A method of fabricating a micro-electrical-mechanical system (MEMS) transducer comprises the steps of forming a membrane (5) on a substrate (3), and forming a back-volume in the substrate. The step of forming a back-volume in the substrate comprises the steps of forming a first back-volume portion (7a) and a second back-volume portion (7b), the first back-volume portion (7a) being separated from the second back-volume portion (7b) by a step in a sidewall of the back-volume. The cross-sectional area of the second back-volume portion (7b) can be made greater than the cross-sectional area of the membrane (5), thereby enabling the back-volume to be increased without being constrained by the cross-sectional area of the membrane (5). The back-volume may comprise a third back-volume portion. The third back-volume portion enables the effective diameter of the membrane to be formed more accurately.