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.

Bibliography

See G. Chedd, Sound (1970).

Sound

Sound is produced by the vibration of some sort of material. In a piano, a hammer strikes a steel string, causing the string to vibrate. A guitar string is plucked and vibrates. A violin string vibrates when it is bowed. In a saxophone, the reed vibrates. In an organ pipe, the column of air vibrates. When a person speaks or sings, two strips of muscular tissue in the throat vibrate. All of the vibrating objects produce sound waves.

Characteristics of Waves

All waves, including sound waves, share certain characteristics: they travel through material at a certain speed, they have frequency, and they have wavelength. The frequency of a wave is the number of waves that pass a point in one second. The wavelength of a wave is the distance between any two corresponding points on the wave. For all waves, there is a simple mathematical relationship between these three quantities called the wave equation. If the frequency is denoted by the symbol f, and the wavelength is denoted by the symbol λ, and the symbol v denotes the velocity, then the wave equation is v = fλ . In a given medium, a sound wave with a shorter wavelength will have a higher frequency.

Waves also have amplitude. Amplitude is the "height" of the wave, or how "big" the wave is. The amplitude of a sound wave determines how loud is the sound.

Longitudinal Waves. Sound waves are longitudinal waves. That means that the part of the medium vibrating moves back and forth instead of up and down or from side to side. Regions where the particles of the medium are pushed together are called compressions. Places where the particles are pulled apart are called rarefactions. A sound wave consists of a series of compressions and rarefactions moving through the medium.

As with all types of waves, the medium is left in the same place after the wave passes. A person listening to a radio across the room receives sound waves that are moving through the air, but the air is not moving across the room.

Speed of Sound Waves. Sound travels through solids, liquids, and gases at different speeds. The speed depends on the springiness of the medium. Steel, for example, is much springier than air, so sound travels through steel about fifteen times faster than it travels through air.

At 0° C, sound travels through dry air at about 331 meters per second. The speed of sound increases with temperature and humidity. The speed of sound in air is related to many important thermodynamic properties of air. Since the speed of sound in air measures how fast a wave of pressure will move through air, anything that depends on air pressure will be expected to behave differently near the speed of sound. This characteristic caused designers of high-speed aircraft many problems. Before the design problems were overcome, several test pilots lost their lives while trying to "break the sound barrier."

The speed of sound changes with temperature. The speed increases by0.6 meters per second (m/s) for each Celsius degree rise in temperature (T u). This information can be used to construct a formula for the speed (v ) of sound at any temperature:

v = (331 + 0.60T )m/s

At average room temperature of 20° C, the speed of sound is close to 343 meters per second.

Frequency and Pitch. Sounds can have different frequencies. The frequency of the sound is the number of times the object vibrates per second. Frequency is measured in vibrations per second, or hertz (Hz). One Hz is one vibration per second. We perceive different frequencies of sound as different pitches; as a consequence, the higher the frequency, the higher the pitch. The normal human ear can detect sounds with frequencies between 20 Hz and 20,000 Hz. As humans age, the upper limit usually drops. Dogs can hear much higher frequencies, up to 50,000 Hz. Bats can detect frequencies up to 100,000 Hz.

Intensity and Perception of Sound

Sound waves, like all waves, transport energy from one place to another. The rate at which energy is delivered is called power and is measured in watts. Sound intensity is measured in watts per square meter (W/m²).

The human ear can detect sound intensity levels as low as 10^-12 W/m²and as high as 1.0 W/m². This is an incredible range. Because of the wide range of sound intensity that humans can hear, humans perceive loudness instead of intensity. A sound with ten times the intensity in watts per square meter is perceived as only about twice as loud.

Since humans do not directly perceive sound intensity, a logarithmic scale for loudness was developed. The unit of loudness is the decibel, named after Alexander Graham Bell. The threshold of hearing—0 dB—represents a sound intensity of 10^-12 W/m². Each tenfold increase in intensity corresponds to 10 dB on the loudness scale. Thus 10 dB is ten times the sound intensity of 0 dB. A sound of 20 dB is ten times the intensity of a 10 dB sound and one hundred times the intensity of a 0 dB sound. The list below shows the loudness of some common sounds.

Even short exposure to sounds above 120 dB will cause permanent damage to hearing. Longer exposure to sounds just below 120 dB will also cause permanent damage.

see also Logarithms; Powers and Exponents.

Elliot Richmond

Bibliography

Epstein, Lewis Carroll. Thinking Physics. San Francisco: Insight Press, 1990.

Giancoli, Douglas C. Physics, 3rd ed. Englewood Cliffs, NJ: Prentice Hall, 1991.

Haber-Schaim, Uri, John A. Dodge, and James A. Walter. PSSC Physics, 7th ed. Dubuque, IA: Kendall/Hunt, 1990.

