Operation of the ultrasonic transducers is based on the echo-sounding principle. Short ultrasonic pulses are sent out by the ultrasonic transducer, reflected from objects in the vicinity and received again by the ultrasonic transducer. The ultrasonic transducer sends the period of time required between sending the ultrasonic pulse and receiving the first echo to the PDC unit which, in turn, calculates the distance to the nearest object from this period of time. Ultrasonic waves propagate at different speeds in gases, liquids and solids. The approximate speed of ultrasonic waves through air at 15 °C is 340 miles.
