Sound

All the sounds we hear have one thing in common. Every sound is produced by vibrations of an object. When an object vibrates, it makes the surrounding air vibrate. The vibrations in the air travel outward in all directions from the object. When the vibrations enter our ears, the brain interprets them as sounds (see Ear). Although many of the sounds we hear travel through the air, sound can move through any material. For example, sound travels well through solid earth. You may have read that American Indians used to put their ears to the ground to listen for distant hoofbeats.

Sound has great importance in our lives. First of all, sound makes it possible for us to communicate with one another through speech. Many sounds, such as music and the singing of birds, provide pleasure. The sounds of radio and television broadcasts bring us entertainment and information. We are warned of danger by such sounds as automobile horns and fire alarms.

Animal sounds. Birds, frogs, and almost all mammals have vocal cords or similar structures and make sounds the same way that people do. A dolphin produces clicks and whistles in air-filled pouches connected to the blowhole, a nostril in the top of its head. The buzzing of bees and flies results from the vibrations of their wings beating against the air. Many other insects produce sounds by rubbing one part of the body against another part. For example, a cricket "sings" by scraping parts of its front wings together.

Some kinds of fishes cluck, croak, grunt, or make other sounds by vibrating a baglike organ, known as a swim bladder or air bladder, that is located below the backbone. Certain kinds of shellfish produce clicking sounds by striking their claws together. The pistol shrimp makes a sound much like a gunshot by snapping one of its claws.

Musical sounds are usually pleasing or interesting sounds. Different kinds of musical instruments produce sounds in different ways.

Certain instruments produce sounds when struck. When the membrane of a drum is hit, for example, it vibrates and produces sound. Such instruments as chimes and xylophones have a series of bars or tubes, each of which sounds a particular note when struck.

The sounds of the cello, violin, harp, and piano are produced when a player makes one or more of their strings vibrate. The vibrating strings in turn cause parts of the body of the instrument to vibrate, setting the surrounding air in motion. The strings of cellos and violins are usually stroked with a bow. A musician plucks the strings of a harp. When the keys of a piano are struck, padded hammers hit strings inside the piano, making them vibrate.

Wind instruments, such as the clarinet, flute, and trumpet, generate sounds by the vibration of columns of air inside the instruments. A clarinet has a flat, thin part called a reed attached to the mouthpiece. The reed vibrates when a player blows on it, which makes the air column inside the clarinet vibrate. The column of air in a flute vibrates when a musician blows across a hole in the flute's mouthpiece. In a trumpet, the vibrating lips of the player make the air column vibrate.

Noises are unpleasant, annoying, and distracting sounds. Most kinds of noises are produced by vibrating objects that send out irregular vibrations at irregular intervals. Such noises include the banging of garbage cans, the barking of a dog, and the roar of a crowd. Many machines and devices, such as air conditioners, vacuum cleaners, and the engines of motor vehicles, produce noise. Natural events also create noise. The shaking of the earth generates the rumble of earthquakes. The crash of thunder is produced by violent vibrations of air that has been heated by lightning.

Some noises consist of impulsive sounds--that is, vibrations which start suddenly and quickly die. Impulsive sounds include the crack of a gunshot and the pop of a firecracker. A power lawn mower produces a series of impulsive sounds. Such noises as the screech of chalk or a fingernail on a blackboard and the wail of a siren consist of a collection of rapid vibrations that do not blend well. See Noise.

Sound waves must travel through a medium. Thus, sound is absent in outer space, which contains no material for a vibrating object to compress and expand.

The nature of a particular sound can be described in terms of (1) frequency and pitch, (2) intensity and loudness, and (3) quality.

Frequency and pitch. The number of condensations or rarefactions produced by a vibrating object each second is called the frequency of the sound waves. The more rapidly an object vibrates, the higher will be the frequency. Scientists use a unit called the hertz to measure frequency. One hertz equals one cycle (vibration) per second (see Hertz). As the frequency of sound waves increases, the wavelength decreases. Wavelength is the distance between any point on one wave and the corresponding point on the next one.

