This application relates to MEMS transducer having a membrane layer (101) and at least one variable vent structure (301). The variable vent structure has a vent hole for venting fluid so as to reduce a pressure differential across the membrane layer and a moveable vent cover (302a, 302b) which, at an equilibrium position, at least partly blocks the vent hole. The vent cover is moveable from its equilibrium position in response to a pressure differential across the vent cover so as to vary the size of a flow path through the vent hole. In various embodiments the vent cover comprises at least a first flap section (302a) and a second flap section (302b), the first flap section being hingedly coupled to the side of the vent hole and the second flap section being hingedly coupled to the first flap section so as to be moveable with respect to the first flap section. In some embodiments the second flap section (302b) may be configured so that it deflects away from the first section (302a) more readily than the first section deflects away from its equilibrium position.