Sound and Ultrasonics
William C. Vergara
The everyday world is filled with all kinds of sounds -- the murmur of voices, the blaring horn of an automobile, the distant putt-putt of a lawn mower, the buzzing of a mosquito. Each of these common sounds is different from the others. And it is easy to tell all of them from hundreds of other sounds.
There are still other sounds, however, that are too high-pitched to be heard by the human ear. These sounds are called ultrasonic sounds, or ultrasound. Some animals, such as dolphins and bats, make and hear these sounds. Scientists have built special instruments to produce and detect ultrasonic sound.
In this article you will read how sounds are produced and why they are different from one another. You will also read how ultrasound has been put to use.
How Sounds Are Produced
All sounds are produced by a certain kind of motion. Suppose you stretch a rubber band and then pluck it. The band moves back and forth so rapidly that its motion appears blurred. The sound you hear comes from its motion. Such very fast, back-and-forth motions are called vibrations. Vibrations are the source of all sounds.
Pluck a guitar string. You will hear a musical sound. If you place your finger lightly on the string, you can feel the vibrations. They produce the sound. When you stop the vibrations, you stop the sound.
Sound Travels Through a Medium
Sound travels from a vibrating object to your ear by means of a medium, or sound carrier. The medium may be a solid, a liquid, or a gas. Air, a mixture of several gases, is the most common carrier of sound as far as people are concerned. If there is no medium to carry the sound to our ears, we cannot hear the sound. For example, the moon has no air. Astronauts on the moon cannot speak directly to each other but must communicate by radio.
The Speed of Sound
A person seated beyond the outfield of a ballpark sees a batter swing at the ball. An instant later the person hears the crack of a bat as it hits the ball. You usually hear a clap of thunder a short time after you see the bolt of lightning that caused the sound. Such common experiences demonstrate that the speed of sound is slower than the speed of light.
Careful measurements show that the speed of sound depends on the medium. For example, sound travels through the air at a speed of about 1,100 feet (335 meters) a second. Sound travels about 4 times as fast in water as it does in air. Sound travels even faster in solids. In steel and in aluminum, the speed of sound is about 15 times faster than it is in air.
Changes in temperature can change the medium enough so that sounds travel through it at a different speed. For example, sound travels slightly faster in warm air than in cold air. However, at a given temperature all sounds in the same medium have the same speed.
Sounds travel from a vibrating object to your ear by means of compression waves in the air. To understand what a compression wave is, you can try an experiment with a coil spring toy made up of many flat metal coils. (One such toy of this type is called a Slinky.) The spring stretches open easily and then returns to its original shape. This flexibility makes it useful for the experiment.
First, find a yardstick or other stick about 3 feet (1 meter) long. Slide it through the center of the spring. Ask a helper to hold one end of the stick firmly. With your right hand hold your end of the stick and one end of the spring at the same time. With your left hand hold the free end of the spring. Now you can stretch the spring back and forth along the stick.
If you slide your left hand back and forth along the stick in a regular motion, a wave will move down the outstretched spring from one end to the other. The individual turns of the spring move only slightly out of position, yet the wave motion travels all along the coils.
To understand how this happens, think about the movement of the spring in slow motion. When you first move the end of the spring toward yourself, the coils nearest your hand are compressed. When you let go, the coils immediately spring apart, or expand, which compresses the next section of coils along the spring.
One such compression and expansion is called a cycle. Scientists call a wave made of many compressions and expansions a compression wave. Many compression waves pass along the spring as your hand continues to jiggle the end of the spring.
Molecules of air behave in much the same flexible way. When pushed by a vibrating object, air molecules bunch up, or compress, and then expand. The effect is passed along to neighboring molecules as compression waves of sound. The invisible compression waves travel through the air, gradually becoming weaker and weaker until the sound waves die out altogether.
Characteristics of Sound Waves
A vibrating object sends out sound waves through the air in all directions. The waves travel outward in ever widening "shells," one inside the other. If you could see sound waves, they would look somewhat like round balloons of different sizes, one inside the other. The balloons would represent the compressions of air molecules, and the space between the balloons would represent the expansions. The vibrating body, the source of the sound, would be in the center of the smallest balloon.
