within the reach of the person skilled in the art and also in different constructional choices a preferred and advantageous embodiment that is shown in fig5 to 6 provides a construction similar to the one of the previous embodiment but in this embodiment the wall 201 is made like a cup or at least with an end face faced towards the membrane that is made hollow like a cup and the shell peripheral wall 501 forms a step like enlargement supporting the outer peripheral band of the membrane the step like enlargement 601 is the same as the flange 5 in the previous embodiment while like the embodiment of fig1 to 3 the end wall member 201 can be axially moved towards the inlet duct i e towards the end wall 101 of the cup like member delimiting the chamber c 1 that is associated to the inlet mouth 2 a spring member for example a coil spring 6 is interposed between the wall 101 and the cup like member i e the end wall 101 associated to the inlet mouth 2 and the faced side of the membrane 4 that is the side thereof faced towards the end wall 101 bearing the inlet mouth 2 the member.