Chapter 16: Sound 517

VOCABULARY

sound p. 517vibration p. 517vacuum p. 521

BEFORE, you learned

• Waves transfer energy• Waves have wavelength,

amplitude, and frequency

NOW, you will learn

• How sound waves are produced and detected

• How sound waves transferenergy

• What affects the speed ofsound waves

KEY CONCEPT

Sound is a wave.

EXPLORE Sound

What is sound?

PROCEDURE

Tie the middle of the string to the spoon handle.

Wrap the string ends around your left and right index fingers. Put the tips of these fingers gently in your ears and hold them there.

Stand over your desk so that the spoon dangleswithout touching your body or the desk. Then move a little to make the spoon tap the desk lightly. Listen to the sound.

WHAT DO YOU THINK?• What did you hear when the spoon tapped the desk? • How did sound travel from the spoon to your ears?

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MATERIALS • piece of string • large metal

spoon

Sound is a type of mechanical wave.In the last chapter, you read that a mechanical wave travels through amaterial medium. Such mediums include air, water, and solid materials.Sound is an example of a mechanical wave. is a wave that is produced by a vibrating object and travels through matter.

The disturbances that travel in a sound wave are vibrations. A is a rapid, back-and-forth motion. Because the medium

vibrates back and forth in the same direction as the wave travels,sound is a longitudinal wave. Like all mechanical waves, sound wavestransfer energy through a medium.

check your reading What do sound waves have in common with other mechanicalwaves? Your answer should include the word energy.

vibration

Sound

OUTLINEStart an outline for thisheading. Remember toleave room for details.

I . Main idea

A. Supporting idea

1. Detail

2. Detail

B. Supporting idea

518 Unit 4: Waves, Sound, and Light

How Sound Waves Are ProducedThe disturbances in a sound wave are vibrations that are usually too smallto see. Vibrations are also required to start sound waves. A vibratingobject pushes and pulls on the medium around it and sends out wavesin all directions.

You have a sound-making instrument within your own body. It isthe set of vocal cords within the voice box, or larynx, in your throat.Put several of your fingers against the front of your throat. Now hum.Do you feel the vibrations of your vocal cords?

Your vocal cords relax when you breathe to allow air to pass in and out of your windpipe. Your vocal cords tense up and draw closetogether when you are about to speak or sing. The illustration belowshows how sound waves are produced by the human vocal cords.

Your muscles push air up from your lungs and through the narrow opening between the vocal cords.

The force of the air causes the vocal cords to vibrate.

The vibrating vocal cords produce sound waves.

check your reading How do human vocal cords produce sound waves?

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reading tip

When you see the wordpush or pull, think of force.

Sound waves are produced by vibrations.

How Vocal Cords Produce Sound

What starts the vibrations in the vocal cords?

enlargement of vocal cords

The vocalcords vibrate in the larynx.

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Air is pushed upfrom the lungs.

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Sound wavesare produced.

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Chapter 16: Sound 519

How Sound Waves Are DetectedThe shape of a human ear helps it collect sound waves. Picture a satellitedish. It collects radio waves from satellites. Your ear works in much the same way. Actually, what we typically call the ear is only the outersection of the ear. The illustration below shows the main parts of thehuman ear.

Your outer ear collects sound waves and reflects them into a tiny tubecalled the ear canal. At the end of the ear canal is a thin, skin-likemembrane stretched tightly over the opening, called the eardrum.When sound waves strike the eardrum, they make it vibrate.

The middle ear contains three tiny, connected bones called thehammer, anvil, and stirrup. These bones carry vibrations from theeardrum to the inner ear.

One of the main parts of the inner ear, the cochlea (KAWK-lee-uh),contains about 30,000 hair cells. Each of these cells has tiny hairson its surface. The hairs bend as a result of the vibrations. Thismovement triggers changes that cause the cell to send electricalsignals along nerves to your brain. Only when your brain receivesand processes these signals do you actually hear a sound.

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1 reading tip

As you read each numbered description here, match it to the number on the illustrationbelow.

