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 warfare. Applications of ultrasonics in physics include the determination of such properties of matter as compressibility, specific heat ratios, and elasticity. Ultrasonics is employed in producing emulsions, such as homogenized milk and photographic film, and for detecting flaws in industrial materials. Strong screen illumination in television is accomplished by using ultrasonic waves modulated by light diffraction. Ultrasound in the gigahertz range can be used to produce an acoustic “microscope,” able to visualize detail down to 1 micrometer. Surface acoustic waves of ultrasonic frequency form an important component of electronic control devices.

In medicine, ultrasonics is used as a diagnostic tool, to destroy diseased tissue, and to repair damaged tissue. Ultrasonic waves have been employed to treat bursitis, various types of rheumatoid arthritis, gout, and muscular injuries and to destroy kidney stones. As a diagnostic tool, ultrasonics is often more revealing than X rays, which do not prove as useful in detecting the subtle density differences found in certain forms of cancer; it is also widely used to produce images of the fetus during pregnancy. When ultrasonic waves are passed through a tissue the waves are reflected in varying degrees, depending on the density and elasticity of the tissue. Using an ultrasonic “scalpel,” a surgeon can make an incision finer than with a conventional surgical knife. Such techniques have been used in delicate surgery on the brain and on the ear. Diathermic devices in which ultrasonic waves are used to produce heat internally as a result of tissue resistance have been used successfully in physical therapy.