stereophonic sound, sound recorded simultaneously through two or more electronic channels. For live recordings, microphones are placed in different positions relative to the sound source. The recorded sound is played back through loudspeakers placed more or less as the recording microphones were placed. The resulting sound does not emanate from a point source, but instead the voices or instruments composing the sound seem to be spread out as they would be naturally. Thus, some of the ambience of the recording hall is recreated. Almost all commercial recordings and motion pictures produced today use stereophonic sound, as do many television and frequency modulation (FM) radio programs. See record player; sound recording; compact disc.

sound recording, process of converting the acoustic energy of sound into some form in which it can be permanently stored and reproduced at any time.

In 1855 the inventor Leon Scott constructed a device called a phonautograph that recorded tracings of the vibrations of sound. Thomas Edison, starting about 1877, made great improvements in mechanical sound recording and was the first inventor to achieve the actually audible reproduction of recorded sound. The greatest advances, however, were made after the adoption in 1925 of electromechanical systems using electronic amplifiers (see record player).

Generally, in recording, the sound waves impinge on a microphone and are converted into an electrical signal that is recorded by a tape recorder. The tape can be edited if desired. When a commercial phonograph record is to be made, a disk of soft acetate composition coated on an aluminum base, called an original, is placed on a rotating turntable. The tape is played back and controls a stylus that cuts a spiral groove starting from the outer edge and moving to the inner edge of the original. For monophonic sound the stylus vibrates from side to side as it cuts the groove. For stereophonic sound the stylus vibrates vertically, as well as from side to side, recording one sound channel in the left wall of the groove and one in the right.

In a series of steps the original is used to make a metal stamper that presses the groove into commercial records. In order to play a commercial record, a stylus, or needle, is placed in the disk's groove while it is in motion on a turntable. The vibrations of the stylus cause the transducer to which it is attached to produce a varying voltage. This voltage is amplified and fed into a loudspeaker.

In magnetic tape systems the varying electrical voltage is converted in a small electromagnet, called a head, into a varying magnetic field that causes magnetic particles embedded in the tape to become aligned in varying degrees as the tape passes through the magnetic field. On playback, the magnetic tape moves past the head, generating a varying voltage in the coil of the head, which is boosted in an amplifier and converted to sound by a loudspeaker.

Compact discs, first introduced commercially in the early 1980s, employ laser technology to inscribe and "read" digital information in a way that avoids actual physical contact between the disc and any type of stylus. The optical properties of the disc's tracks are measured by a sensor and converted to digital signals and then to sound. Compact discs have the advantage of minimal wear and a greater possible dynamic range. Compact disc technology was superceded by digital versatile disc (DVD) technology with the introduction of the DVD-Audio format in 1999. Using a similar optical technology, DVD offers greater storage capacity and even more accurate sound reproduction. Recorded music and other sound may also be stored on and played from computer disks using several different computer-program file formats.

Motion picture soundtracks are called optical recordings. The sound to be recorded is converted into an electrical signal that is used to modulate the intensity of a beam of light. This modulated beam exposes a moving film to make a recording of the sound. Reproduction is effected by shining a steady beam of light through the developed film that is the sound track. As the film moves across the light beam, some of the light passes through it into a photocell, the amplified output of which activates a loudspeaker. See tape recorder.

sound barrier: see aerodynamics.

sound, any disturbance that travels through an elastic medium such as air, ground, or water to be heard by the human ear. When a body vibrates, or moves back and forth (see vibration), the oscillation causes a periodic disturbance of the surrounding air or other medium that radiates outward in straight lines in the form of a pressure wave. The effect these waves produce upon the ear is perceived as sound. From the point of view of physics, sound is considered to be the waves of vibratory motion themselves, whether or not they are heard by the human ear.

Generation of Sound Waves

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.

Characteristics of Sound Waves

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.

Characteristics of Musical Sounds

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.

