ACOUSTICS

the science concerned with the production, control, transmission, reception, and effects of sound. Sound is one of the pervasive elements of man's environment. The sound in any given setting is characterized by a number of qualities, including intensity (or loudness), information content, reverberation, clarity, and so on. The acoustical engineer attempts to find optimal values of these qualities for particular settings and purposes and then to design physical means for attaining those values. Acoustics is generally divided into several branches, the principal ones of which are architectural acoustics and environmental acoustics. Other significant fields include musical acoustics, which deals with the principles governing the operation and design of musical instruments and the way musical sounds affect listeners; engineering acoustics, which is concerned chiefly with the development of high-fidelity sound recording and reproduction systems; and ultrasonics, which involves research into acoustical phenomena whose vibration rate is above the audible range (i.e., roughly 20,000 hertz) and their application in industry and biomedical science. Architectural acoustics focuses on the behaviour of sound waves in closed spaces and on those factors crucial to creating optimal acoustical conditions for different applications. The best-known application of acoustical engineering is in the design of concert halls and auditoriums, where the quality of sound is of critical importance. The characteristics of loudness and distribution of sound, reverberation time, and balance of direct and reflected sound will depend on such design variables as the form and dimensions of the hall, the materials used to finish the walls and especially the ceiling, the arrangement of seats and the materials used to cover them, the placement of special reflecting surfaces, the use of amplification, and the presence of such stage furnishings as curtains and draperies. Environmental acoustics deals primarily with the problem of noise control. Noise from jet aircraft, factories, heavy construction machinery, and automobiles has become a problem in large urban areas. Control of such forms of noise pollution ranges from efforts to produce quieter machines through the use of absorbent mountings to more careful fabrication of moving parts so as to reduce vibrations that give rise to noise. Noise in office and residential buildings has been curbed by the utilization of improved insulation materials in walls, the erection of suitably designed room partitions, and other procedures. the science concerned with the production, control, transmission, reception, and effects of sound. Sound is one of the pervasive elements of man's environment. The sound in any given setting is characterized by a number of qualities, including intensity (or loudness), information content, reverberation, clarity, and so on. The acoustical engineer attempts to find optimal values of these qualities for particular settings and purposes and then to design physical means for attaining those values. Acoustics is generally divided into several branches, the principal ones of which are architectural acoustics and environmental acoustics. Other significant fields include musical acoustics, which deals with the principles governing the operation and design of musical instruments and the way musical sounds affect listeners; engineering acoustics, which is concerned chiefly with the development of high-fidelity sound recording and reproduction systems; and ultrasonics, which involves research into acoustical phenomena whose vibration rate is above the audible range (i.e., roughly 20,000 hertz) and their application in industry and biomedical science. Architectural acoustics focuses on the behaviour of sound waves in closed spaces and on those factors crucial to creating optimal acoustical conditions for different applications. The best-known application of acoustical engineering is in the design of concert halls and auditoriums, where the quality of sound is of critical importance. The characteristics of loudness and distribution of sound, reverberation time, and balance of direct and reflected sound will depend on such design variables as the form and dimensions of the hall, the materials used to finish the walls and especially the ceiling, the arrangement of seats and the materials used to cover them, the placement of special reflecting surfaces, the use of amplification, and the presence of such stage furnishings as curtains and draperies. Environmental acoustics deals primarily with the problem of noise control. Noise from jet aircraft, factories, heavy construction machinery, and automobiles has become a problem in large urban areas. Control of such forms of noise pollution ranges from efforts to produce quieter machines through the use of absorbent mountings to more careful fabrication of moving parts so as to reduce vibrations that give rise to noise. Noise in office and residential buildings has been curbed by the utilization of improved insulation materials in walls, the erection of suitably designed room partitions, and other procedures. the science concerned with the production, control, transmission, reception, and effects of sound. The term is derived from the Greek akoustos, meaning "hearing." Beginning with its origins in the study of mechanical vibrations and the radiation of these vibrations through mechanical waves, acoustics has had important applications in almost every area of