Hewitt, Paul G. Conceptual Physics. Menlo Park, CA: Addison-Wesley, 1992.

sound¹ / sound/ • n. vibrations that travel through the air or another medium and can be heard when they reach a person's or animal's ear: light travels faster than sound. ∎  a group of vibrations of this kind; a thing that can be heard: she heard the sound of voices in the hall don't make a sound. ∎  the area or distance within which something can be heard: we were always within sound of the train whistles. ∎  short for speech sound. ∎  the ideas or impressions conveyed by words: you've had a hard day, by the sound of it. ∎  (also musical sound) sound produced by continuous and regular vibrations, as opposed to noise. ∎  music, speech, and sound effects when recorded, used to accompany a film or video production, or broadcast: [as adj.] a sound studio. ∎  broadcasting by radio as distinct from television. ∎  the distinctive quality of the music of a particular composer or performer or of the sound produced by a particular musical instrument: the sound of the Beatles. ∎  (sounds) inf. music, esp. popular music: sounds of the sixties. • v. [intr.] emit sound: a loud buzzer sounded. ∎  [tr.] cause (something) to emit sound: she sounded the horn. ∎  [tr.] give an audible signal to warn of or indicate (something): a different bell begins to sound midnight. ∎  [tr.] say (something); utter: he sounded a warning that a coup was imminent. ∎  convey a specified impression when heard: he sounded worried. ∎  (of something or someone that has been described to one) convey a specified impression: it sounds as though you really do believe that | the house sounds lovely. ∎  [tr.] test (the lungs or another body cavity) by noting the sound they produce: the doctor sounded her chest. PHRASAL VERBS: sound off express one's opinions in a loud or forceful manner.DERIVATIVES: sound·less adj. sound·less·ly adv. sound·less·ness n. sound² • adj. 1. in good condition; not damaged, injured, or diseased: they returned safe and sound he was not of sound mind. ∎  based on reason, sense, or judgment: sound advice for healthy living the scientific content is sound. ∎  competent, reliable, or holding acceptable views: he's a bit stuffy, but he's very sound on his law. ∎  financially secure: she could get her business on a sound footing for the first time. 2. (of sleep) deep and undisturbed. ∎  (of a person) tending to sleep deeply: I am a sound sleeper. 3. severe: such people should be given a sound thrashing. • adv. soundly: he was sound asleep. PHRASES: (as) sound as a bell in perfect condition.DERIVATIVES: sound·ly adv. sound·ness n. sound³ • v. 1. [tr.] ascertain (the depth of water), typically by means of a line or pole or using sound echoes. ∎  Med. examine (a person's bladder or other internal cavity) with a long surgical probe. 2. [tr.] question (someone), typically in a cautious or discreet way, as to their opinions or feelings on a subject: we'll sound out our representatives first. ∎  inquire into (someone's opinions of feelings) in this way: officials arrived to sound out public opinion at meetings in factories. 3. [intr.] (esp. of a whale) dive down steeply to a great depth. • n. a long surgical probe, typically with a curved, blunt end. DERIVATIVES: sound·er n. sound⁴ • n. a narrow stretch of water forming an inlet or connecting two wider areas of water such as two seas or a sea and a lake.

380. Sound

See also 198. HEARING ; 236. LANGUAGE ; 284. MUSIC ; 330. PRONUNCIATION ; 378. SONGS and SINGING ; 382. SPEECH ; 394. THUNDER .

acoustics

1. Physics. the study of sound and sound waves.

2. the qualities or characteristics of a space, as an auditorium, that deter-mine the audibility and fidelity of sounds in it. —acoustician, n. —acoustic, adj.

anacamptics

Obsolete, the study of the reflection of sounds. —anacamptic, adj.

assonance

likeness or approximate similarity in sound.

bombilation

Rare. a rumbling sound.

bombination

Rare. a buzzing or humming sound.

cacophony

1. a harshness of sound.

2. discordant noise. —cacophonic, cacophonous, adj.

crepitation

a crackling sound.

diacoustics

Rare. the science of sounds refracted through various media.

echolocation

the fixing of the position of an object by transmitting a signal and measuring the time required for it to bounce back, typically done by radar or sonar and by bats.

echometry

the measurement of the duration of and intervals between sounds. —echometer, n.

euphony

1. an agreeableness in sounds; a pleasantness to the ear; harmoniousness.

2. Phonetics. a harmoniousness in speech sounds, especially in word choices emphasizing various patterns of consonants or vowels. —euphonic, euphonical, euphonious, adj.

harmonometer

an instrument for measuring the relationships between sounds.

homonymy

the state or quality of sounding identical, whether spelled identically or not, as bear and bare.

homophony

the state or condition of a letter, word, or symbol having the same sound as another but a different meaning, regardless of sameness or difference in spelling, as choirlquire. —homophonic, homophonous, adj.

kaleidophon, kaleidophone

an instrument for the visual representation of sound waves.

monotony

dullness or uniformity, similar to that experienced from a repeated sound. —monotonous, adj.

onomatopoeia

the state or condition of a word formed to imitate the sound of its intended meaning, as rustle. —onomatopoeic, onomatopoetic, onoma-topoietic, onomatopoeial, adj.

oxyphonia, oxyphony

an unusually sharp quality or pitch of sound or voice.

phonology

1. the study of speech sounds, from either or both the phonetic and phonemic viewpoints.

