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500 Chapter 17

17.1 Mechanical Waves

Reading StrategyPreviewing Copy the web diagram below.Use Figure 2 to complete the diagram. Thenuse Figures 3 and 4 to make similar diagramsfor longitudinal waves and surface waves.

Key ConceptsWhat causes mechanicalwaves?

What are the three maintypes of mechanicalwaves?

Vocabulary◆ mechanical wave◆ medium◆ crest◆ trough◆ transverse wave◆ compression◆ rarefaction◆ longitudinal wave◆ surface wave

Have you ever gone to a wave pool at an amusement park? You canhear the laughter and screams as wave after wave passes by, giving thepeople a wild ride. It’s obvious that waves are moving through thewater, but you may not realize that the screams and laughter are alsocarried by waves. In this chapter, you will learn about the differentkinds of mechanical waves, including sound waves.

What Are Mechanical Waves?A mechanical wave is a disturbance in matter that carries energy fromone place to another. Recall that energy is the ability to do work. In awave pool, each wave carries energy across the pool. You can see theeffects of a wave’s energy when the wave lifts people in the water.

Mechanical waves require matter to travelthrough. The material through which a wave trav-els is called a medium. Solids, liquids, and gases allcan act as mediums. In a wave pool, waves travelalong the surface of the water. Water is the medium.Waves travel through a rope when you shake oneend of it. In that case, the medium is the rope.

A mechanical wave is created when asource of energy causes a vibration to travelthrough a medium. A vibration is a repeatingback-and-forth motion. When you shake a rope,you add energy at one end. The wave that resultsis a vibration that carries energy along the rope.

a. ?

b. ?Crests

TransverseWaves

Figure 1 In a wave pool, thewaves carry energy across the pool.

500 Chapter 17

FOCUS

Objectives17.1.1 Define mechanical waves and

relate waves to energy.17.1.2 Describe transverse,

longitudinal, and surface wavesand discuss how they areproduced.

17.1.3 Identify examples of transverseand longitudinal waves.

17.1.4 Analyze the motion of amedium as each kind ofmechanical wave passesthrough it.

Build VocabularyParaphrase This section containsseveral words that may not be familiarto students: medium, crest, trough,transverse, longitudinal, compression, andrarefaction. Have students paraphrasethese words using words they know. For example, they might construct asentence such as, “In other words, amedium is the stuff carrying the wave.”

Reading Strategya. Troughs b. Rest position; LongitudinalWave: Compressions, Rarefactions, Restposition, Direction; Surface Wave:Circular motion that returns to sameposition, Direction of wave

INSTRUCT

What Are MechanicalWaves?Build Science SkillsInferring Have students look at Figure 1. Ask, What happens to theswimmers in the pool as a wavepasses? (The swimmers move up anddown and back and forth.) What kind ofenergy do the swimmers have as theybob up and down? (Kinetic) Fromwhere do they get this energy?(From the wave)Logical, Visual

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Reading Focus

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Section 17.1

Print• Reading and Study Workbook With

Math Support, Section 17.1 • Transparencies, Chapter Pretest and

Section 17.1

Technology• Interactive Textbook, Section 17.1• Presentation Pro CD-ROM, Chapter Pretest

and Section 17.1• Go Online, NSTA SciLinks, Vibrations

and waves

Section Resources

Mechanical Waves and Sound 501

Types of Mechanical WavesMechanical waves are classified by the way they move through amedium. The three main types of mechanical waves are transversewaves, longitudinal waves, and surface waves.

Transverse Waves When you shake one end of a rope up anddown, the vibration causes a wave. Figure 2 shows a wave in a rope atthree points in time. Before the wave starts, every point on the rope isin its rest position, represented by the dashed line. The highest pointof the wave above the rest position is the crest. The lowest point belowthe rest position is the trough (TRAWF). You can see from the ribbonattached to the rope that crests and troughs are not fixed points on awave. In Figure 2A, the ribbon is at a crest. In Figure 2C, the ribbon isat a trough. The motion of a single point on the rope is like the motionof a yo-yo. The point vibrates up and down between a maximum andminimum height.

Notice that the wave carries energy from left to right, in a directionperpendicular to the up-and-down motion of the rope. This is a trans-verse wave. A transverse wave is a wave that causes the medium tovibrate at right angles to the direction in which the wave travels.

Have you ever shaken crumbs off a picnic blanket? This is anotherexample of a transverse wave. Shaking one end of the blanket up anddown sends a transverse wave through the blanket. The up and downmotion of the blanket helps to shake off the crumbs.