Sound waves are detected in the human ear, beginning with vibrations of the eardrum.

middleear

outer ear

inner ear

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Sound waves causethe eardrum tovibrate.

Cells in the cochleadetect the vibrations and send a message tothe brain.

Enlargement of hairs on asingle cell in the cochlea(magnified 2185x)

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The vibrations travelthrough the hammer, anvil,and stirrup to theinner ear.

How do vibrations get from the eardrum to the cochlea?

How the Ear Detects Sound

Sound waves vibrate particles.You can see the motion of waves in water. You can even ride themwith a surfboard. But you cannot see air. How, then, can you picturesound waves moving through air? Sound waves transfer the motion of particles too small to see from one place to another.

For example, think about a drum that has been struck. What happens between the time the drum is struck and the sound is heard?

• The drum skin vibrates rapidly. It pushes out and then in, overand over again. Of course, this happens very, very fast. Thevibrating drum skin pushes against nearby particles in the air.The particles in the air become bunched together, or compressed.

• When the drum skin pushes the opposite way, a space opens upbetween the drum’s surface and the particles. The particles rushback in to fill the space.

• The back-and-forth movement, or vibration, of the particles isthe disturbance that travels to the listener. Both the bunched upareas, or compressions, and the spaces between the compressionsare parts of the wave.

Notice that the waves consist of repeating patterns of compressionsand spaces between the compressions. The compressions are areas ofhigh air pressure. The spaces between the compressions are areas oflow air pressure. The high- and low-pressure air pushes and pulls onthe surrounding air, which then pushes and pulls on the air aroundthat. Soon a sound wave has traveled through the air and has transferredkinetic energy from one place to another.

check your reading Summarize in your own words how sound travels through air.

compression space between compressions

vibratingdrum skin

520 Unit 4: Waves, Sound, and Light

reminder

Kinetic energy is the energy of motion.

direction of sound waveparticles in

the air

In the middle 1600s, scientists began to do experiments to learnmore about air. They used pumps to force the air out of enclosedspaces to produce a vacuum. A is empty space. It has no particles—or very, very few of them. Robert Boyle, a British scientist,designed an experiment to find out if sound moves through a vacuum.

Boyle put a ticking clock in a sealed jar. He pumped some air outof the jar and still heard the clock ticking. Then he pumped more airout. The ticking grew quieter. Finally, when Boyle had pumped outalmost all the air, he could hear no ticking at all. Boyle’s experimentdemonstrated that sound does not travel through a vacuum.

The photograph at the right shows equipment that is set up to perform an experiment similar to Boyle’s. A bell is placed in asealed jar and powered through the electrical connections at thetop. The sound of the loudly ringing bell becomes quieter as airis pumped out through the vacuum plate.

Sound is a mechanical wave. It can move only through a mediumthat is made up of matter. Sound waves can travel through air, solidmaterials, and liquids, such as water, because all of these mediums aremade up of particles. Sound waves cannot travel through a vacuum.

check your reading How did Boyle’s experiment show that sound cannot travelthrough a vacuum?

vacuum

How does sound transfer energy?PROCEDURE

Sprinkle a few grains of salt into the jar. Put the jar on a flat surface in awell-lit place.

Cut off the neck of the balloon with the scissors.

Stretch the balloon over the mouth of the jar and pull the sides down pastthe rim of the jar’s mouth. Use a rubber band to make a tight fit.

Tap the balloon with the eraser end of the pencil. Observe what happens to the salt on the bottom of the jar.

WHAT DO YOU THINK?• What happens to the salt?

• How can you explain what you observed?

CHALLENGE Suppose you could pump all the air out of the jar and could leave the salt grains in the jar and the tight rubber cover on top. If you repeated the experiment, do you think the results would be different? Explain your answer.

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Sound EnergySound EnergySKILL FOCUS Observing

MATERIALS • clean jar• table salt• balloon• scissors• rubber band• pencil with good

eraser end

TIME10 minutes

salt

Chapter 16: Sound 521

INFER As air is pumpedout of the jar, the soundof the bell becomes quieter. Why do you thinkthe bell is suspended?

connections

sealedjar

bell

vacuumplate

Sound Experiment

The speed of sound depends on its medium.Suppose you are in the baseball stands during an exciting game. Apitch flies from the mound toward home plate, and you see the batterdraw back, swing, and hit the ball high. A split second later you hearthe crack of the bat meeting the ball. You notice that the sound ofthe hit comes later than the sight. Just how fast does sound travel?

