This application relates to MEMS devices, particularly MEMS capacitive transducers, and processes to form such MEMS transducers that provide increased robustness and elasticity to acoustic shock. This application describes a transducer structure having a flexible membrane 101 supported between a first volume 109 and a second volume 110. The transducer structure includes at least one variable vent structure 401 in communication with at least one of the first and second volumes, the variable vent structure comprises at least one movable part, and the movable part is It can move to change the size of the flow path through the vent structure in response to the pressure difference across it. A variable vent can be formed through the membrane, and the movable portion can be a portion of the membrane defined by one or more channels that can be deflected away from the surface of the membrane. The variable vent is preferably closed at a normal range of pressure differentials, but at high pressure differentials it opens to provide faster equalization of air above and below the membrane.
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