Wavelength and Frequency
The distance between one compression and the next is called the wavelength of a sound wave. The human ear can hear sounds whose wavelengths in air are from a little less than 3/4 inch (2 centimeters) to about 69 feet (21 meters) long.
An object that vibrates 500 times per second causes a sound wave that vibrates at the rate of 500 cycles per second. This rate is called the frequency of the sound wave. Instead of cycles per second, the term hertz (abbreviated Hz) is often used. This unit was named for Heinrich Hertz, a German scientist. For example, a bell or a piano string that vibrates 250 times per second produces a sound wave with a frequency of 250 hertz.
Depending on the rate of vibration, sound waves are described as being of low, medium, or high frequency. Thus, a 22-hertz wave is considered a low-frequency wave, while a wave of 7,000 hertz is a high-frequency wave.
There is a relationship between the wavelength and the frequency. Short wavelengths have a high frequency. Long wavelengths have a low frequency. There is also a connection between the frequency and the pitch.
A whistling or screeching sound has a high pitch. A deep or rumbling sound, such as you get from the low notes on a piano, has a low pitch. The pitch of a sound depends on its frequency. A high frequency (many cycles per second) produces a high pitch. A lower frequency (fewer cycles per second) produces a lower pitch.
You can show that the pitch of a sound depends on the frequency by plucking a stretched rubber band. If you pluck a loosely stretched rubber band, the frequency of the sound waves produced may be so low that you can see the vibrations of the rubber band. The sound you hear has a low pitch. Stretch the rubber band tightly and pluck it again. Its vibrations are so fast that you see only a blur, and the sound has a higher pitch.
Think of how an electric fan sounds when you turn on the switch. At first the fan turns slowly, and the air particles around it vibrate slowly. A low-frequency sound wave is produced, and you hear a low-pitched hum. As the fan whirls faster, the air particles vibrate faster. A higher-frequency sound wave results, and you hear a higher-pitched hum.
Amplitude, Loudness, and Intensity
A sound wave is a disturbance of the particles in the medium, or sound carrier. As each particle is disturbed, it moves back and forth a tiny distance. The distance each particle travels from its undisturbed position is called the amplitude of the sound wave. In general, the greater the amplitude, the louder the sound.
Pluck a stretched rubber band gently. It vibrates gently and strikes the air particles gently. Each air particle is made to move only a short distance -- that is, the sound wave has a small amplitude. You hear a soft, gentle sound. If you pluck the rubber band with a greater force, each air particle moves a greater distance. The sound wave has a greater amplitude, and you hear a louder sound.
Loudness depends on each person's sense of hearing. Loudness may not be the same for two people sitting next to each other and listening to the same sound. One person may have a keener sense of hearing than the other. Therefore scientists cannot measure the loudness of a sound because it may be different for each person who hears it.
Scientists can, however, measure the intensity of a sound. Intensity refers to the amount of energy in the sound, no matter who hears it.
Think back to how you made a rubber band produce sounds of different loudness. When you plucked the rubber band gently, it produced a soft sound. When the rubber band was plucked with greater force, you put more energy into the sound. That produced a sound wave of greater amplitude. The sound had a greater intensity and was louder to your ears. It would sound louder to most people with normal hearing.
Measuring Intensity of Sound
Scientists can measure the intensity of a sound by measuring the amplitude of the sound wave. This intensity is called the sound level. It is measured in units called decibels. One decibel is the smallest change in intensity that can be detected by most people.
Here are the decibel units of some sample sounds:
Faintest whisper most people can hear: 0 decibels.
Whisper loud enough to be heard 5 feet (1.5 meters) away: 20 decibels.
Ordinary conversation at a distance of 3 feet (1 meter): 65 decibels.
Traffic on a busy street: 80 decibels.
Noisiest spot at Niagara Falls: 95 decibels.
A loud, amplified rock band: 100 decibels.