Viscount Melville Sound, 250 mi (402 km) long and 100 mi (161 km) wide, arm of the Arctic Ocean, Northwest Territories and Nunavut, Canada, between Victoria and Prince of Wales islands on the south and Melville and Bathurst islands on the north. It is a section of the Northwest Passage. Through McClure Strait on the west it is linked with the Beaufort Sea; Barrow Strait and McClintock Channel lead east and southeast. A crossroads of arctic waterways, it is navigable only under favorable weather conditions. The western part was explored (1850-53) by Sir Robert McClure.

Sound, the: see Øresund, Denmark and Sweden.

Scoresby Sound, arm of the Greenland Sea, E Greenland. It has numerous fjords that branch out generally westward to the ice cap. Some of the branches extend more than 180 mi (290 km) inland. At its mouth is the settlement of Scoresbysund (1995 pop. 483), the center of the district of Scoresbysund. The town is a fishing and hunting base.

Puget Sound, arm of the Pacific Ocean, NW Wash., connected with the Pacific by Juan de Fuca Strait, entered through the Admiralty Inlet and extending in two arms c.100 mi (160 km) S to Olympia. The sound, which receives many streams from the Cascade Range, has numerous islands and is navigable for large ships. Along its shores are important ports and commercial cities; the Puget Sound Naval Shipyard is at Bremerton. The Puget Sound lowland, which extends south from the sound, is the most densely populated area of Washington; Seattle and Tacoma are the principal cities. The sound was discovered in 1792 by English Capt. George Vancouver and named for his aide, Peter Puget, who explored it.

Prince William Sound, large, irregular, islanded inlet of the Gulf of Alaska, S Alaska, E of the Kenai peninsula. It has many bays and good harbors; the large Columbia Glacier flows into Columbia Bay, in the N central portion. Shipping is focused at the port of Valdez, which is the southern terminus of the trans-Alaskan pipeline linked to Prudhoe Bay. Access to the interior of the sound is by highway and railroad. Fishing, forestry, and some mining are prevalent activities in the area. Valdez and Cordova are the largest towns on the sound. On Mar. 24, 1989, the Exxon Valdez hit a reef nearby and spilled 11 million gallons of oil into Prince William Sound. Clean-up efforts ensued; however, much of the region's wildlife was killed or endangered as a result of the environmental disaster.

Port Royal Sound, arm of the Atlantic Ocean, between St. Helena and Parris islands to the north and Hilton Head Island to the south, in S S.C.; it receives the Broad River. The sound was named in 1562 by French explorer Jean Ribaut, founder of a short-lived Huguenot settlement on Parris Island. In Nov., 1861, during the Civil War, Union Comdr. Samuel F. Du Pont reduced the forts guarding the sound, and the area remained in Union hands for the rest of the war, becoming a major naval base.

Plymouth Sound, deep inlet of the English Channel, Devon and Cornwall, SW England. It is a famous roadstead and forms a bay c.3 mi (5 km) wide. It receives the Tamar River through the Hamoaze estuary and the Plym River through the Cattewater estuary. A breakwater, c.2 mi (3 km) long, protects the harbor. The inlet has been protected by forts for centuries. Plymouth, long a major port, is located at its head.

Parry Sound, town (1991 pop. 6,125), S Ont., Canada, on Parry Sound, an inlet of Georgian Bay of Lake Huron. It is an active port and the center of a popular vacation area.

Pamlico Sound, lagoon, 80 mi (129 km) long and 15 to 30 mi (24-48 km) wide, E N.C., separated from the Atlantic Ocean by a row of low, sandy barrier islands; largest lagoon along the U.S. East Coast. It receives the Neuse and Pamlico rivers and is linked on the N with Albemarle Sound. Cape Hatteras National Seashore is located on the barrier islands. Along the coastal areas are numerous waterfowl nesting sites, and there is some commercial fishing in the waters.

