A MEMS capacitive transducer with increased robustness and resilience to acoustic shock. The transducer structure includes a flexible membrane supported between a first volume and a second volume, and at least one variable vent structure in communication with at least one of the first and second volumes. The variable vent structure includes at least one moveable portion which is moveable in response to a pressure differential across the moveable portion so as to vary the size of a flow path through the vent structure. The variable vent may be formed through the membrane and the moveable portion may be a part of the membrane, defined by one or more channels, that is deflectable away from the surface of the membrane. The variable vent is preferably closed in the normal range of pressure differentials but opens at high pressure differentials to provide more rapid equalization of the air volumes above and below the membrane.