A sound level of 120 or more decibels is painful to the ears of most people. If you stood near an 11-inch (280-millimeter) heavy military gun that was being fired, the sound would be very painful to your ears. The sound level is about 230 decibels. For this reason, soldiers usually use ear protectors when they are near a big gun or cannon that is about to be fired.
Both the intensity and the loudness of a sound decrease rapidly as you move away from the sound source. Sound waves have energy. As the waves move away from their source, the energy spreads out over an ever increasing space. At a great distance from the source of the sound, the energy is so spread out that any one spot receives only a tiny bit of energy. If you are standing a great distance from the source of the sound, only a very small amount of the energy reaches your ears, and you hear only a tiny sound.
Behavior of Sound
Sound waves can behave in ways that produce a number of different special effects. Some of these effects may cause the sound to be repeated or to be louder or blurred. If you have ever heard an echo, you have heard one of these effects.
Suppose you shout across a canyon. Part of the sound wave you have produced with your voice strikes the opposite wall of the canyon and bounces back. The sound is reflected from the canyon wall. You hear the sound of your voice returning. This returned sound is what is known as an echo.
In order for you to detect a definite echo, the reflecting surface must be at least 55 feet (16.75 meters) away from you. The reason for this has to do with the speed of sound and the way we hear.
When you shout, at first you hear your own voice. This sensation of sound lasts in your mind for about 1/10 second. Thus, if you are to hear a separate echo, the sound must take at least 1/10 second to cross the canyon and return. This is the minimum time that must pass before the sensation of the original shout has died out in your mind.
In this minimum time of 1/10 second, sound travels about 110 feet (33.5 meters). If the canyon wall is 55 feet away, the sound will make a round trip of about 110 feet. You will be able to tell the difference between the shout and its echo, but you cannot do this if the wall is any closer. If the wall is more than 55 feet away, you may hear a clear, distinct echo of your shout as the sound wave bounces back.
The reflection of sound sometimes causes an effect called reverberation. In some large auditoriums and gymnasiums, for example, sounds are reflected many times from the floor, walls, and ceiling. When the sound waves bounce around between these surfaces, the original sound is repeated many times. But no distinct, separate echoes are heard. Each echo has mixed with the others that are reflected from various parts of the room. This overall effect is called reverberation.
Reverberation is measured by making a sharp sound, like the crack of a pistol, and measuring the time it takes for all the echoes and re-echoes to die out. This time is called the time of reverberation. If the time of reverberation in an auditorium is too long, speech sounds run together and it is hard to understand the speaker. If it is too short, the room sounds "dead" and not enough sound reaches the listeners. A certain amount of reverberation is necessary in auditoriums, especially for the enjoyment of music.
Acoustics is the science that is concerned with the behavior of sound. It also means the way sounds echo or reverberate in an auditorium or concert hall. People say, "The acoustics of this concert hall are very good," or "This auditorium has poor acoustics."
Engineers and architects must take into account the problems of echoes and reverberation when they plan auditoriums, concert halls, and other public places. They must design the room so that there are no echoes. But they do want some reverberation, which is necessary for good acoustics.
The amount of reverberation can be controlled by placing special materials on the floor, walls, and ceiling of a room. Porous materials, such as wool, felt, and cork, are good sound absorbers. When sound waves enter the pores of these materials, the vibrations of the particles of the air are stopped. Sound-reflecting materials can be positioned in a room, especially auditoriums and concert halls, to help create desirable reverberations.
Sound-absorbing tiles, called acoustical tiles, line the ceilings of many restaurants, reducing the clatter of dishes and the noise made by the voices of many people. Soft draperies, carpets, and wall hangings help to reduce reverberation in auditoriums and concert halls.
The presence of people in an auditorium also affects the acoustics. The clothing and bodies of people absorb some of the sound waves. In an empty auditorium, listen to the way your voice sounds. It sounds different than when most of the seats are filled.
For special experiments, scientists often use a room called an anechoic chamber. Anechoic means without echoes. The walls, ceiling, and floor of such a room are constructed of sound-absorbing materials built into compartments of various sizes. Different size compartments trap sounds of different wavelengths. The result is a room in which almost no sound is reflected. Such a chamber reveals the true sound quality of instruments and voices. The anechoic chamber is useful for making very accurate measurements of sound and for testing microphones and other devices.