Owen Sound, city (1991 pop. 21,674), SE Ont., Canada, on Owen Sound. It is a port and railroad terminal in a farming region, and it has large grain elevators. There are printing and other industries.

Norton Sound, inlet of the Bering Sea, c.150 mi (240 km) long and 125 mi (200 km) across at its widest point, W Alaska, S of the Seward Peninsula. Norton Bay is its northeast arm. Nome is on the north shore and the Yukon River flows into the sound from the south. It is navigable from May to October.

Nootka Sound, inlet of the Pacific Ocean and natural harbor on the west coast of Vancouver Island, SW British Columbia, Canada, lying between the mainland and Nootka Island (206 sq mi/534 sq km). The mouth of the sound was sighted (1774) by Juan Pérez, the Spanish explorer. The sound itself was visited by Capt. James Cook (1778), who was the first European to land in that region. John Meares, the British explorer, established a trading post on Nootka Sound in 1788. Its seizure by Spaniards in 1789 became the subject of a controversy between Spain and England over claims in the region. The Nootka Convention (1790) resolved the dispute and opened the N Pacific coast to British settlement.

Mississippi Sound, arm of the Gulf of Mexico, c.100 mi (160 km) long and from 7 to 15 mi (11-24 km) wide, extending from Lake Borgne in Louisiana on the west to Mobile Bay in Alabama on the east. It is part of the Intracoastal Waterway and is separated from the Gulf by a series of narrow islands and sandbars. Cat, Horn, Petit Bois, and Dauphin are the main islands. Gulfport, Biloxi, and Pascagoula, Miss., are on the Mississippi Sound.

Milford Sound, inlet of the Tasman Sea, indenting SW South Island, New Zealand. Part of Fiordland National Park, it is a well-known resort area. Mountains rise steeply from the shore to a height of 9,042 ft (2,756 m).

Melville Sound, Canada: see Viscount Melville Sound.

McMurdo Sound, Antarctica: see Ross Sea.

Long Island Sound, arm of the Atlantic Ocean, c.90 mi (145 km) long and 3-20 mi (5-32 km) wide, separating Long Island, N.Y., from the SE New York mainland and Connecticut. On the W the East River joins it with New York Bay. The sound is fed from the north by the Housatonic, Connecticut, and Thames rivers. The Long Island Sound is a popular leisure-boating center. New Haven, New London, and Bridgeport, Conn., are the largest port cities; many residential communities line the sound. Federal and state environmental studies in the mid-1990s identified serious problems in the sound and recommended a massive cleanup.

Lancaster Sound, arm of Baffin Bay, c.200 mi (320 km) long and 40 mi (60 km) wide, Nunavut Territory, Canada. It extends west between Devon and Baffin islands and is part of the shortest water route across N Canada to the Beaufort Sea. It was visited in 1616 by William Baffin, the English explorer.

Albemarle Sound, large inland body of generally fresh water, c.55 mi (90 km) long, from 3 to 14 mi (4.8-22 km) wide, NE N.C. Shallow and tideless, the sound is separated from the Atlantic Ocean by a long, narrow barrier island. The Chowan and Roanoke rivers are the largest of many streams flowing into the sound. Albemarle Sound forms a vital link in the Intracoastal Waterway; canals connect it with Chesapeake Bay. Fort Raleigh National Historic Site on Roanoke Island and Wright Brothers National Memorial at Kitty Hawk are at the western end of the sound (see National Parks and Monuments, table).

Quality of sound that distinguishes one instrument, voice, or other sound source from another. Timbre largely results from a characteristic combination of overtones produced by different instruments. This distinctive combination (which usually varies across the range of pitches) is what principally permits a listener to distinguish a clarinet from a flute, an alto from a tenor, or even a Stradivarius violin from a Guarneri violin, when both are sounding the same pitch. One element of timbre results from the differing methods of producing the sounds (blowing, bowing, striking, etc.), especially audible at the moment a note begins.

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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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formerly Melville Sound

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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