the material. As described, a sheet of polyethylene terephthalate is stretched at 150-200 DEG C., coated by vacuum deposition with e.g. aluminium, cadmium, tin or zinc, heat-treated at 150-170 DEG C., and subjected to a heat-treatment at 150-190 DEG C. while under radial tension to relieve the material of surface waviness while retaining the surface corrugations. The diaphragm may be used in pressure-gradient electrostatic microphones, and electrostatic loudspeaker units. Alternatively, it may be provided with a dome by moulding or by applying the dome as a separate component, and used in a moving coil microphone. In a further embodiment, an oblong sheet acts as a single diaphragm for two or more microphones. Specification 881,584 is referred to.