For: Links on vibrations and waves

Visit: www.SciLinks.org

Web Code: ccn-2171

Direction of wave

Crest

Restposition

Direction ofvibration

Trough

A

B

C

Figure 2 A transverse wave causesthe medium to vibrate in a directionperpendicular to the direction inwhich the wave travels. In the waveshown here, each point on the ropevibrates up and down between amaximum and minimum height.A The ribbon is at a crest. B Theribbon is at the rest position. C Theribbon is at a trough. Comparing and Contrasting Howdoes the direction of the wavecompare with the direction in whichthe ribbon moves?

Wave Dance

Purpose Students will simulatetransverse and longitudinal waves.

Materials 10 chairs, 10 students,space to move around

Class Time 20 minutes

Procedure Prepare students for thedemonstration ahead of time. To simulatea transverse wave, students will do awave like the one fans do at a football orbaseball game. Place the chairs in a lineand have each student sit in a chair. Eachstudent rises when the student in front ofhim or her is fully standing. To simulate alongitudinal wave, have students stand ina line, arm-length apart. The first studentsteps forward and back, then taps thenext student. When each student feels atap, he or she takes a step forward andthen a step back. These steps can berepeated as a dance.

Expected Outcome Students willrecognize that waves occur when adisturbance moves through a medium,and students will distinguish betweentwo different kinds of disturbance.Kinesthetic, Visual

Types of MechanicalWavesUse VisualsFigure 2 Have students examine theposition of the ribbon in Figure 2A andFigure 2B. Ask, How has the ribbonmoved? (It has moved down. There is noleft-to-right motion.) How does thiscompare to the direction of the wave?(Perpendicular)Visual

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Customize for English Language Learners

Simplify the PresentationIn English, one word may have several differentmeanings. For that reason, students who arelearning English may have additional difficultygrasping the scientific meanings of such words.

Identify the multiple-meaning words in thissection, such as wave, matter, and medium.Discuss with students the different meanings ofeach word and then explain which meaning isused in this section.

Answer to . . .

Figure 2 The movement of the ribbonis up and down, which is perpendicularto the direction of the wave.

Download a worksheet on vibrationsand waves for students to complete,and find additional teacher supportfrom NSTA SciLinks.

0498_hsps09te_Ch17.qxp 4/19/07 9:00 AM Page 501

502 Chapter 17

A

B

Push

Pull Direction of wave

Compression

Compression

Rarefaction

Rest position

Longitudinal Waves Figure 3 shows a wave in a spring toy attwo points in time. To start the wave, add energy to the spring bypushing and pulling the end of the spring. The wave carries energyalong the spring from left to right. You can see in Figure 3A that whenthe wave starts, some of the coils are closer together than they wouldbe in the rest position. An area where the particles in a medium arespaced close together is called a compression (kum PRESH un). As thecompression moves to the right in Figure 3B, coils behind it are spreadout more than they were in the rest position. An area where the particlesin a medium are spread out is called a rarefaction (rehr uh FAK shun).

Look at the ribbon tied to one of the coils. The ribbon is first in acompression and then in a rarefaction. However, the ribbon and thecoil it is tied to do not move along the spring. As compressions and rar-efactions travel along the spring toward the right, each coil vibrates backand forth around its rest position. In this wave, the vibration is a back-and-forth motion of the coil that is parallel to, or in the same directionas, the direction in which the wave moves. This is a longitudinal wave.A longitudinal wave (lawn juh TOO duh nul) is a wave in which thevibration of the medium is parallel to the direction the wave travels.

Waves in springs are not the only kind of longitudinal waves. P waves(originally called primary waves) are longitudinal waves produced byearthquakes. Because P waves can travel through Earth, scientists canuse these waves to map Earth’s unseen interior.

What are compressions and rarefactions?

Observing Wavesin a MediumProcedure

1. Fill a large, clear, square orrectangular containerhalfway with water. Add adrop of food coloring inthe center of the container.

2. At the side of the container,submerge a ruler length-wise. Move the ruler upand down to make waves.

3. Observe and record howthe waves and the foodcoloring move.

Analyze and Conclude

1. Comparing andContrasting Compare themovement of the waveswith the movement of thefood coloring.

2. Formulating HypothesesGenerate one or morehypotheses to explain theobserved motion of thefood coloring.

Figure 3 A longitudinal wavecauses the medium to vibrate in adirection parallel to the directionin which the wave travels. Eachpoint on the spring vibrates backand forth about its rest position.A When the end of the spring ispushed, a compression starts tomove along the spring. B Whenthe end of the spring is pulled, ararefaction follows thecompression along the spring.

502 Chapter 17

Observing Waves in a Medium

ObjectiveAfter completing this activity, studentswill be able to • describe a mechanical wave as a

passage of energy through a medium,with no net movement of the medium.

This lab can dispel the misconceptionthat waves are parts of the medium thattravel with the wave.

