Sound waves are generated by any vibrating body. For example, when a violin string vibrates upon being bowed or plucked, its movement in one direction pushes the molecules of the air before it, crowding them together in its path. When it moves back again past its original position and on to the other side, it leaves behind it a nearly empty space, i.e., a space with relatively few molecules in it. In the meantime, however, the molecules which were at first crowded together have transmitted some of their energy of motion to other molecules still farther on and are returning to fill again the space originally occupied and now left empty by the retreating violin string. In other words, the vibratory motion set up by the violin string causes alternately in a given space a crowding together of the molecules of air (a condensation) and a thinning out of the molecules (a rarefaction). Taken together a condensation and a rarefaction make up a sound wave; such a wave is called longitudinal, or compressional, because the vibratory motion is forward and backward along the direction that the wave is following. Because such a wave travels by disturbing the particles of a material medium, sound waves cannot travel through a vacuum.
Sounds are generally audible to the human ear if their frequency (number of vibrations per second) lies between 20 and 20,000 vibrations per second, but the range varies considerably with the individual. Sound waves with frequencies less than those of audible waves are called subsonic; those with frequencies above the audible range are called ultrasonic (see ultrasonics).
A sound wave is usually represented graphically by a wavy, horizontal line; the upper part of the wave (the crest) indicates a condensation and the lower part (the trough) indicates a rarefaction. This graph, however, is merely a representation and is not an actual picture of a wave. The length of a sound wave, or the wavelength, is measured as the distance from one point of greatest condensation to the next following it or from any point on one wave to the corresponding point on the next in a train of waves. The wavelength depends upon the velocity of sound in a given medium at a given temperature and upon the frequency of vibration. The wavelength of a sound can be determined by dividing the numerical value for the velocity of sound in the given medium at the given temperature by the frequency of vibration. For example, if the velocity of sound in air is 1,130 ft per second and the frequency of vibration is 256, then the wave length is approximately 4.4 ft.
The velocity of sound is not constant, however, for it varies in different media and in the same medium at different temperatures. For example, in air at 0°C;. it is approximately 1,089 ft per second, but at 20°C;. it is increased to about 1,130 ft per second, or an increase of about 2 ft per second for every centigrade degree rise in temperature. Sound travels more slowly in gases than in liquids, and more slowly in liquids than in solids. Since the ability to conduct sound is dependent on the density of the medium, solids are better conductors than liquids, liquids are better conductors than gases.
Sound waves can be reflected, refracted (or bent), and absorbed as light waves can be. The reflection of sound waves can result in an echo—an important factor in the acoustics of theaters and auditoriums. A sound wave can be reinforced with waves from a body having the same frequency of vibration, but the combination of waves of different frequencies of vibration may produce "beats" or pulsations or may result in other forms of interference.
Musical sounds are distinguished from noises in that they are composed of regular, uniform vibrations, while noises are irregular and disordered vibrations. Composers, however, frequently use noises as well as musical sounds. One musical tone is distinguished from another on the basis of pitch, intensity, or loudness, and quality, or timbre. Pitch describes how high or low a tone is and depends upon the rapidity with which a sounding body vibrates, i.e., upon the frequency of vibration. The higher the frequency of vibration, the higher the tone; the pitch of a siren gets higher and higher as the frequency of vibration increases. The apparent change in the pitch of a sound as a source approaches or moves away from an observer is described by the Doppler effect. The intensity or loudness of a sound depends upon the extent to which the sounding body vibrates, i.e., the amplitude of vibration. A sound is louder as the amplitude of vibration is greater, and the intensity decreases as the distance from the source increases. Loudness is measured in units called decibels. The sound waves given off by different vibrating bodies differ in quality, or timbre. A note from a saxophone, for instance, differs from a note of the same pitch and intensity produced by a violin or a xylophone; similarly vibrating reeds, columns of air, and strings all differ. Quality is dependent on the number and relative intensity of overtones produced by the vibrating body (see harmonic), and these in turn depend upon the nature of the vibrating body.
See G. Chedd, Sound (1970).
A mixture of three pure tones (top) yields a complex resultant tone (bottom), such as might be elipsis
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Artificial imitation of sound to accompany action and supply realism in a dramatic production. Sound effects were first used in the theatre, where they can represent a range of action too vast or difficult to present onstage, from battles and gunshots to trotting horses and rainstorms. Various methods were devised by backstage technicians to reproduce sounds (e.g., rattling sheet metal to create thunder); today most sound effects are reproduced by recordings. An important part of old-fashioned radio dramas, sound effects are still painstakingly added to television and movie soundtracks.
