The recording of sound waves in a two-dimensional pattern (the hologram) and the use of the hologram to reconstruct the entire sound field throughout a three-dimensional region of space. Acoustical holography, which first appeared in the 1960s in studies in ultrasonics, is an outgrowth of optical holography, invented by Dennis Gabor in 1948. The wave nature of both light and sound make holography possible. The objective of optical holography is to observe (reconstruct) three-dimensional images of the sources of reflected light (visible or nonvisible). Acoustical holography involves reconstruction of the sound field that arises due to radiation of sound at a boundary, such as the vibrating body of a violin, the fuselage of an aircraft, or the surface of a submarine. This reconstruction represents a solution to an inverse wave propagation problem explained heuristically using Huygens' principle. Both acoustical holography and optical holography rely on the acquisition of an interferogram, a two-dimensional recording at a single frequency of the phase and amplitude of an acoustic or electromagnetic field, usually in a plane. Gabor called this interferogram a hologram. See also: Holography; Huygens' principle; Inverse scattering theory

A method of nondestructive testing and materials characterization that uses mechanical waves moving through materials. It is similar to seismology, except in being concerned with the scale of engineering structures, such as aircraft, bridges, and chemical tanks. When a structure is subjected to external force (or stress), a defect (for example, a crack or welding flaw) on the structure is activated and enlarged dynamically, and thus generates waves, which spread through materials at a certain speed. Such waves, known as acoustic emission signals, are detected by sensors attached on the surfaces of the structure. Mechanical vibration due to acoustic emission signals is weak and requires high-sensitivity sensors and electronic amplification before it can be analyzed. See also: Seismology

An instrument that is sensitive to the interference of two or more acoustic waves. It provides information on acoustic wavelengths that is useful in determining the velocity and absorption of sound in samples of gases, liquids, and materials, and it yields information on the nonlinear properties of solids.

The use of intense acoustic waves to hold a body that is immersed in a fluid medium against the force of gravity without obvious mechanical support. A common demonstration of the phenomenon of acoustic levitation is suspending droplets in a column of air via the pressure of sound waves (see illustration). See also: Air; Pressure; Sound; Sound intensity; Sound pressure

An instrument that utilizes focused acoustic waves to produce images of surface and subsurface features in materials, and to measure elastic properties on a microscopic scale. It has been used to image and measure local elastic properties in metals, ceramics, semiconductor integrated circuits, polymeric materials, and biological materials including individual cells.

Unwanted sound. Noise control is the process of obtaining an acceptable noise environment for people in different situations. These definitions and the words “unwanted” and “acceptable” suggest that criteria need to be established to determine when noise from different sources is unwanted and that these criteria could or should be used to decide on acceptable noise limits. Understanding noise and its control, then, requires a knowledge of the major sources of noise, sound propagation, human response to noise, and the physics of methods of controlling noise. The increase in noise levels from human activities in industrialized societies led to the term "noise pollution." Different governments have passed legislation and created regulations to control noise. Noise as an unwanted by-product of an industrialized society affects not only the operators of machines (Fig. 1) and vehicles, but also other occupants of buildings in which machines are installed, passengers of vehicles, and most importantly the communities in which machines, factories, and vehicles are operated. See also: Machine

The discipline of acoustic phonetics can be narrowly defined as the study of the acoustic output of the vocal tract for speech, but ultimately it encompasses much more. Acoustic phonetics makes direct contact with, and in many cases is the foundation of, areas of study such as speech synthesis, automatic (computer) recognition of speech, speech perception, phonological analysis and theory, and speech pathology.

The net pressure exerted on a surface or interface by an acoustic wave. One might presume that the back-and-forth oscillation of fluid caused by the passage of an acoustic wave will not exert any net force on an object, and this is true for sound waves normally encountered. (The sound power of a normal speaking voice is less than one-millionth of the electric power of a 100-W light.) Intense sound waves, however, can exert net forces in one direction of sufficient magnitude (proportional to the sound intensity) to balance gravitational forces and thus levitate an object in air. These acoustic forces, generated by acoustic radiation pressure, are generally not of the magnitude to lift a table off the floor, although such is not inconceivable.

A device to measure the acoustic power or intensity of a sound beam by means of the force or torque that the beam exerts on an inserted object or interface. The underlying theory involves the concept of radiation pressure, which is the time-independent part of the pressure associated with a nominally sinusoidal acoustic disturbance. Such pressure occurs, for example, when a plane sound wave is partially reflected at an interface between two materials, with the nonlinear interaction between the incident and reflected waves giving rise to a steady pressure on the interface. If a narrow beam is incident on the interface and the transmitted wave is fully absorbed by the second material, the magnitude of the radiation force F (area integral of radiation pressure) equals a constant times W/c, where W is the power of the sound beam and c is the sound speed. The multiplicative constant is of the order of unity and depends on the thermodynamic properties of the fluid. The force acts in the vector direction of propagation of the sound wave. When the inserted object is of small size, the force is not given by as simple an expression, but it can nevertheless be predicted from basic principles. Thus, acoustic radiometers do not require external calibration and are themselves sometimes used in the calibration of transducers.

The science of sound, which in its most general form endeavors to describe and interpret the phenomena associated with motional disturbances from equilibrium of elastic media. An elastic medium is one such that if any part of it is displaced from its original position with respect to the rest, as for example by an impact, it will return to its original state when the disturbing influence is removed. Acoustics was originally limited to the human experience produced by the stimulation of the human ear by sound incident from the surrounding air. Modern acoustics, however, deals with all sorts of sounds which have no relation to the human ear, for example, seismological disturbances and ultrasonics.