Skills Focus Inferring

Prep Time 15 minutes

Materials large, clear container; foodcoloring; ruler; droppers (optional)

Advance Prep Dilute food coloringwith water. Use droppers to dropcolored water into the container.


Safety Students should wear alaboratory apron to avoid stains.

Teaching Tips• Instruct students to observe the wave

and the drop from the side.

Expected Outcome The drop of foodcoloring sits on the bottom of the con-tainer as the water wave moves backand forth. Some small currents disturbthe food coloring.

Analyze and Conclude1. The wave moved. The food coloringstayed in place on the bottom.2. Students may hypothesize that waterat the bottom is not disturbed by asurface wave, or that the disturbancedepends on the depth of the water.Visual, Logical

For Enrichment

Float bits of cork on the water. Generatewaves and observe the motion of thecork bits. Students should observe up-and-down motion and also side-to-sidemotion of the cork bits. Students shouldvary the depths of the water to see if thisaffects the results.Visual

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Section 17.1 (continued)

Surfing Surfing originated in Polynesia andwas developed in Hawaii. Native Hawaiiansrode long, heavy, carved wooden surfboards.Surfing is possible because of the properties ofwaves as they enter shallower water. The speedof a wave in shallow water depends on thedepth of the water. As a wave approaches theshore, it slows down. In shallow water with asloping bottom, the front portion of the waveis in shallower water and moving slower than

the rear portion, so the back of the wavecatches up with the front. This causes the waveto “break.” Surfers ride the breaking wave asthe surfboard slides down the steep front ofthe wave. Modern surfboards are made from a foam core covered with plastic resin. Theselighter and smaller surfboards are much easierto handle than the heavy wooden boards usedby early Hawaiians.

Facts and Figures

Section 17.1 Assessment

Reviewing Concepts1. Describe how mechanical waves

are produced.

2. List the three main types of mechanical waves.

3. For each type of wave, compare the vibrationof the medium to the direction of the wave.

4. Name one example of each type of wave.

Critical Thinking5. Comparing and Contrasting How are

transverse and longitudinal waves similar?How are they different?

6. Applying Concepts A spring hangs from theceiling. Describe how a single coil moves as alongitudal wave passes through the spring.

7. Interpreting Diagrams In Figure 4, why isthe first position of the bobber the same as thefifth position of the bobber?

Surface Waves If you ask people to describe waves, mostlikely they will describe ocean waves before they think of the wavesthat travel in a rope or a spring. Ocean waves are the most famil-iar kind of surface waves. A surface wave is a wave that travelsalong a surface separating two media.

The ocean wave in Figure 4 travels at the surface between waterand air. The floating fishing bobber helps to visualize the motion ofthe medium as the wave carries energy from left to right. When acrest passes the bobber, the bobber moves up. When a troughpasses, the bobber moves down. This up-and-down motion, likethe motion of a transverse wave, is perpendicular to the directionin which the wave travels. But the bobber also is pushed back andforth by the surface wave. This back-and-forth motion, like themotion of a longitudinal wave, is parallel to the direction in whichthe wave travels. When these two motions combine in deep water,the bobber moves in a circle.

If you watched the bobber for ten minutes, it would not movecloser to shore. Most waves do not transport matter from oneplace to another. But when ocean waves approach the shore, theybehave differently. Perhaps you have seen seaweed washed ashoreby breaking waves. As a wave enters shallow water, it topples overon itself because friction with the shore slows down the bottom ofthe wave. The top of the wave continues forward at its originalspeed. As a result, the wave carries the medium, along with any-thing floating in it, toward the shore.


Wave direction

Figure 4 As the ocean wave moves to theright, the bobber moves in a circle,returning to its original position. Making Generalizations If these werebreaking waves near the shore, whatwould happen to the bobber over time?

Energy Review potential and kineticenergy in Section 15.1. Then, describe theenergy changes in a single coil of a springas longitudinal waves pass through it.

Build Reading LiteracySummarize Refer to page 598D inChapter 20, which provides theguidelines for summarizing.

Have students construct a table tosummarize what they have learnedabout mechanical waves. The rowscould be labeled with the kind of waveand the columns could be labeled withthe wave properties.Logical

ASSESSEvaluate UnderstandingHave students write a paragraphsummarizing the content of this section.

ReteachUse Figures 2 and 3 to summarize thesimilarities and differences betweentransverse and longitudinal waves.

The coil gains elastic potential energyand has the least kinetic energy when itis in a compression. In a rarefaction, thecoil is moving fastest, has its greatestkinetic energy, and its elastic potentialenergy is at a minimum.

If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 17.1.

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Answer to . . .

Figure 4 The bobber would be pushedto shore by breaking waves.

Compressions are areaswhere particles in a

medium are spaced close together.Rarefactions are areas where particlesin a medium are spread out.

