Ultrasonics
Ultrasonics, branch of physics dealing with high-frequency sound waves, usually in the range above 20,000 hertz (Hz), that is, above the audible range. It is to be distinguished from supersonics (see Aerodynamics), which deals with phenomena arising when the velocity of a solid body exceeds the speed of sound. Modern ultrasonic generators can produce frequencies up to more than several gigahertz (1 GHz = 1 billion Hz) by transforming alternating electric currents into mechanical oscillations. Detecting and measuring ultrasonic waves are accomplished mainly through the use of a piezoelectric receiver or by optical means (see Crystal ), because ultrasonic waves are rendered visible by the diffraction of light. The science of ultrasonics has many applications in various fields of physics, chemistry, technology, and medicine. Ultrasonic waves have long been used for detection and communication devices called sonar, of great importance in present-day navigation, and especially in submarine