Most people can hear sounds with frequencies from about 20 to 20,000 hertz. Bats, dogs, and many other kinds of animals can hear sounds with frequencies far above 20,000 hertz. Different sounds have different frequencies. For example, the sound of jingling keys ranges from 700 to 15,000 hertz. A person's voice can produce frequencies from 85 to 1,100 hertz. The tones of a piano have frequencies ranging from about 30 to 15,000 hertz.

The frequency of a sound determines its pitch--the degree of highness or lowness of the sound as perceived by a listener (see Pitch). High-pitched sounds have higher frequencies than low-pitched sounds. Musical instruments can produce a wide range of pitches. For example, a trumpet has valves that can shorten or lengthen the vibrating column of air inside the instrument. A short column produces a high-frequency, high-pitched sound. A long column results in a note of low frequency and low pitch.

Intensity and loudness. The intensity of a sound is related to the amount of energy flowing in the sound waves. Intensity depends on the amplitude of the vibrations producing the waves. Amplitude is the distance that a vibrating object moves from its position of rest as it vibrates. The larger the amplitude of vibration is, the more intense will be the sound.

The loudness of a sound refers to how strong the sound seems to us when it strikes our ears. At a given frequency, the more intense a sound is, the louder it seems. But equally intense sounds of different frequencies are not equally loud. The ear has low sensitivity to sounds near the upper and lower limits of the range of frequencies we can hear. Thus, a high-frequency or low-frequency sound does not seem as loud as a sound of the same intensity in the middle of the frequency range.

Water waves in a pond get weaker as they travel away from their source. In the same way, sound waves lose intensity as they spread outward in all directions from their source. Thus, the loudness of a sound decreases as the distance increases between a person and the source of the sound. You can observe this effect in a large field by walking away from a friend who is talking at a constant level. As you move farther and farther away, the voice of your friend gets fainter and fainter.

Sound quality, also called timbre, is a characteristic of musical sounds. Quality distinguishes between sounds of the same frequency and intensity produced by different musical instruments.

Almost every musical sound consists of a combination of the actual note sounded and a number of higher tones related to it. The actual note played is the fundamental. The higher tones are overtones of the fundamental. For example, when a note is produced by a violin string, the string vibrates as a whole and produces the fundamental. But the string also vibrates in separate sections at the same time. It may vibrate in two, three, four, or more parts. Each of these vibrations produces an overtone of higher frequency and pitch than the fundamental. The greater the number of vibrating parts is, the higher will be the frequency of the overtone.

The number and strength of the overtones help determine the characteristic sound quality of a musical instrument. For instance, a note on the flute sounds soft and sweet because it has only a few, weak overtones. The same note played on the trumpet has many, strong overtones and thus seems powerful and bright.

In general, liquids and solids are denser than air. But they are also far less compressible. Therefore, sound travels faster through liquids and solids than it does through air. Compared with its speed through air, sound travels about 4 times faster through water and about 15 times faster through steel. The speed of sound through air is commonly measured at sea level at 59 ºF. (15 ºC). At that temperature, sound travels 1,116 feet (340 meters) per second. However, the speed of sound increases as the air temperature rises. For instance, sound travels 1,268 feet (386 meters) per second through air at 212 ºF. (100 ºC).

The speed of sound is much slower than the speed of light. In a vacuum, light travels 186,282 miles (299,792 kilometers) per second--almost a million times faster than sound. As a result, we see the flash of lightning during a storm before we hear the thunder. If you watch a carpenter hammering on a distant building, you will see the hammer strike before you hear the sound of the blow.

You may have noticed that the pitch of a train whistle seems higher as the train approaches and lower after the train passes and moves away. The sound waves produced by the whistle travel through the air at a constant speed, regardless of the speed of the train. But as the train approaches, each successive wave produced by the whistle travels a shorter distance to your ears. The waves arrive more frequently, and the pitch of the whistle appears higher. As the train moves away, each successive wave travels a longer distance to your ears. The waves arrive less frequently, producing a lower apparent pitch. This apparent change in pitch produced by moving objects is called the Doppler effect. To a listener on the train, the pitch of the whistle does not change.

