14 should be very soft and skin like to the touch and should have a low durometer preferably dome 14 is constructed of silicone rubber having the following properties table 2______________________________________dome propertiesproperty range test method______________________________________ultimate tensile 1250 1350 astm d412 die cstrength psiultimate elongation 600 750 astm d412 die ccompression set 3 5 astm d395 method b 22 hours at 158 u00b0 f 300 modulus of 600 700 astm d412 die celasticity psihardness shore a 35 50 astm d2240 64tdurometer preferably 35 40tear strength ppi 190 210 astm d624 die c______________________________________ the foregoing combination of properties in the rim and dome portions of the devices of this invention has been found to result in a springless diaphragm which gives improved performance and which can be manufactured without the low efficiency and the high costs which have plagued the prior art devices it will therefore be seen that the rim 18 and dome 14 of this invention are constructed of different materials which have distinctly different properties unlike the prior art which formed spring containing devices using materials of construction to mold these devices which were the same throughout the molded article exemplary of the especially preferred silicone rubbers useful in construction of the devices of this invention are dimethyl and methyl vinyl siloxane copolymers and blends of dimethyl polysiloxane and methyl vinyl polysiloxane reinforced with fumed silica fillers.