The measurement of vibration of lightweight structures requires that any loading due to a vibration transducer be eliminated. A noncontacting displacement sensor has been developed to assist in the understanding of the dynamics of lightweight air cushion vehicle skirts. A remote vibration sensor is required to monitor the unsteady motion of these membranelike structures. A low‐cost acoustic displacement sensor has been developed. A piezoelectric driver aimed at the vibrating surface; the reflected beam is detected by a microphone. The motion of the reflector introduces a “Doppler shift” which is detected by a lock‐in amplifier whose internal electronics transform this information into a signal proportional to displacement. The resolution of the system is governed by the wavelength of the incident beam, high frequencies (f0 < 20 kHz) affording peak to peak values of the order of 10 mm; for larger excursions lower frequencies are used. The system is sensitive to changes in air temperature, the speed of sound figuring in the Doppler factor. In addition the directivity of the transducer plays a role, the resultant amplitude modulated signal affording additional sensitivity. The displacement transducer is readily configured from standard components found in many laboratories and is ideal for the detection of low‐frequency displacements (f < 100 Hz). [Work supported by the Natural Science and Engineering Research Council and Transport Canada.]