Sound is a sensation that we hear. A sound originates in the vibration of an object. This vibration, in turn, makes the air or some other substance surrounding the object vibrate. The vibrations in the substance travel as waves, moving outward from the object in all directions. When the waves enter our ears, our organs of hearing translate them into nerve impulses. The impulses travel to the brain, which interprets them as a sound. The term sound also refers to the traveling waves.
Waves of sound can travel in any kind of substance. Most of the sounds that we hear travel in air, which scientists classify as a gas. But sound can also travel in liquids and solids. Sound travels most rapidly in solids, and more rapidly in liquids than in gases.
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A substance in which sound waves travel is called a sound medium. Where no sound medium is present, there can be no sound. There is no sound in outer space because outer space contains no sound medium.
How some familiar sounds are produced
The human voice
is produced in the larynx, a part of the throat. Two small folds of tissue stretch across the larynx. These folds, the vocal cords, have a slitlike opening between them. When we speak, muscles in the larynx tighten the vocal cords, narrowing the opening. Air from the lungs rushes past the tightened cords, causing them to vibrate. The vibrations produce the vocal sounds. The tighter the vocal cords are, the more rapidly they vibrate and the higher are the sounds produced.
Animal sounds.
Birds, frogs, and almost all mammals have vocal cords or similar structures. These animals therefore make sounds as people do. But many animals produce sounds in different ways. A dolphin produces clicks and whistles in air-filled pouches connected to its blowhole, a nostril in the top of its head. Bees buzz as they fly because their wings move rapidly. The wings make the air vibrate, producing the buzzing sound. Other insects produce sounds by rubbing one body part against another. A cricket “sings” by scraping parts of its front wings together.
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Some kinds of fishes vibrate a swim bladder or air bladder, a baglike organ below the backbone. The vibrations produce clucks, croaks, grunts, and other sounds. Certain kinds of shellfish produce clicks by striking their claws together.
Musical sounds
are produced in various ways. Certain instruments make sounds when struck. For example, when a drummer hits the membrane of a drum, the membrane vibrates, producing sound. Xylophones have a series of bars, each of which sounds a particular note when struck.
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A stringed instrument, such as a cello, violin, or harp, produces sound when a player makes one or more of its strings vibrate. This vibration causes parts of the instrument’s body to vibrate, creating sound waves in the air.
A wind instrument, such as a clarinet, flute, or trumpet, generates sound when a player makes a column of air inside the instrument vibrate. A clarinet has a flat, thin part called a reed attached to its mouthpiece. The reed vibrates when a player blows across it. The vibration of the reed, in turn, makes the air column 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.
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Noises
are unpleasant, annoying, and distracting sounds. Many manufactured products are noisy. An automobile makes noise when its engine vibrates and makes other parts of the vehicle vibrate. Natural events also create noise. Thunder occurs when lightning heats the air, causing the air to vibrate. Some noises consist of impulsive sounds—that is, sounds that start suddenly and end quickly. Impulsive sounds include the crack of a gunshot and the bang of a firecracker.
The nature of sound
Sound waves resemble the waves that travel across the surface of a body of water. You can create such surface waves by dropping a small pebble into a tub of water. When the pebble strikes the surface, the water will react by producing a series of waves. You will see the waves as expanding circles with the pebble’s point of entry in the center. 
But there is a major difference between the shape of water waves and the shape of sound waves. Water waves travel in two dimensions, moving along the plane of the water’s surface. Sound waves, by contrast, travel in three dimensions. Although you cannot see sound waves, you can imagine them as expanding spheres with the vibrating object in the center.
An individual sound wave consists of a region in which the sound medium is denser than normal and a region in which the medium is less dense than normal. As a vibrating object moves outward from its position of rest, it compresses the medium, making it denser. The resulting region of compression is called a condensation. As the vibrating object then moves inward, the medium fills in the space formerly occupied by the object. The resulting region, called a rarefaction, is less dense than normal. As the object continues to move outward and inward, a succession of condensations and rarefactions travels away from the object.
Scientists describe sound in terms of (1) frequency and pitch, (2) wavelength, (3) intensity and loudness, and (4) quality.
Frequency and pitch.
The frequency of a sound is the number of waves that pass a given point each second. The more rapidly an object vibrates, the greater is the frequency of the sound that it makes. Scientists use a unit called the hertz to measure frequency. One hertz equals one cycle (vibration, or sound wave) per second. 
The frequency of a sound determines its pitch—the degree of highness or lowness of the sound as we hear it. A high-pitched sound has a higher frequency than a low-pitched sound.
Most people can hear sounds that have frequencies from about 20 to 20,000 hertz. Bats, dogs, and many other kinds of animals can hear sounds with frequencies much higher than 20,000 hertz.