2. the phonetic and phonemic systems of a language. See also 247. LINGUISTICS . —phonologist, n. —phonological, adj.

phonomania

an abnormal love of noise.

phonophobia

an abnormal f ear of noise.

plangency

the condition or quality of producing a deep or loud sound. —plangent, adj.

psychoacoustics

the study of the relationship between sounds and their perception by the listener, especially with regard to how the perception depends on the physical characteristics of the sound rather than on the mind of the listener. —psychoacoustician, n. —psychoacoustic, adj.

raucity

the state or quality of sounding hoarse or harsh. —raucous, adj.

sibilancy, sibilance

the state or quality of a hissing sound. —sibilant, adj.

stridulation

1. the producing of a shrill, grating noise by chafing a serrated part of the body against a hard part.

2. the noise so produced. —stridulator, n. —stridulant, stridulatory, adj.

susurration

1. the act or process of whispering.

2. a whispering sound or soft rustling. Also susurrus. —susurrant, susurrous, adj.

tautophony

repetition of the same sound. —tautophonic, tautophonical, adj.

ultrasonics

the science or study of ultrasonic vibrations, those belonging to a frequency above the audio range. —ultrasonic, adj.

ululation

1. the act of wailing or hooting.

2. the sound thus produced. —ululant, adj.

sound Physiological sensation perceived by the brain via the ear, caused by an oscillating source, and transmitted through a material medium as a sound wave. The velocity at which a sound wave travels through a medium depends on the elasticity of the medium and its density. If the medium is a gas, the sound wave is longitudinal and its velocity depends on the gas temperature. The speed of sound in dry air at standard temperature and pressure (STP) is 331mps (750mph) and depends on the height above sea level. Pure sounds are characterized by pitch (highness or lowness), timbre (additional notes with frequencies like the basic note), and intensity (the rate of flow of sound energy).

SOUND.
1. In technical terms, vibrations that travel through the air at some 1,087 feet (331 metres) per second at sea level and are heard through their stimulation of organs in the ear.

2. A particular effect of such vibrations; the sound of bells; a speech sound.

3. In PHONETICS, the audible result of an utterance: the b-sound in ‘big’. Although the nouns sound and noise can often be used interchangeably (What was that sound/noise?), sound usually relates to regular and harmonious vibrations, noise to irregular and discordant vibrations. See SPEECH, TONE, VOICE.

sound3 that which is or may be heard. XIII. ME. sun, so(u)n — AN. s(o)un, (O)F. son :- L. sonus.
So sound vb. cause to make a sound XIII; emit a sound XIV. ME. sune, so(u)ne — AN. suner, OF. soner (mod. sonner) :- L. sonāre. The form with -d appears XV, and is established XVI.

sound4 †penetrate XIV; intr. and trans. ascertain the depth of water XV; measure or examine as by sounding XVI. — (O)F. sonder use the sounding-lead :- Rom. *subundāre, f. L. SUB- + unda wave.
So sound sb. †act of sounding XVI; (surg.) instrument for probing XVIII.

sound2 unhurt, uninjured XII; healthy XIII; based on fact or good grounds XV; solid, ample XVI. ME. sund, aphetic of isund, OE. ġesund = OS. gisund (Du. gezond), OHG. gisunt (G. gesund) :- WGmc. *ʒasundaz.
Hence adv. fast asleep XIV.

sound (sownd) (in surgery)
1. n. a long rodlike instrument, often with a curved end, used to explore body cavities or to dilate strictures in the urethra or other canals.

2. vb. to explore a cavity using a sound.

sound1 †swimming; †sea, water OE.; swimming-bladder of fish XIV; (from Scand.) narrow channel of water XV. OE. sund = ON. sund swimming, strait :- Gmc. *sundam, f. *sum- *swem- SWIM
.

sound v.
1. ascertain (the depth of water), typically by means of a line or pole or using sound echoes.

2. find the depth of water in (a ship's hold).

Sound / sound/ (the Sound) another name for Øresund.

Sound, see ACOUSTICS.

sound •abound, aground, around, astound, bound, compound, confound, dumbfound, expound, found, ground, hound, impound, interwound, mound, pound, profound, propound, redound, round, sound, stoneground, surround, theatre-in-the-round (US theater-in-the-round), underground, wound •spellbound • westbound • casebound •eastbound • windbound • hidebound •fogbound • stormbound •northbound • housebound •outbound • southbound • snowbound •weatherbound • earthbound •hellhound • greyhound • foxhound •newshound • wolfhound •bloodhound • background •battleground • campground •fairground • playground •whip-round • foreground •showground • merry-go-round •runaround • turnaround • ultrasound •pre-owned, unowned •unchaperoned • poind • untuned •Lund