life. It has been fundamental to many developments in the arts-some of which, especially in the area of musical scales and instruments, took place after long experimentation by artists and were only much later explained as theory by scientists. For example, much of what is now known about architectural acoustics was actually learned by trial and error over centuries of experience and was only recently formalized into a science. Other applications of acoustic technology are in the study of geologic, atmospheric, and underwater phenomena. Psychoacoustics, the study of the physical effects of sound on biological systems, has been of interest since Pythagorus first heard the sounds of vibrating strings and of hammers hitting anvils in the 6th century BC, but the application of modern ultrasonic technology has only recently provided some of the most exciting developments in medicine. Even today, research continues into many aspects of the fundamental physical processes involved in waves and sound and into possible applications of these processes in modern life. Sound waves follow physical principles that can be applied to the study of all waves; these principles are discussed thoroughly in the article mechanics of solids. The article ear explains in detail the physiological process of hearing-that is, receiving certain wave vibrations and interpreting them as sound. Richard E. Berg Additional reading General works Comprehensive discussions of the propagation and perception of sound, many containing sections on the ear, on sound recording and reproduction, and on architectural acoustics, are offered in the following books, which require almost no mathematical background: John Backus, The Acoustical Foundations of Music, 2nd ed. (1977); Murray Campbell and Clive Greated, The Musician's Guide to Acoustics (1987); John R. Pierce, The Science of Musical Sound, rev. ed. (1992); Michael J. Moravcsik, Musical Sound: An Introduction to the Physics of Music (1987); and Ian Johnston, Measured Tones: The Interplay of Physics and Music (1989). Books requiring an elementary understanding of mathematics include Harvey E. White and Donald H. White, Physics and Music: The Science of Musical Sound (1980); Richard E. Berg and David G. Stork, The Physics of Sound (1982), with separate sections demanding considerable knowledge of musical notation and instruments; William J. Strong and George R. Plitnik, Music, Speech, High-Fidelity, 2nd ed. (1983); John S. Rigden, Physics and the Sound of Music, 2nd ed. (1985); and Donald E. Hall, Musical Acoustics, 2nd ed. (1991). A somewhat higher level of mathematics is needed for the comprehensive Arthur H. Benade, Fundamentals of Musical Acoustics (1976, reissued 1990), a relatively sophisticated classic in the field; and Thomas D. Rossing, The Science of Sound, 2nd ed. (1990), covering virtually every area of acoustics.Important advanced texts include the following classics: Leo L. Beranek, Acoustics (1954, reissued 1986); R. Bruce Lindsay, Mechanical Radiation (1960); and Harry F. Olson, Music, Physics, and Engineering, 2nd ed. (1967). More recent advanced comprehensive studies are Allan D. Pierce, Acoustics: An Introduction to Its Physical Principles and Applications (1981, reissued 1989); F.B. Stumpf, Analytical Acoustics (1980); Lawrence E. Kinsler et al., Fundamentals of Acoustics, 3rd ed. (1982); Donald E. Hall, Basic Acoustics (1987); and S.N. Sen, Acoustics, Waves and Oscillations (1990). Thomas D. Rossing (ed.), Musical Acoustics (1988); and Carleen Maley Hutchins (ed.), The Physics of Music: Readings from Scientific American (1978), are collections of articles.Contemporary research in areas related to sound and its application is covered in periodicals: see The Journal of the Acoustical Society of America (monthly); Acustica (monthly); Journal of Sound and Vibration (biweekly); and Soviet Physics: Acoustics (bimonthly). History of acoustics John William Strutt (Baron Rayleigh), The Theory of Sound, 2nd ed., rev. and enlarged, 2 vol. (1894-96, reissued 1945), remains a most important historical authority on nearly all aspects of theoretical acoustics. Herman L.F. Helmholtz, On the Sensations of Tone as a Physiological Basis for the Theory of Music, 2nd English ed. (1885, reprinted 1954; originally published in German, 4th German ed., 1877), is the historical magnum opus in the field of psychoacoustics. Excellent collections of papers of historical interest include R. Bruce Lindsay (ed.), Acoustics: Historical and Philosophical Development (1973), and Physical Acoustics (1974); and Stephen G. Brush (ed.), History of Physics: Selected Reprints (1988). Architectural acoustics An important survey of the subject is found in Wallace C. Sabine, Collected Papers on Acoustics (1964). Leo L. Beranek (ed.), Noise and Vibration Control, rev. ed. (1988), contains excellent sections applying to concert halls; and Leo L. Beranek, Music, Acoustics & Architecture (1962, reprinted 1979), discusses more than 50 existing concert halls, relating physical properties of sound waves in auditoriums to their subjective effects. Eighty-seven concert halls are surveyed in the illustrated work by Richard H. Talaske, Ewart A. Wetherill, and William J. Cavanaugh (eds.), Halls for Music Performance: Two Decades of Experience, 1962-1982 (1982). Lothar Cremer and Helmut A. Mller, Principles and Applications of Room Acoustics, 2 vol. (1982; originally published in German, 2nd ed., 1976-78), is a detailed and advanced treatment. Richard E. Berg