Sounds from different sources have different frequencies. For example, the sound of jingling keys ranges from 700 to 15,000 hertz. Human voices produce frequencies from about 85 to 1,100 hertz. The tones of a piano have frequencies from about 30 to 15,000 hertz.
Musicians use various techniques to change the pitch of the tones produced by their instruments. For example, a trumpet player presses and releases valves that 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.
Wavelength
is the distance between any point on one wave and the corresponding point on the next wave. Wavelength is related to frequency: The greater the frequency of a wave, the shorter the wavelength.
Intensity and loudness.
The intensity of a sound is a measure of the power of its sound waves. Sound intensity can be defined as the amount of sound power striking a unit of surface area—such as a square millimeter of the surface of an eardrum. Sound intensity can also be defined in terms of energy. Because power is a rate of energy flow, sound intensity is also a measure of the sound energy striking a unit of surface area each second. Scientists commonly refer to a sound’s intensity as its sound pressure level. 
Sound intensity depends partly on the amplitude of the vibrations creating the waves. Amplitude is the longest distance that an object moves from its position of rest as it vibrates. For an object vibrating at a given frequency, intensity increases as amplitude increases.
Scientists use a unit called the decibel (dB) to measure sound intensity. An increase of 10 dB represents a tenfold increase in power. Thus, a 50-dB sound delivers 10 times as much power per unit of area as a 40-dB sound.
A 3,000-hertz tone of 0 dB marks the threshold of audibility—the softest sound that the normal human ear can hear. A whisper amounts to about 20 dB. Ordinary conversation occurs at about 60 dB. Loud rock music can produce up to 120 dB. A level of 140 dB is the threshold of pain. Sounds of 140 dB or more can make a person’s ears hurt.
Loudness refers to how strong a sound seems to be when we hear it. For sounds of a given frequency, the more intense a sound is, the louder it seems. But equally intense sounds that have different frequencies are not equally loud. The ear has a low sensitivity to sounds near the upper and lower ends 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.
Scientists often use a unit called the phon << FON >> to measure loudness. Measured in phons, the loudness of a tone is equal to the intensity level in decibels of a 1,000-hertz tone that seems equally loud. For example, a tone with an intensity of 80 dB and a frequency of 20 hertz seems as loud as a 20-dB tone with a frequency of 1,000 hertz. Thus, the 80-dB tone has a loudness level of 20 phons.
Sound quality,
or timbre, distinguishes between sounds of the same pitch and intensity produced by different musical instruments. Almost every musical sound is a combination of the actual tone sounded and a number of higher tones related to it. The tone played is the fundamental. The higher tones are overtones.
In many musical instruments, the overtones are harmonics. The frequency of a harmonic is an integer multiple of the frequency of the fundamental. That is, the frequency of the harmonic equals the fundamental frequency multiplied by a whole number (positive integer).
Suppose, for example, a violinist plays the note A above middle C. The A string of the violin will produce the fundamental by vibrating at 440 hertz over its entire length. The string will also produce a harmonic by vibrating in two segments, each equal to half the length of the string. Each segment will vibrate at a rate equal to twice the fundamental frequency—that is, at 880 hertz. The string will produce another harmonic by vibrating in three segments, each equal to one-third of the string’s length. The frequency of this harmonic will be three times that of the fundamental, or 1,320 hertz. Other harmonics will have frequencies of four and more times that of the fundamental.
The number, intensity, and frequency of the overtones help determine the characteristic sound quality of an instrument. A note on the flute sounds soft and sweet because it has only a few weak harmonics. The same note played on the trumpet sounds powerful and bright because it has many strong overtones.
How sound behaves
The speed of sound
depends on the medium’s density and its compressibility, a measure of how easily it can be squeezed into a smaller volume. If two mediums are equally dense, but one is more compressible than the other, sound will travel more slowly through the more compressible medium. If two mediums are equally compressible, but one is denser than the other, sound will travel more slowly through the denser medium.
In general, liquids and solids are denser than air. But they are also much less compressible. Therefore, sound travels faster through liquids and solids than it does through air.
At sea level and a temperature of 59 °F (15 °C), sound travels in air at a speed of 1,116 feet (340 meters) per second. As the air temperature increases, the speed of sound also increases. For example, sound travels 1,268 feet (386 meters) per second in air at 212 °F (100 °C).
The speed of sound is related to frequency and wavelength by the equation: v = f × λ, where v is the speed of sound, f is frequency, and λ (the Greek letter lambda) is wavelength. Thus, for example, where the speed of sound is 1,116 feet (340 meters) per second, the musical note A—whose frequency is 440 hertz—has a wavelength of about 21/2 feet (0.8 meter).