Jet airplanes sometimes fly at supersonic speeds. A plane flying faster than the speed of sound creates shock waves, strong pressure disturbances that build up around the aircraft. People on the ground hear a loud noise, known as a sonic boom, when the shock waves from the plane sweep over them.

Reflection. If you shout toward a large brick wall at least 30 feet (9 meters) away, you will hear an echo. The echo is produced when the sound waves are reflected from the wall to your ears. Generally, when sound waves in one medium strike a large object of another medium--such as the waves in air hitting the brick wall--some of the sound is reflected. The remainder is sent into the new medium. The speed of sound in the two mediums and the densities of the mediums help determine the amount of reflection. If sound travels at about the same speed in both materials and both have about the same density, little sound will be reflected. Instead, most of the sound will be transmitted into the new medium. If the speed differs greatly in the two mediums and their densities are greatly different, most of the sound will be reflected. Sound waves travel much more slowly through air than through brick, and brick is much denser than air. Thus when you shout at the brick wall, most of the sound is reflected. See Echo.

Refraction. When sound waves leave one medium and enter another in which the speed of sound differs, the direction of the waves is altered. This change in direction results from a change in the speed of the waves and is called refraction. If sound waves travel slower in the second medium, the waves will be refracted toward the normal. The normal is an imaginary line perpendicular to the boundary between the mediums. If sound travels faster in the second medium, the waves will be refracted away from the normal.

Sound waves can also be refracted if the speed of sound changes according to their position in a medium. The waves bend toward the region of slower speed. You may have noticed that sounds carry farther at night than during a sunny day. During the day, air near the ground is warmer than the air above. Sound waves in the air are bent away from the ground into the cooler air above, where their speed is slower. This bending of the waves results in weaker sound near the ground. At night, air near the ground becomes cooler than the air above. Sound waves are bent toward the ground, enabling sound near the ground to be heard over longer distances.

Diffraction. Sound waves traveling along the side of a building spread out around the corner of the building. When sound waves pass through a doorway, they spread out around its edges. This spreading out of waves as they pass by the edge of an obstacle or through an opening is called diffraction. Diffraction occurs whenever a sound wave encounters an obstacle or opening. But it is most evident when the wavelength of the sound wave is long compared with the size of the obstacle or opening. Diffraction enables you to hear a sound from around a corner, even though no straight path exists from the source of the sound to your ears. See Diffraction.

Resonance is the reinforcing of sound. It occurs when a small repeated force produces larger and larger vibrations in an object. To produce resonance, the repeated force must be applied with the same frequency as the resonance frequency of the object. Resonance frequency is approximately the frequency at which an object would vibrate naturally if disturbed in some way. It is said that some opera singers can shatter a wineglass by singing a note with a frequency equal to the resonance frequency of the glass. The vibrations produced in the glass get larger and larger until it breaks.

You can demonstrate resonance by holding a vibrating tuning fork over a tube that is open at one end and closed at the other end. If the tube is almost exactly one-fourth as long as the wavelength of the sound waves from the fork, the waves will travel down the column of air inside the tube and be reflected from the bottom. The original waves and the reflected waves combine and form wave patterns that appear to stand still. Such patterns are called standing waves. When standing waves form in the tube, the air column and the tuning fork are in resonance. The standing waves in the tube cause the surrounding air to vibrate with a larger amplitude, resulting in a louder sound.

Resonance increases the loudness of the sounds produced by many musical instruments. For example, a wind instrument produces resonance in the same way as the tuning fork and the tube. Standing waves are set up in the column of air inside the instrument. The air column resonates with the vibrations at the mouthpiece, amplifying the sound of the instrument.

Beats. When two tones of slightly different frequencies are sounded at the same time, you hear a single sound that gets louder and softer at regular intervals. These periodic variations of loudness are called beats. Beats are produced because the sound waves of the two tones overlap and interfere with each other.

