A direction finding and ranging (DIFAR) sensor is provided in which a relatively light sensing element (30) that undergoes motion in response to an acoustic wave is disclosed. The motion of the sensing element relative to an inertial mass (2, 4) is measured along two mutually perpendicular directions to determine the direction of propagation of the acoustic wave. The construction of the DIFAR sensor enables the contact between the sensing element (30) and internally disposed relative motion transducers to be adjusted after assembly of the DIFAR sensor.