A recent development in the field of ultrasonic wind measurement is based on acoustic resonance [4] [5] . While conventional ultrasonic anemometers are based on measuring the time of flight, wind sensors with acoustic resonance reflect an ultrasonic wave in a small cavity.
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The cavity contains several vibrating membranes that generate and receive ultrasonic acoustic waves. The repeated reflection between the reflectors creates a quasi-standing wave perpendicular to the wind direction and a transverse wave parallel to the wind direction. If air flows along the axis between the reflectors, this influences the propagation speed of the wave and creates a phase shift that is measured. To compensate for changes in the speed of sound, for example due to temperature changes, the ultrasonic frequency is continuously adjusted so that the cavity operates at resonance. This makes the measurement of the air speed independent of the variable speed of sound.
From successive measurements of different pairs of membranes, the vector components of the air flow can be determined and thus wind speed and direction can be mathematically calculated.