The interference of the combined waves is called constructive if condensations coincide with condensations and rarefactions meet rarefactions. The waves reinforce each other, producing a louder sound. The interference between the waves is destructive if condensations coincide with rarefactions. A weaker sound or silence results. If the periods of constructive and destructive interference alternate, the loudness of the sound increases and decreases, thus producing beats. See Interference.

The number of beats per second, called the beat frequency, equals the difference between the frequencies of the two tones. For example, if a 256-hertz tone and a 257-hertz tone are sounded together, one beat will be heard each second.

A unit called the phon is often used to measure the loudness level of tones. The loudness level in phons of any tone is the intensity level in decibels of a 1,000-hertz tone that seems equally loud. For example, a tone of 20 decibels with a frequency of 1,000 hertz has a loudness level of 20 phons. A tone of any frequency and intensity that seems equally loud is also assigned a loudness level of 20 phons. For instance, a tone of 80 decibels with a frequency of 20 hertz seems as loud as the 20-decibel tone at 1,000 hertz. Thus, the 80-decibel tone has a loudness level of 20 phons.

Controlling sound. The science of acoustics deals with sound and its effects on people. A major field of acoustics is environmental acoustics, which involves control of noise pollution.

We are continually exposed to noise from a variety of sources, such as airplanes, construction projects, industries, motor vehicles, and household appliances. People exposed to loud noise for long periods may suffer temporary or permanent loss of hearing. Loud sounds of short duration, such as the noise of a gunshot or a firecracker, can also damage the ear. Constant noise--even if it is not extremely loud--can cause fatigue, headaches, hearing loss, irritability, nausea, and tension.

Noise pollution can be controlled in a number of ways. Acoustical engineers have quieted the noise made by many devices. For example, mufflers help quiet automobile engines. And in buildings, thick, heavy walls, well-sealed doors and windows, and various other means may be used to block noise. Industrial workers and other people exposed to intense noise should wear some form of ear protectors to help prevent hearing loss.

Acoustics also involves providing good conditions for producing and listening to speech and music in such places as auditoriums and concert halls. For example, acoustical engineers work to control reverberation--the bouncing back and forth of sound against the ceiling, walls, floor, and other surfaces of an auditorium or hall. Some reverberation is necessary to produce pleasing sounds. But too much reverberation can blur the voice of a speaker or the sound of a musical instrument. Engineers use such sound-absorbing items as acoustical tiles, carpets, draperies, and upholstered furniture to control reverberation. See Acoustics.

Using sound. Sound has many uses in science and industry. Geophysicists often use sound in exploring for minerals and petroleum. In one technique, they set off a small explosion on or just below the earth's surface. The resulting sound waves bounce off underground layers of rock. The nature of each echo and the time it takes for the waves to reach the surface indicate the type and thickness of each rock layer present. Geophysicists can thus locate possible mineral- or oil-bearing rock formations. A device called sonar uses sound waves to detect underwater objects (see Sonar). Warships can locate enemy submarines with sonar. Fishing boats use sonar systems to detect schools of fish.

Sound with frequencies above the range of human hearing is called ultrasound. It is used to clean watches and other delicate instruments. Manufacturers also use ultrasonic waves to test metals, plastics, and other materials. Physicians can diagnose brain tumors, gallstones, liver diseases, and other disorders with ultrasound. Ultrasound also provides a relatively safe means to check the development of unborn children. See Ultrasound.

Scientists and engineers have developed several devices for recording and reproducing sound. These devices include the microphone, the speaker, and the amplifier. A microphone changes sound waves into electric signals that correspond to the pattern of the waves. A speaker changes electric signals, such as those produced by a microphone, back into sound. An amplifier is used in most sound-reproduction systems in order to strengthen the electric signals and make them powerful enough to operate the speaker. Every phonograph, public address system, radio, tape recorder, and television set has at least one amplifier. See Electronics; Microphone; Speaker.

In recording music, engineers sometimes make two or more separate recordings from microphones placed at various points around the source. If these recordings are played back together correctly, they produce stereophonic sound. Stereophonic sound has qualities of depth and direction similar to those of the original sound. To reproduce stereophonic sound, a sound system must have an amplifier and a speaker for each of the recordings. See Stereophonic Sound System.

