‘Sonics’ suggests a mechanical wave or an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of human hearing. Sound wave propagation with the frequencies beyond the limit of human hearing is referred to as ‘ultrasonic’. The history of ultrasonic wave propagation owes its origin to Paul Langevin, who in 1917 transmitted sound waves in sea water. Langevin’s work depended on another discovery, by Pierre and Jacques Curie, of the piezoelectric effect. They discovered that when a stress is applied to crystalline material, such as quartz, an electric charge is produced. They also found that, conversely, an electric charge applied to the material produces a change in the material’s dimensions. Hence, an alternating voltage applied to the crystal can produce vibrations that generate sound waves in a surrounding medium. This system is known as piezoelectric transducer. The earliest form of an ultrasonic transducer was a whistle developed by Francis Galton in 1883 to investigate the threshold frequency of human hearing. He produced a gas-driven transducer that generated sound of known frequencies and was able to determine that the normal limit of human hearing was around 18 kHz to 20 kHz.

Sound is transmitted through a medium by vibration of the molecules through which the wave is travelling. Sound wave propagation can be categorised into  three types; audio, low and high ultrasonic and diagnostic ultrasound. The audio range falls between 16 Hz and 20 kHz, and this range is important because its frequencies can be detected by the human ear. The useful ultrasound range refers to the frequencies between 20 kHz and 2 MHz, for both low power and high power ultrasonics. This range is used in a wide range of applications such as manufacturing, sonochemistry and medicine. Diagnostic ultrasound frequencies of 2 MHz to 10 MHz are mostly used in medical applications such as ultrasonography and elastography. One of the chief medical applications is obstetric sonography, which is commonly used during foetal examinations.

Low and high ultrasonics have been used to solve problems in such diverse areas as engineering, physics, chemistry, medicine, microscopy, underwater, ranging and navigation. The first application of the ultrasonic frequencies was in the sonar detection of submarines during World War I. Wood and Loomis established techniques for generating high acoustic powers to study the effects of high intensity sound. The first practical application of ultrasound, outside sonar, was an ultrasonic flaw detection system which used the Wood and Loomis techniques. Later, during World War II, ultrasonic vibrations were applied in timing devices, anti-jamming devices, and moving target detecting systems which focus attention on the appearance of moving targets.

At the end of World War II considerable expertise had been developed in sending and receiving high frequency ultrasonic pulses to investigate many physical problems such as oscillatory magnetoacoustic phenomena in metals and the effect of dislocations and impurities on sound wave propagation and relaxation processes in gases, liquids and solids. During post-war years, the introduction of the piezoelectric ceramics occurred, replacing quartz as a piezoelectric material. New applications in measurement and control were found in medicine, cleaning and machining.