The Doppler effect.
You may have noticed that the pitch of a train whistle is relatively high as the train approaches and relatively low as 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 decrease in distance causes the waves to arrive more frequently than they would if the train were not moving. Thus, the frequency, and therefore the pitch, of the whistle is higher than it would be if the train were standing still.
As the train moves away, each successive sound wave created by the whistle travels a longer distance to your ears. The increase in distance causes the waves to arrive less frequently, producing a lower pitch.
This apparent change in pitch produced by moving objects is called the Doppler effect. A listener on the train does not experience this effect because the train is not moving relative to him or her. Loading the player...
Doppler effect
Supersonic speed.
Jet airplanes sometimes fly at supersonic speeds—that is, faster than the speed of sound. A supersonic plane creates shock waves, strong pressure disturbances that build up around the aircraft and travel slightly faster than less intense sound. When the shock waves from the plane sweep over people on the ground, the people hear a loud noise that is known as a sonic boom.
Reflection.
If you shout toward a large brick wall at least 30 feet (9 meters) away, you will hear an echo. The echo will occur because the wall will reflect most of the sound waves that you create when you shout.
Generally, when sound waves in one medium strike a large object of another medium, some of the sound is reflected. The remainder enters the new medium. The speed of sound in the two mediums and the densities of the mediums help determine the amount of reflection. If the speed differs greatly in the two mediums and their densities are much 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.
Refraction.
When sound waves leave one medium and enter another, the waves can be refracted (bent). For refraction to occur, the waves must enter the second medium at an angle other than 90°, and the speed of sound must be different in the two mediums. If sound travels more slowly in the second medium, the waves will bend 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 bend away from the normal.
Refraction can also occur in a single medium if the speed of sound is not the same throughout the medium. In this kind of medium, sound waves will bend toward a region in which the speed of sound is lower. Such refraction accounts for the fact that sounds carry farther at night than during a sunny day. During the day, air near the ground is warmer than the air above. Because the speed of sound is lower in the cooler air, the waves bend upward. As a result, the sound near the ground is relatively weak. But at night, air near the ground becomes cooler than the air above. Sound waves bend toward the ground, and so sound near the ground can be heard over longer distances.
Diffraction.
When sound waves pass through a doorway, they spread out around its edges. The spreading of waves as they pass by the edge of an obstacle or through an opening is called diffraction. Diffraction enables you to hear a sound from around a corner. 
Resonance.
Any object will vibrate if it is disturbed. This natural vibration is called resonance, and its frequency is the object’s resonance frequency. The quality of the sound produced by the vibration depends on the shape of the object and the material of which it is made. For example, a thin goblet will ring if struck with a fingernail, but a block of wood will make only a brief, dull sound.
If you apply a small force to a vibrating object at the object’s resonance frequency, you will increase the amplitude of the vibration. The amplitude can become quite large.
You can demonstrate resonance with a tuning fork and a tube that is open at one end. The length of the tube must be one-fourth as long as the wavelength of the sound produced by the fork.
First, hold the tuning fork away from the tube and strike the fork. The fork will make a soft sound. Then strike the fork again and hold it above the open end of the tube. The sound waves will travel down the column of air inside the tube, and the closed end will reflect them. The original waves and the reflected waves will combine, forming standing waves in the tube. The air column and the tuning fork will be in resonance, and so the amplitude of the standing waves will grow. The standing waves will cause the surrounding air to vibrate with a larger and larger amplitude, resulting in a louder and louder sound.
Resonance makes most musical instruments louder than they would be otherwise. A wind instrument produces resonance in the same way as a tuning fork and tube. A violin produces a resonance between its strings, its body, and the space inside its body.
Beats.
If two tones that have slightly different frequencies are sounded at the same time, you will hear a single tone. This tone will become louder and softer at regular intervals. The variations in loudness are called beats. 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, you will hear one beat each second.
Beats occur because the sound waves of the two tones overlap and interfere with each other in a certain way. An interference of waves is called constructive if the condensations of the waves of one tone coincide with the condensations of the waves of the other tone. When constructive interference occurs, the waves reinforce each other, producing a louder sound. But if the condensations of one tone coincide with the rarefactions of the other tone, the interference is destructive, resulting in a weaker sound or silence. And if periods of constructive and destructive interference alternate, the loudness of the sound increases and decreases periodically, producing beats.
Working with sound
Controlling sound.
The science of acoustics deals with sound and its effects on people. We are continually exposed to noise from a variety of sources, such as airplanes, construction projects, factories, 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.
Acoustical engineers have developed many ways to quiet noise. For example, mufflers help quiet automobile engines. In buildings, thick, heavy walls and well-sealed doors and windows can block out noise. In addition, industrial workers and other people exposed to intense noise can wear earplugs to help prevent hearing loss.
