Sound can be defined as a wave which is created by vibrating objects and propagated through a medium from one location to another. Unlike electromagnetic waves, sound waves are mechanical waves, which exhibit compression and rarefactions along the “wave”. When sound, carried through the air, encounters a solid object, some of the sound is reflected back from the object, some is absorbed by the material of which the object is made, and the rest is transmitted through the object. In general, hard, less porous materials reflect more sound than do more porous materials, and more porous materials absorb more sound than do less porous ones. Because of this nature sound waves cannot travel in vacuum and travel faster in denser medium.

Humans can hear frequencies of sounds in the range of 20 Hz to 20000 Hz. Ultrasound is sound whose pitch is higher than the human ear can hear— frequencies higher than 20 kHz. They find many applications in defence, industry, medicine and other areas.

Bats use ultrasound to locate their prey using sophisticated echolocation principle. Echolocation technique can be effectively employed for ranging applications. SONAR is such an important application that was first used in the 1940s during World War II.

Transducers and sensors play a key role in various ranging applications using ultrasound. The innovation started with the invention of Piezo Electric effect by the Curies in 1880 using natural quartz crystal. In simple terms, it is the principle of converting energy by applying pressure to a crystal. The reverse of this Piezo Electric effect converts the energy back into its original form. Ultrasound sensors are based on these principles.

The frequency range of natural quartz crystals cannot be controlled. However, manmade crystals can have predetermined fundamental frequencies. Ultrasound transducers and sensors make use of such artificial crystals.

In a general ranging application, a wave pulse is transmitted through a transducer. A part of this energy is reflected back from a ‘target’. This reflected energy is captured by another transducer and utilised for further processing. Ultrasonic ranging employs the same principle of Pulse – Echo. Here electrical energy is converted into sound energy, by the transducer and transmitted as a pulse. The reflected sound energy is converted back to electrical energy by the receiving transducer. The time difference between the transmitted pulse and the received echo is proportional to the distance of the target that partially reflected back the sound energy.

All ultrasound systems use this principle for various applications.