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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 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 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.
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