Ultrasonic transducers can transmit ultrasonic waves through media, and receive the echo waves, which are reflected by the object interface. The distance between the transceiver and the object interface can be calculated by time of flight (the time ultrasound transmitted and received by the transceiver), regardless of the composition, shape and color of the subject. Non-contact ultrasonic proximity measurement is where the ultrasonic transceiver transmits ultrasonic waves traveling through the air to the object interface and receives its reflected wave.