When you strike an object and make it vibrate, the force of your blow determines the intensity of the sound wave. But the frequency of the sound wave does not depend on the force of your blow. The frequency depends on the size, shape, and material of the object. Every vibrating object has its own frequency of vibration. This is called its natural frequency.
For example, the metal of a bell vibrates at its own natural frequency no matter how hard the clapper inside the bell strikes the metal. A tuning fork -- a two-pronged "fork" used by musicians and singers to get a pure tone -- is constructed to vibrate at a certain frequency. If it is constructed to vibrate at 440 cycles per second, this is its natural frequency.
Suppose two neighboring objects have the same natural frequency. One of the objects is made to vibrate. Its sound waves reach the second object and strike against it. Because the frequency of the sound waves is the same as the natural frequency of the second object, the waves start the second object vibrating. Both produce the same note. The second object is in sympathetic vibration with the first.
If you have two tuning forks of the same frequency, you can hear sympathetic vibrations. Hold each tuning fork at its stem and make one vibrate by striking it against the edge of a table. After the tuning fork has sounded for a few seconds, stop its vibration by moving your hand up to the prongs. You can then hear the same sound coming from the second fork, because it is in sympathetic vibration with the first.
Sympathetic vibrations can cause an avalanche. In a mountainous region, a large mass of rocks and snow may be piled loosely on a steep slope. The sound waves from a human voice or from some other source may cause some rock and snow particles to vibrate sympathetically. This small motion is sometimes enough to start the large mass of loosely piled material tumbling down the mountainside.
An effect called resonance occurs when a number of small repeated pushes cause a large vibration. Resonance increases the amplitude, and thus the intensity, of a sound.
For an example of how resonance occurs, think of a child sitting on a swing. You give the swing a push, and it starts moving in an arc. If you continue giving a small push each time the swing is at its highest point, the swing soon moves through a wide arc. Although each push is small, you have timed the pushes so that their effect adds up, producing the large back-and-forth motion.
The swing has a natural frequency at which it moves back and forth. Your pushes, if timed to this natural frequency, add up to make the wider arc. If you push at a faster or a slower rate, the swing comes almost to a stop. Resonance takes place only when you push at the natural frequency of the swing.
The sounds made by wind instruments in music are reinforced (strengthened) by resonance. The resonance comes from a column of air inside the instrument. Thus, the sound of a trumpet or a bugle is reinforced by resonance of the air inside the instrument.
Sometimes an object can be made to vibrate at a frequency other than its natural frequency. Its back-and-forth motion is then called forced vibration. A common example of forced vibration is in the eardrum. When sound waves strike the eardrum, they cause it to vibrate at the frequency of the received sound wave.
You can use a tuning fork to produce forced vibrations. Strike the tuning fork to make the prongs vibrate, and listen to the sound for a moment. The sound is quite soft. Now place the stem of the fork against the top of a table. The sound at once becomes much louder. The vibrations of the fork force the tabletop to vibrate at the same frequency as the tuning fork. The table's larger size sets more air in motion than the tuning fork alone does, and you hear a louder sound.
Many musical instruments are designed so that forced vibrations amplify their sounds. In a violin, for example, vibrating strings force the wood to vibrate -- thereby increasing the sound made by the strings. The forced vibration of the sounding board of a piano amplifies the sound made when a note is struck.
Interference and Beats
If two musicians play a tone of the same frequency, the two sounds combine. The result is a louder sound. But if two tones of only slightly different frequencies are played together, a series of loud and soft sounds is heard. This effect -- of loud and soft sounds occurring one after the other -- is called beats.
Beats are caused by the interference of two sound waves. When the compressions and expansions of two different sound waves reach your ear at the same time, they reinforce each other. The sound becomes louder. But when the compressions of one wave reach your ear at the same time as the expansions of the other, they interfere with each other. For a moment, almost no sound is heard.
