An implantable microphone is disclosed having an external diaphragm and housing that forming chamber capable of being pressurized by deformational movement of the diaphragm induced by pressure waves (e.g., acoustic signals) propagating through overlying tissue. The chamber is shaped such that the volume of the chamber upon deflection of the diaphragm is reduced compared to a static volume of the chamber (i.e., volume of the chamber with no diaphragm deflection). As a result, the change in pressure within the chamber for a given diaphragm displacement is greater than it would be within a chamber having a cylindrical volume, leading to greater microphone sensitivity. In one arrangement, the chamber is shaped such that it is deeper at its center than at its edges, for example, to form a conical or paraboloidal volume.