ultrasonics, study and application of the energy of sound waves vibrating at frequencies greater than 20,000 cycles per second, i.e., beyond the range of human hearing. The application of sound energy in the audible range is limited almost entirely to communications, since increasing the pressure, or intensity, of sound waves increases loudness and therefore causes discomfort to human beings. Ultrasonic waves, however, being inaudible, have little or no effect on the ear even at high intensities. They are produced, commonly, by a transducer containing a piezoelectric substance, e.g., a quartz-crystal oscillator that converts high-frequency electric current into vibrating ultrasonic waves. Ultrasonics has found wide industrial use. For nondestructive testing an object is irradiated with ultrasonic waves; variation in velocity or echo of the transmitted waves indicates a flaw. Fine machine parts, ball bearings, surgical instruments, and many other objects can be cleaned ultrasonically. They are placed in a liquid, e.g., a detergent solution or a solvent, into which ultrasonic waves are introduced. By a phenomenon called cavitation, the vibrations cause large numbers of invisible bubbles to explode with great force on the surfaces of the objects. Film or dirt is thus removed even from normally inaccessible holes, cracks, and corners. Radioactive scale is similarly removed from nuclear reactor fuel and control rods. In medicine ultrasonic devices are used to examine internal organs without surgery and are safer to genetic material than X rays. The waves with which the body is irradiated are reflected and refracted; these are recorded by a sonograph for use in diagnosis (see ultrasound for further description of medical uses). Metals can be welded together by placing their surfaces in contact with each other and irradiating the contact with ultrasound. The molecules are stimulated into rearranged crystalline form, making a permanent bond. Ultrasonic whistles, which cannot be heard by human beings, are audible to dogs and are used to summon them.
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