You can hear beats by using two tuning forks that have the same frequency. Change the frequency of one fork slightly by putting a thin rubber band around one of its prongs. Then strike the forks. You will hear the repeated loudness and softness as regular pulses. These are beats.
It is annoying to hear more than four or five beats per second, and beats can spoil music. Musicians in an orchestra tune their instruments in unison so that all will produce the same pitch, or frequency. This eliminates the possibility of beats on identical pitches, and the instruments are said to be "in tune."
However, this applies only to notes of the same name, such as all "A" notes. In modern, or "tempered," tuning, all other intervals are very slightly out of tune and actually generate beats. This system is used to equalize the various intervals between the half-steps of the musical scale. These beats, described as "shimmers," give the characteristic richness to groups of notes played together as chords.
Have you ever noticed the change in pitch of a train whistle or automobile horn as the vehicle approaches and then passes you? As the sound source comes toward you, the pitch sounds higher. When the source is nearest you, you hear the sound at its normal pitch. Then as the source of sound moves away from you the pitch drops. This change in pitch is called the Doppler effect, after Christian Doppler, a 19th-century Austrian scientist.
An automobile horn, like a train whistle, always vibrates at the same frequency. The apparent change in frequency in the sound wave is caused by the motion of the automobile. When the vehicle approaches you, it is moving in the same direction as the sound waves coming from the horn. The sound waves are pushed closer together. Your ear receives more vibrations each second than the horn actually sends out. This gives the effect of a higher frequency, and you hear a higher-pitched sound.
After the car passes you, the opposite effect takes place. The horn still sends out the same sound waves but is moving away from the waves. The sound waves are spaced farther apart, and your ear receives fewer vibrations a second than the horn sends out. This gives the effect of a lower frequency, and you hear a lower-pitched sound.
The Doppler effect occurs whenever distance is changing between the source of sound and the listener. For example, you will hear a change in pitch if you are driving past a parked car whose horn is blowing. The amount of change depends on how fast the sound source or the listener is moving. If the speed of the sound source or the listener is great, the change in pitch is great. If the speed is slow, the change in pitch is less.
What determines whether a vibrating object will produce a noise, a pleasant sound, or a musical tone?
When compressions and expansions of a sound wave follow each other in an even, regular order, the sound is pleasant. People usually call it music. When the compressions and expansions follow each other in an uneven, irregular pattern, the sound we hear is a noise.
Hammering on a steel tank produces a loud noise. The metal of the tank vibrates unevenly. It sends out compressions and expansions that do not follow each other in a regular pattern. A note played on the piano is a musical tone because the strings that produce the tone send out a regular pattern of compressions and expansions. A skilled pianist plays enjoyable music by producing a series of regular vibrations that blend well together. A child banging on the piano keyboard produces unpleasant noises.
An exception to this rule is the sound of percussion instruments. Cymbals and most drums produce random combinations of frequencies, usually considered noises. However, when the bursts of noise are skillfully played in organized combinations, noise is converted into music.
Fundamentals and Overtones
A tuning fork is a specially constructed device that vibrates at only one particular frequency. However, most vibrating objects vibrate at several frequencies at the same time.
To understand how an object can vibrate at several frequencies at once, here's an example. A string is stretched between two posts. When plucked in the middle, the string vibrates as a whole . When plucked halfway between the middle and either end, the string again vibrates as a whole. But each half of the string vibrates separately as well. Depending on where it is plucked, the string can be made to vibrate as a whole and also in three or more separate parts. The parts vibrate at a different frequency from the vibration of the whole string.
The tone produced by the vibration of the string as a whole is called the fundamental tone of the string. The tones produced by the vibrations of the parts are called overtones or harmonics.
Musical instruments are designed to produce both a fundamental tone and overtones. In stringed instruments the strings vibrate as a whole and in sections, producing fundamentals and overtones. When a wind or woodwind instrument is played, the column of air in the instrument vibrates as a whole and in sections, producing both fundamentals and overtones. The metal or skin of percussion instruments produces overtones as well as fundamentals.
The different combinations of fundamentals and overtones give the sound of each musical instrument its special quality. Two instruments playing the same note sound different because each instrument produces different overtones. You might say that each has its own "recipe" of fundamentals and overtones.