Sound travels more slowly than light, and it does not always travel at the same speed. Two main factors affect the speed of sound: the material that makes up the medium—such as air or water—and thetemperature. If we know the medium and the temperature, however,we can predict the speed of sound.

check your reading Which two factors affect the speed of sound?

The Effect of the MaterialYou have probably heard sounds in more than one medium.Think about the medium in which you most often hear sound—air.You listen to a radio or a compact disk player. You hear the siren of afire truck. These sound waves travel through air, a mixture of gases.

Now think about going swimming. You dip below the water’ssurface briefly. Someone jumps into the water nearby and splashes water

against the pool wall. You hear strange underwatersounds. These sound waves travel through water,a liquid.

Sound travels faster through liquids than itdoes through gases because liquids are denser thangases. That means that the particles are packedcloser together. It takes less time for a water particle to push on the water particles around itbecause the particles are already closer togetherthan are the particles in air. As a result, diversunderwater would hear a sound sooner than people above water would.

Sound can also travel through solid materialsthat are elastic, which means they can vibrate backand forth. In solid materials, the particles arepacked even closer together than they are in liquidsor gases. Steel is an example of an elastic materialthat is very dense. Sound travels very rapidlythrough steel. Look at the chart on the left.Compare the speed of sound in air with the speedof sound in steel.

Medium State Speed of Sound

Air (20°C) Gas 344 m/s (769 mi/h)

Water (20°C) Liquid 1,400 m/s (3,130 mi/h)

Steel (20°C) Solid 5,000 m/s (11,200 mi/h)

Materials and Sound Speeds

These divers can hear themotor of a distant boatbefore their friends abovewater hear it.

522 Unit 4: Waves, Sound, and Light

Chapter 16: Sound 523

The Effect of TemperatureSound also travels faster through a medium at higher temperatures than at lower ones.Consider the medium of air, a mixture ofgases. Gas particles are not held tightly togetheras are particles in solids. Instead, the gas particles bounce all around. The higher thetemperature, the more the gas particles wiggleand bounce. It takes less time for particles thatare already moving quickly to push against theparticles around them than it takes particlesthat are moving slowly. Sound, therefore,travels faster in hot air than in cold air.

Look at the picture of the snowboarders.The sound waves they make by yelling willtravel more slowly through air than similarsounds made on a hot day. If you could bearto stand in air at a temperature of 100°C(212°F—the boiling point of water) and listento the same person yelling, you might noticethat the sound of the person’s voice reaches you faster.

The chart on the right shows the speed of sound in air at two different temperatures. Compare the speed of sound at the temperatureat which water freezes with the speed of sound at the temperature at which water boils. Sound travels about 17 percent faster in air at100°C than in air at 0°C.

check your reading What is the difference between the speed of sound in air at 0ºCand at 100ºC?

KEY CONCEPTS1. Describe how sound waves

are produced.

2. Describe how particles move asenergy is transferred through asound wave.

3. Explain how temperatureaffects the speed of sound.

CRITICAL THINKING4. Predict Would the sound

from a distant train travelfaster through air or throughsteel train tracks? Explain.

5. Evaluate Suppose an audience watching a sciencefiction movie hears a loud roaras a spaceship explodes inouter space. Why is this scene unrealistic?

CHALLENGE6. Evaluate A famous riddle

asks this question: If a tree fallsin the forest and there is noone there to hear it, is thereany sound? What do youthink? Give reasons for youranswer.

Medium Temperature Speed of Sound

Air 0ºC (32ºF) 331 m/s (741 mi/h)

Air 100ºC (212ºF) 386 m/s (864 mi/h)

Temperature and Sound Speeds

These snowboarders’shouts reach theirfriends more slowlyin this cold air thanthey would in hot air.