SOUND

a mechanical disturbance from a state of equilibrium that propagates through an elastic material medium. A purely subjective definition of sound is also possible, as that which is perceived by the ear, but such a definition is not particularly illuminating and is unduly restrictive, for it is useful to speak of sounds that cannot be heard by the human ear, such as those that are produced by dog whistles or by sonar equipment. The study of sound should begin with the properties of sound waves. There are two basic types of wave, transverse and longitudinal, differentiated by the way in which the wave is propagated. In a transverse wave, such as the wave generated in a stretched rope when one end is wiggled back and forth, the motion that constitutes the wave is perpendicular, or transverse, to the direction (along the rope) in which the wave is moving. An important family of transverse waves is generated by electromagnetic sources such as light or radio, in which the electric and magnetic fields constituting the wave oscillate perpendicular to the direction of propagation. Sound propagates through air or other mediums as a longitudinal wave, in which the mechanical vibration constituting the wave occurs along the direction of propagation of the wave. A longitudinal wave can be created in a coiled spring by squeezing several of the turns together to form a compression and then releasing them, allowing the compression to travel the length of the spring. Air can be viewed as being composed of layers analogous to such coils, with a sound wave propagating as layers of air push and pull at one another much like the compression moving down the spring. Figure 1: Graphic representations of a sound wave. (A) Air at equilibrium, in the absence of Figure 1: Graphic representations of a sound wave. (A) Air at equilibrium, in the absence of A sound wave thus consists of alternating compressions and rarefactions, or regions of high pressure and low pressure, moving at a certain speed. Put another way, it consists of a periodic (that is, oscillating or vibrating) variation of pressure occurring around the equilibrium pressure prevailing at a particular time and place. Equilibrium pressure and the sinusoidal variations caused by passage of a pure sound wave (that is, a wave of a single frequency) are represented in Figure 1A and 1B, respectively. Richard E. Berg a mechanical disturbance from a state of equilibrium that propagates through an elastic material medium. A purely subjective definition of sound is also possible, as that which is perceived by the ear, but it is not particularly illuminating and is unduly restrictive, for it is useful to speak of sounds that cannot be heard by the human ear, such as those that are produced by dog whistles or by sonar equipment. Sound is treated in a number of articles. For the physics of sound generation, propagation, and detection, see acoustics. For a more general discussion of vibration and wave motion, see mechanics. For a discussion of biological mechanisms of sound production and reception, see phonetics; sensory reception. For a discussion of musical sounds, see music; musical instrument. For a discussion of the use of acoustical waves as a tool of research, see earthquake. Most solids, liquids, and gases of ordinary experience can serve as media for sound. A mechanical disturbance may be produced in any number of ways but will consist of a sudden increase in pressure at some point. Since the material is elastic, the compression is not permanent; once the disturbing influence is removed, the compressed region will rebound, but in doing so it will compress an adjacent region. The result of this cycle repeating itself is the generation of a compression wave, followed by a rarefaction wave as each region of elastic material rebounds. These waves are longitudinal, i.e., the displacement of a particle of the medium is in the direction of wave motion (note, however, that there is no net transport of material). The waves thus generated travel through the medium at a speed that is a function of the equilibrium pressure and density of the material and, to various extents, of the specific heat (for a gas), the elasticity (for liquids and solids), and the temperature of the medium and of the frequency of the wave. In dry air (at 0 C and a sea-level pressure of 14.7 pounds per square inch [1013.25 millibars]) the speed of sound is 331.29 metres per second (741.1 miles per hour). (This calculation, made in 1986, corrected an earlier calculation made about 1942.) The speed of sound in seawater is 1,490 metres (4,889 feet) per second and in steel 5,000 metres (16,405 feet) per second. A great many of the sounds encountered in daily life are periodic, that is, the sound waves associated with them occur in patterns that repeat with regularity over time. Such sounds are characterized by a dominant frequency, or pitch, which may be defined as the number of waves that pass a fixed point per unit time. The simplest form of sound wave, called a pure tone, can be represented by a sine wave; more complex but still relatively simple forms, such as notes played on various musical instruments, can be represented by more complex harmonic curves, with numbers of harmonic tones or overtones superimposed on the fundamental tone. The complexity of a sound wave can become so great that no particular pitch can be discerned; white noise, so called by analogy with white light (a mixture of all frequencies, or colours, of visible light), is an example. Another parameter of sound, in addition to velocity and frequency, is intensity, which is defined as the average flow of energy per unit time through a unit area of the medium. Intensity is usually measured in watts per square centimetre. The standard usually used for the quietest sound audible to the human ear has an intensity of 10-16 watt per square centimetre. Closely related to the intensity of a sound is the sound pressure, or pressure excess over equilibrium caused by a sound wave. Sound pressure is measured in dynes per square centimetre or, in the SI system of units, in pascals. The sound pressure of the human voice at ordinary conversational level, measured directly in front of the mouth, is about 0.1 pascal, which may be compared with the standard pressure of the atmosphere of about 105 pascals. Also related to sound intensity, although not simply, is loudness. Loudness is a subjective phenomenon and can be measured only comparatively, by means of a standard reference sound under specified conditions. Sound waves behave in many ways as light waves do, and optical analogies are frequently useful in describing acoustical phenomena. Thus, sound waves can be reflected, refracted, diffracted, and scattered, and many of the differences in behaviour between sound and light waves reflect simply the very great difference in wavelengths ordinarily dealt with. Additional reading An enormous amount of physical data on such topics as the velocity of sound and the elastic properties of materials, as well as surveys of important theories in the field, are found in the following reference books: Herbert L. Anderson (ed.), A Physicist's Desk Reference (1989); Dwight E. Gray (ed.), American Institute of Physics Handbook, 3rd ed. (1972); and Rita G. Lerner and George L. Trigg (eds.), Encyclopedia of Physics, 2nd ed. (1991). For biographies of scientists who worked in the field of acoustics, see Charles Coulston Gillispie (ed.), Dictionary of Scientific Biography, 16 vol. (197080).A most important modern work on the physiology of hearing is Georg von Bksy, Experiments in Hearing (1960, reprinted 1980). Juan G. Roederer, Introduction to the Physics and Psychophysics of Music, 2nd ed. (1975), thoroughly and clearly discusses the ear and hearing, using only basic mathematics. An excellent survey of psychoacoustics is provided in Brian C.J. Moore, An Introduction to the Psychology of Hearing, 3rd ed. (1989). Modern experiments in hearing are described in Reinier Plomp, Aspects of Tone Sensation: A Psychophysical Study (1976).Data on hearing ranges in animals is collected in Richard R. Fay, Hearing in Vertebrates: A Psychophysics Databook (1988). Chandler S. Robbins, Bertel Bruun, and Herbert S. Zim, Birds of North America, expanded rev. ed. (1983), includes audio spectrographs of bird calls. Richard E. Berg