Sound is a mechanical disturbance of the medium in which it propagates. It encompasses a wide range of frequencies. The audible range is from approximately 20 Hz to 20 000 Hz, and is generally referredto as “sonic”. Frequencies less than 50 Hz are usually termed “subsonic”, and those above 15 000 Hz “ultrasonic”. Transit-time systems operate in the ultrasonic range at frequencies typically between 100 kHz and 1,5 MHz.