Acoustical engineers also provide good conditions for producing and listening to speech and music. For example, they work to control reverberation—the persistence of a sound caused by its bouncing back and forth against the ceiling, walls, floor, and other surfaces of an auditorium. Some reverberation is necessary to produce pleasing sounds. But too much reverberation can blur the voice of a speaker or the sound of an instrument. Engineers use such sound-absorbing items as carpets, draperies, acoustical tiles, and upholstered furniture to control reverberation.
Using sound.
Sound has many uses in science and industry. Geophysicists 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 the echo takes 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. Fishing boats use sonar to detect schools of fish. Warships can locate enemy submarines with sonar.
There are many uses for ultrasound, sound with frequencies above the range of human hearing. Technicians use ultrasound to clean watches and other delicate instruments. Manufacturers use ultrasonic waves to detect flaws in 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. In addition, doctors use ultrasound to break up kidney stones nonsurgically.
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 strengthens the electric signals, making them powerful enough to operate the speaker.
In recording music, engineers make two or more separate recordings from microphones placed at various points around the sound source or sources. When these recordings are played back together, they produce stereophonic sound. This kind of sound has qualities of depth and direction that are similar to those of the original sound. To reproduce stereophonic sound, a sound system must have two or more amplifiers and speakers.
History
Early ideas about sound.
The study of sound began in ancient times. As early as the 500’s B.C., Pythagoras, a Greek philosopher and mathematician, experimented on the sounds of vibrating strings. About 400 B.C., a Greek scholar named Archytas may have observed that faster motions result in higher pitched sounds. About 50 years later, the Greek philosopher Aristotle suggested that the movement of air carries sound to our ears. From then until about A.D. 1300, most investigation of sound dealt with its relationship to music.
The study of waves.
European scientists did not begin extensive experiments on the nature of sound until the early 1600’s. About that time, Italian astronomer and physicist Galileo demonstrated that the frequency of sound waves determines their pitch. Galileo scraped a chisel across a brass plate and noticed this action often produced a screech. During the screech, filings would gather as lines on the plate. Galileo reasoned that waves within the plate concentrated the filings into the lines. He then worked out the mathematical relationship between the spacing of the lines and the frequency and pitch of the screeches.
About 1640, Marin Mersenne, a French mathematician, attempted to measure 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 as easily if placed in a jar from which almost all the air had been removed. During the late 1600’s, the English scientist Isaac Newton formulated a 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, explained that a string could vibrate at more than one frequency at the same time. In the early 1800’s, French mathematician Jean Baptiste Fourier developed a mathematical technique for analyzing waves. Fourier’s technique can break down complex sound waves into the pure, single-frequency tones that make them up. During the 1860’s, Hermann von Helmholtz, a German physicist and physiologist, investigated the perception of sound.
The recording of sound.
In 1877, American inventor Thomas A. Edison invented the first practical phonograph. This device recorded sound on tinfoil wrapped around a small metal cylinder, and it could replay the sound. In 1887, Emile Berliner, a German immigrant to the United States, invented a phonograph that used discs instead of cylinders. Stereophonic phonographs and discs appeared in 1958. Audio compact discs were introduced in Japan and Europe in 1982, and in the United States in 1983.
Tape recorders were in wide use in the radio and recording industries by 1950. In the mid-1950’s, manufacturers began to produce stereophonic tape recorders for use in the home. By the mid-1960’s, tape cassettes were competing with phonograph records.
Synchronized sound came to motion pictures in the mid-1920’s, when engineers in Germany and the United States demonstrated a few systems. In these systems, the sound from a disc was mechanically matched with the film. This method was soon replaced by one in which the sound was recorded on the film. The sound-on-film system is still in use.
Modern acoustics.
In 1878, the British physicist Lord Rayleigh described many of the important principles of acoustics in the book The Theory of Sound. Although many 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 and physiologist, showed how the ear distinguishes between sounds. In the 1960’s, the field of environmental acoustics expanded rapidly in response to concern over the physical and psychological effects of noise.
Acoustical research of the 1970’s included the study of new uses of ultrasound and the development of better ultrasonic equipment. During the 1980’s, research included the development of computers that can understand and reproduce speech.
By the early 2000’s, two important areas of acoustics were active noise cancellation (ANC) and active structural-acoustic control (ASAC). In the simplest form of ANC, a speaker produces sound waves that interfere destructively with the waves of an unwanted sound. As a result, the sound from the speaker cancels out the unwanted sound. In ASAC, a device called an actuator applies force to an object whose vibration is producing an unwanted sound. The force changes the nature of the vibration, making the resulting sound less objectionable.