Like different musical instruments, each human voice has its own sound-wave recipe. The vocal cords in each person's throat vibrate with frequencies and overtones that depend on the size, thickness, length, and tension of the cords. The amount and force of the air passing between the cords as a person speaks also helps determine the kind of sound waves produced. This makes it possible for you to identify many different voices, just as you can tell the difference between the sound of a piano and a flute.
The human ear generally cannot hear sound waves whose frequency is higher than about 20,000 hertz. Such high-frequency waves are called ultrasound or ultrasonic waves. ("Ultra" means "beyond," and "sonic" means "sound.") Ultrasonic waves may have frequencies as high as millions of hertz. They can never be heard by a person.
Ultrasonic waves can be produced in several ways. One method requires a thin slice of a quartz crystal. (Quartz is a common mineral. It has the unusual property of responding to a changing, or alternating, electric current by vibrating in step with the current.) High-frequency alternating current is fed into the quartz. The crystal vibrates in step with the current and produces an ultrasonic wave. Another method makes use of an electromagnet with a special core usually made of the metal nickel. When a high-frequency alternating current is passed through the electromagnet, the nickel core expands and contracts slightly. This sets up ultrasonic vibrations in the surrounding medium.
Uses of Ultrasound
Ultrasonic waves have a very short wavelength. (Short wavelengths have a high frequency.) Because of this, many waves -- and their energy -- are packed into a small space. Scientists have found ways of putting this concentrated energy to use in scientific research, in industry, and in medicine.
Ordinary sound waves move outward in all directions. But the very short wavelengths of ultrasonic waves make it possible to beam ultrasound in a straight line. Scientists have used straight ultrasonic beams in systems called sonar to locate submarines, schools of fish, and other solid objects underwater.
In sonar systems, sound reflections, or echoes, are used to detect the solid objects. A pulse, or burst, of ultrasonic waves is sent out from an instrument on a ship. The ultrasonic waves travel underwater in a straight line until they strike a solid object. The waves are then reflected and received at an instrument on board the ship.
To measure the depth of the ocean a type of sonar system called a fathometer is used. An operator aboard a ship beams an ultrasonic wave down into the water. The time it takes for the echo of the sound wave to return to the instrument on the ship gives the operator the depth of the water.
Ultrasound is used in industry to detect flaws hidden deep inside metals or other solid materials. An ultrasonic wave is beamed into the part to be tested. The way in which the beam is echoed back into a receiving device tells whether there is a defect in the material and where the defect is.
An ultrasonic wave passed through a liquid makes the liquid vibrate very fast. The vibration can shake up paint to mix it thoroughly, homogenize milk by breaking up the fat particles, and clean tools, machine parts, dishes, and other solid objects.
Ultrasound is used experimentally to break down the cell walls of bacteria. In medicine it is used to help detect brain injuries, to trace blood flow through various parts of the body, and for certain kinds of surgery. In dentistry, a dental drill controlled by ultrasonic vibrations can neatly penetrate or cut into tooth enamel, creating very little friction or heat in the gum tissue. Certain ultrasound frequencies can be used to penetrate muscles and create heat, thereby stimulating circulation.
An interesting application of ultrasound is in ultrasonic holography, a method of producing three-dimensional pictures with ultrasound. This is possible because ultrasound waves can easily travel through soft body tissues. When they reach denser layers of bone and cartilage, the waves are reflected back. Computers store and compare the images to generate three-dimensional pictures. With this technique it is even possible to safely detect the form of an unborn baby inside its mother's womb.
Ultrasonics and Animals
Some animals can hear ultrasonic sounds. For example, dogs can hear the high-pitched sound of a dog whistle that most people cannot hear. There are some animals that have built-in sonar systems. Dolphins have a sort of sonar system that they use to locate fish and obstacles in the water. Bats, too, have a sonar system. They find their way about in darkness by uttering ultrasonic squeaks that are echoed back to them from solid objects like walls. Because animal sonar is so efficient, scientists are studying these animals to learn their "ultrasonic secrets."