Abstract: An anemometer comprises an acoustic resonant cavity 1 between two plates 2 and 3 and open in the direction of flow of a fluid, the cavity having at least one pair of electro acoustic transducers 5 which are energised at an Eigenfrequency to produce a standing wave perpendicular to the direction of flow of the fluid and a travelling wave perpendicular to the standing wave. By using the time delays or phase shifts between the signal energising a transducer and the signal generated by the response of the other transducer the velocity of flow of the fluid can be calculated. Compensation is provided for changes in the Eigenfrequency caused by changes in the environment.