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Sharp rise in aerodynamic drag that occurs as an aircraft approaches the speed of sound. At sea level the speed of sound is about 750 miles (1,200 km) per hour, and at 36,000 feet (11,000 metres) it is about 650 miles (1,050 km) per hour. The sound barrier was formerly an obstacle to supersonic flight. If an aircraft flies at somewhat less than sonic speed, the pressure waves (sound waves) it creates outspeed their sources and spread out ahead of it. Once the aircraft reaches sonic speed the waves are unable to get out of its way. Strong local shock waves form on the wings and body; airflow around the craft becomes unsteady, and severe buffeting may result, with serious stability difficulties and loss of control over flight characteristics. Generally, aircraft properly designed for supersonic flight have little difficulty in passing through the sound barrier, but the effect on those designed for efficient operation at subsonic speeds may become extremely dangerous. The first pilot to break the sound barrier was Chuck Yeager (1947), in the experimental X-1 aircraft.
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Mechanical disturbance that propagates as a longitudinal wave through a solid, liquid, or gas. A sound wave is generated by a vibrating object. The vibrations cause alternating compressions (regions of crowding) and rarefactions (regions of scarcity) in the particles of the medium. The particles move back and forth in the direction of propagation of the wave. The speed of sound through a medium depends on the medium's elasticity, density, and temperature. In dry air at 32 °F (0 °C), the speed of sound is 1,086 feet (331 metres) per second. The frequency of a sound wave, perceived as pitch, is the number of compressions (or rarefactions) that pass a fixed point per unit time. The frequencies audible to the human ear range from approximately 20 hertz to 20 kilohertz. Intensity is the average flow of energy per unit time through a given area of the medium and is related to loudness. Seealso acoustics; ear; hearing; ultrasonics.
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Body of water, northern Canada. Located in the Arctic Archipelago, between Melville and Victoria islands, the sound is 250 mi (400 km) long and 100 mi (160 km) wide. Its discovery, when reached from the east (1819–20) by William E. Parry and from the west (1850–54) by Robert McClure, proved the existence of the Northwest Passage. The sound is navigable only under favourable weather conditions.
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Deep inlet, Norwegian Sea, eastern central coast of Greenland. It runs inland for 70 mi (110 km) and has numerous fjords (the longest is 280 mi, or 451 km) and two large islands. It was charted by William Scoresby in 1822.
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Arm of the Pacific Ocean indenting northwestern Washington, U.S. It was explored by the British navigator George Vancouver in 1792 and named by him for Peter Puget, a second lieutenant in his expedition, who probed the main channel. It has many deepwater harbours, including Seattle, Tacoma, Everett, and Port Townsend, which are shipping ports for the rich farmlands along the river estuaries. It provides a sheltered area for recreational boating and salmon fishing.
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Inlet of the Gulf of Alaska, southern Alaska, U.S. It lies east of the Kenai Peninsula and spans 90–100 mi (145–160 km). It was named by the British captain George Vancouver in 1778 to honour a son of George III. In 1989 one of the largest oil spills in history occurred when the tanker Exxon Valdez ran aground on Bligh Reef and lost 10.9 million gallons of crude oil into the sound, with disastrous effects on its ecology.
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Shallow body of water, eastern shore of North Carolina, U.S. It is separated from the Atlantic Ocean by the Outer Banks. It extends 80 mi (130 km) south from Roanoke Island and is 8–30 mi (13–48 km) wide. Numerous waterfowl nest along the coastal waters; there is some commercial fishing, especially for oysters.
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Inlet of the Tasman Sea, southwestern coast of South Island, New Zealand. About 2 mi (3 km) wide, the sound extends inland for 12 mi (19 km). It was named by a whaler in the 1820s for its resemblance to Milford Haven in Wales. It is the northernmost fjord in Fiordland National Park and is the site of Milford Sound town, one of the region's few permanently inhabited places.
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Bay, western extension of the Ross Sea, Antarctica. Lying at the edge of the Ross Ice Shelf, the channel is 92 mi (148 km) long and up to 46 mi (74 km) wide; it has been a major centre for Antarctic explorations. First discovered in 1841 by Scottish explorer James C. Ross, it served as one of the main access routes to the Antarctic continent. Ross Island, on the shores of the sound, was the site of headquarters for British explorers Robert Falcon Scott and Ernest Shackleton.
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Body of water between the southern shore of Connecticut and the northern shore of Long Island, New York, U.S. It connects with the East River and with Block Island Sound. Covering 1,180 sq mi (3,056 sq km), it is 90 mi (145 km) long and 3–20 mi (5–32 km) wide. Its shores have many residential communities and summer resorts.
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Coastal inlet, northeastern North Carolina, U.S. Protected from the Atlantic Ocean by the Outer Banks, it is about 50 mi (80 km) long and 5–14 mi (8–23 km) wide. It is connected with Chesapeake Bay by the Dismal Swamp Canal and the Albemarle and Chesapeake Canal. Elizabeth City is its chief port. Explored by Ralph Lane in 1586, it was later named for George Monck, duke of Albemarle.
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