The wave theory. The understanding that sound travels in the form of waves may have originated with the Italian artist Leonardo da Vinci about 1500. But European scientists did not begin extensive experiments on the nature of sound until the early 1600's. About that time, the Italian astronomer and physicist Galileo demonstrated that the frequency of sound waves determines pitch. Galileo scraped a chisel across a brass plate, producing a screech. He then related the spacing of the grooves made by the chisel to the pitch of the screech.

About 1640, Marin Mersenne, a French mathematician, obtained the first measurement of the speed of sound in air. About 20 years later, the Irish chemist and physicist Robert Boyle demonstrated that sound waves must travel in a medium. Boyle showed that a ringing bell could not be heard if placed in a jar from which as much air had been removed as possible. During the late 1600's, the English scientist Sir Isaac Newton formulated an almost correct relationship between the speed of sound in a medium and the density and compressibility of the medium.

In the mid-1700's, Daniel Bernoulli, a Swiss mathematician and physicist, explained that a string could vibrate at more than one frequency at the same time. In the early 1800's, a French mathematician named Jean Baptiste Fourier developed a mathematical technique that could be used to break down complex sound waves into the pure tones that make them up. During the 1860's, Hermann von Helmholtz, a German physicist, investigated the interference of sound waves, the production of beats, and the relationship of both to the ear's perception of sound.

Recent developments. Much of modern acoustics is based on the principles of sound described in The Theory of Sound, a book published by the British physicist Lord Rayleigh in 1878. Although many of the properties of sound have thus been long established, the science of acoustics has continued to expand into new areas. In the 1940's, Georg von Bekesy, an American physicist, showed how the ear distinguishes between sounds. In the 1960's, the field of environmental acoustics expanded rapidly in response to growing concern over the physical and psychological effects of noise pollution.

Acoustical research of the 1970's included the study of new uses of ultrasound, and the development of better ultrasonic equipment. During the early 1980's, research included the design of better sound-reproducing equipment and the development of computers that can understand and reproduce speech. Acoustical engineers also studied possible uses of infrasound--that is, sound with frequencies below the range of human hearing.

Contributor: Alan B. Coppens, Ph.D., Associate Prof. of Physics, Naval Post-graduate School. and James V. Sanders, Ph.D., Associate Prof. of Physics, Naval Postgraduate School.

Related articles include:

Principles of sound
Acoustics; Decibel; Doppler Effect; Ear; Echo; Harmonics; Interference; Larynx; Noise; Pitch; Tone; Ultrasound; Vibration; Voice; Waves.

Sound instruments and devices
Compact disc; Electronics; Fathometer; Hearing Aid; Microphone; Oscilloscope; Phonograph; Radio; Sonar; Speaker; Stereophonic Sound System; Stethoscope; Tape Recorder; Telephone; Television; Tuning Fork; Voiceprint.

Other related articles
Bell, Alexander Graham; Berliner, Emile; Deafness; Edison, Thomas Alva; Frequency Modulation; Helmholtz, Hermann Ludwig; Mach, Ernst; Muffler; Music; Phonetics; Singing.

Why does sound travel faster through liquids and solids than through air?

How do wind instruments generate tones?

Why do acoustical engineers try to control the amount of reverberation in auditoriums and concert halls?

How can noise pollution affect people?

How did Robert Boyle demonstrate that sound waves must travel in a medium?

Why do sounds carry farther at night than during a sunny day?

How would it be possible for an opera singer to shatter a wineglass by singing?

Why does a note on the flute sound different from the same note played on the trumpet?

Why are sound waves absent in outer space?

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Cash, Terry, and Taylor, Barbara. Sound. Warwick Pr., 1989.

Taylor, Barbara. Sound. Gloucester Pr., 1992.

Gardner, Robert. Experimenting with Sound. Watts, 1991.

Lampton, Christopher. Sound. Enslow, 1992.

Pierce, John R. The Science of Musical Sound. Rev. ed. W. H. Freeman, 1992.

White, Glenn D. The Audio Dictionary. 2nd ed. Univ. of Washington Pr., 1991.

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