The material from which the diaphragm is made and the thickness thereof may vary depending upon the requirements in and particular case and it may be formed of metal or metal alloy such as beryllium copper or stainless steel or of a suitable plastics material such as P.T.F.E. Disposed above the diaphragm and also clamped between the parts 11, 12 is a plate 13 the underside of which is profiled as