ULTRASONICS

vibrations of frequencies greater than the upper limit of the audible range for humansthat is, greater than about 20 kilohertz. The term sonic is applied to ultrasound waves of very high amplitudes. Hypersound, sometimes called praetersound or microsound, is sound waves of frequencies greater than 1013 hertz. At such high frequencies it is very difficult for a sound wave to propagate efficiently; indeed, above a frequency of about 1.25 1013 hertz, it is impossible for longitudinal waves to propagate at all, even in a liquid or a solid, because the molecules of the material in which the waves are traveling cannot pass the vibration along rapidly enough. Many animals have the ability to hear sounds in the human ultrasonic frequency range. Some ranges of hearing for mammals and insects are compared with those of humans in the Table. A presumed sensitivity of roaches and rodents to frequencies in the 40 kilohertz region has led to the manufacture of pest controllers that emit loud sounds in that frequency range to drive the pests away, but they do not appear to work as advertised. Richard E. Berg vibrational or stress waves in elastic media that have a frequency above those of sound waves that can be detected by the human eari.e., above 20 kilohertz. Such waves may be longitudinal waves of the same type as the sound waves that travel in air, or, in solids, they may be transverse or shear waves. In addition, some ultrasonic waves propagate along the surface of a solid (Rayleigh waves) or in thin rods and sections of material (Lamb waves). There are numerous means of generating and detecting ultrasonic vibration. The most common are magnetostrictive or piezoelectric transducers, which convert high-frequency alternating magnetic fields or electric currents into mechanical vibrations. A distinction is made between high-power and low-power ultrasonics on the basis of whether the waves produce a distortion in the medium. High-power applications include ultrasonic welding; ultrasonic drilling, in which the direction of vibration and thus of cutting is perpendicular to the surface of the material to be cut, so that holes may be drilled in any shape; and ultrasonic irradiation of fluid suspensions, which may be used for the clarification of wine by precipitation or the coagulation of particles suspended in factory exhaust fumes. High-amplitude ultrasonic waves applied to a liquid may cause the liquid to cavitate and generate shock waves that produce vigorous streaming and shear stresses. These effects can be exploited in the production of emulsions, the cleaning of surfaces, and the disruption of biological structures. Low-power ultrasonic waves are used in sonar devices for underwater detection and navigation and for mapping the seabed profile and composition. Similar pulse-echo techniques are used in the nondestructive testing of industrial materials and structures (e.g., reinforced plastics, railway lines, aircraft, and reactor vessels); ultrasonic waves are scattered by discontinuities in a test object and can thus be used to detect cavities or cracks or to measure thickness. In medicine, low-power ultrasonics can be used in place of X rays to produce an image of internal bodily structures. (See ultrasound.) Ultrasonic waves are used in the laboratory to study certain properties of materials, including the compressibility of molecules in solution, the elastic moduli of solids, and the molecular structure of gases and liquids. At very high frequencies (100 megahertz and upward), ultrasonic microscopy gives visualization of small structures with resolution comparable to that of optical microscopy. Ultrasonics at these frequencies are also used in solid-state physics. Additional reading Ultrasonics Classic works in the field of ultrasonics include Basil Brown and John E. Goodman, High-Intensity Ultrasonics: Industrial Applications (1965); Robert T. Beyer and Stephen V. Letcher, Physical Ultrasonics (1969); Dale Ensminger, Ultrasonics: Fundamentals, Technology, Applications, 2nd ed., rev. and expanded (1988); and Robert T. Beyer, Nonlinear Acoustics (1974). P.N.T. Wells, Biomedical Ultrasonics (1977), provides a summary of biomedical applications through the time of publication. Another survey of developments of the period is Robert T. Beyer, A New Wave of Acoustics, Physics Today, 34(11):145157 (November 1981). James R. Matthews (ed.) Acoustic Emission (1983), describes in great detail modern techniques for testing materials with ultrasonic emissions. A variety of applications are studied in A.P. Cracknell, Ultrasonics (1980); Kenneth S. Suslick (ed.), Ultrasound: Its Chemical, Physical, and Biological Effects (1988); and D. Stansfield, Underwater Electroacoustic Transducers: A Handbook for Users and Designers (1990). B.P. Hildebrand and B.B. Brenden, An Introduction to Acoustical Holography (1972), surveys holographic techniques and the basis for later development in medical imaging. Harvey Feigenbaum, Echocardiography , 4th ed. (1986), discusses ultrasonic cardiography. Russel K. Hobbie (ed.), Medical Physics: Selected Reprints (1986), collects articles on advances in medical ultrasonics. Information on later research activity in the field is found in B.R. McAvoy (ed.), IEEE 1990 Ultrasonics Symposium: Proceedings, 3 vol. (1989); and in the materials published in the serial Physical Acoustics: Principles and Methods (irregular), ed. by Warren P. Mason and R.N. Thurston. Periodicals include Ultrasonics (quarterly); IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control (bimonthly), covering topics of ongoing scientific conferences in all areas of sound and ultrasonics; and JAMA: Journal of the American Medical Association (weekly), for medical applications of ultrasound. Infrasonics Infrasonic waves in nature are studied in Earl E. Gossard and William H. Hooke, Waves in the Atmosphere: Atmospheric Infrasound and Gravity Waves: Their Generation and Propagation (1975). Experimental infrasonic techniques are discussed in A.F. Yakushova, Geology with the Elements of Geomorphology (1986; originally published in Russian, 2nd rev. ed., 1983); and Bruce A. Bolt (ed.), Earthquakes and Volcanoes: Readings from Scientific American (1980). Environmental noise U.S. law and public policy regarding environmental noise are described in United States Office of Noise Abatement and Control, Public Health and Welfare Criteria for Noise (1973). A wealth of data is accumulated in Cyril M. Harris (ed.), Handbook of Noise Control, 2nd ed. (1979). Noise control applications are examined in P.O.A.L. Davies, M. Heckl, and G.L. Koopman, Noise Generation and Control in Mechanical Engineering (1982); Lewis H. Bell et al., Industrial Noise Control: Fundamentals and Applications (1982); and John E.K. Foreman, Sound Analysis and Noise Control (1990). Richard E. Berg