Abstract: The invention relates to hydrophones which require liquid backing of the sensing plate, and more particularly, to compliance enhancing means for use in such hydrophones. In a practical embodiment, in which a compliant sensing plate in the form of a thin diaphragm is employed, liquid backing of the sensing plate is provided to balance the hydrostatic pressure across the sensing plate. To increase the compliance of the hydrophone, reduced by the presence of the backing oil, and to do so in a linear manner, a bellows is introduced into the backing chamber having the desired compliance and pressure rating. The invention is applicable to hydrophones having either compliant or stiff sensing plates. An improvement in hydrophone linearity and sensitivity is obtainable over a significant range of hydrostatic pressures.