A housing (A) which is connected with an internal combustion engine to be vibrated therewith. A diaphragm (B) is mounted in an interior chamber of the housing to undergo vibration therewith. The diaphragm is constructed to have a natural resonant frequency which substantially matches the knock frequency of the engine model for which the knock sensor is designed. The diaphragm is mounted to the housing by a central portion (20) such that an edge portion (24) undergoes maximum resonant displacement. To facilitate the passage of air from one side of the diaphragm to the other during near resonant displacement, a plurality of apertures (26) are provided through the diaphragm edge portion. In this manner, viscous drag is reduced which increases the magnitude of the resonant displacement and decreases the bandwidth or range of frequencies over which the diaphragm undergoes significant displacement. A piezoelectric element (30) is connected with the diaphragm to provide an electrical output signal which varies with the diaphragm displacement.