The device includes a base (3) and an ultrasound transmitter (11) and receiver (21). Either the transmitter or receiver is mounted for rotation about a fixed axis (8) w.r.t. the base and is driven by an electric motor (7). The other component is fixed to the base. The components are mounted so the sound emitted by the transmitter is directed tangentially w.r.t. its circular trajectory so that it is periodically received at the receiver. The receiver provides a signal representative of the ultrasound frequency received. An analogue sampling switch is connected to the receiver so that it only operates when the transmitter is within the required section of the circular trajectory.
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