4. Transverse wave: shaking the end of a ropeup and down; Longitudinal wave:compressions and rarefactions movingthrough a spring; Surface wave: deep waterwave in an ocean5. Both kinds of waves carry energy througha medium without transferring matter. Intransverse waves, the medium vibratesperpendicular to the direction in which thewave travels, while in longitudinal waves, themedium vibrates parallel to the direction inwhich the wave travels.

6. The coil will move up and down about itsrest position as compressions and rarefactionspass through.7. The bobber has moved in a full circle thatreturns it back to its starting position(because one complete surface wave haspassed through the bobber’s position).


1. Mechanical waves are formed when asource of energy causes a vibration to travelthrough a medium.2. Transverse, longitudinal, and surface waves3. Transverse wave: medium vibratesperpendicular to the direction the wave travels;Longitudinal wave: medium vibrates parallel tothe direction the wave travels; Surface wave:medium vibrates both perpendicular andparallel to wave direction (circular motion)


504 Chapter 17

17.2 Properties of Mechanical Waves

Will it be a good day for surfing? You might not think that a surferwould check the Internet to find out. But some Web sites now updateocean wave data every hour. Of course, fishing boats and naval vesselsalso need this information. Usually, the properties used to describewaves are period, frequency, wavelength, speed, and amplitude.

Frequency and PeriodHow do surfers know when the next wave is coming? If they count thetime between two successive crests, the next crest usually will comeafter this same time interval. Any motion that repeats at regular timeintervals is called periodic motion. The time required for one cycle, acomplete motion that returns to its starting point, is called the period.For an ocean wave, the period is the time between two successive crests.

Any periodic motion has a frequency, which is the number ofcomplete cycles in a given time. For a wave, the frequency is thenumber of wave cycles that pass a point in a given time. Frequency ismeasured in cycles per second, or hertz (Hz).

A wave’s frequency equals the frequency of the vibratingsource producing the wave. The rope in Figure 5A is shaken with afrequency of one vibration per second, so the wave frequency is onecycle per second, or 1 hertz. In Figure 5B, the vibration is twice as fast,so the frequency is two cycles per second, or 2 hertz.

Reading StrategyBuilding Vocabulary Copy and expand thetable below. As you read, write a definition inyour own words for each term.

Key ConceptsWhat determines thefrequency of a wave?

How are frequency,wavelength, andspeed related?

How is the amplitude ofa wave related to thewave’s energy?

Vocabulary◆ periodic motion◆ period◆ frequency◆ hertz◆ wavelength◆ amplitude Period

Frequency

Wavelength

Amplitude

Vocabulary Term Definition

b. ?

a. ?

c. ?

d. ?

Frequency = 1.0 hertz

Rest position

One cycle per second

Frequency = 2.0 hertz

Two cycles per second

Figure 5 Frequency is thenumber of complete cycles in agiven time. A A wave vibrating atone cycle per second has afrequency of 1.0 Hz. B A wavevibrating at two cycles per secondhas a frequency of 2.0 Hz.

A

B

504 Chapter 17

FOCUS

Objectives17.2.1 Define frequency, period,

wavelength, and wave speedand describe these propertiesfor different kinds of waves.

17.2.2 Solve equations relating wavespeed to wavelength andfrequency or period.

17.2.3 Describe how to measureamplitude and relate amplitudeto the energy of a wave.

Build VocabularyWord Forms The term amplitudecontains the root ampl-, which comesfrom the Latin amplus. Have studentsthink of other English words that containampl- (amplify, ample). What do thesewords have in common? (All have to dowith largeness or fullness.)

Reading Strategya. The time required for one cycle b. The number of complete cycles in agiven time c. The distance between apoint on a wave and the same point on the next cycle of the wave d. Themaximum displacement of a mediumfrom its rest position

INSTRUCT

Frequency and PeriodUse VisualsFigure 5 Have students compare thenumber of complete cycles per second inFigures 5A and 5B. Ask, What happensto the frequency when the number of cycles per second increases? (Itincreases.) If a wave has a frequency of 0.5 Hz, how many cycles per secondis it vibrating? (It is vibrating at one-halfcycle per second.) What is the period of a 0.5-Hz wave? (2 seconds)Logical

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Section 17.2


Math Support, Section 17.2 and Math Skill: Calculating Wave Properties

• Math Skills and Problem SolvingWorkbook, Section 17.2

• Transparencies, Section 17.2

Technology• Interactive Textbook, Section 17.2• Presentation Pro CD-ROM, Section 17.2• Go Online, NSTA SciLinks, Wave properties

Section Resources


WavelengthWavelength is the distance between a point on one wave and the samepoint on the next cycle of the wave. For a transverse wave, wavelengthis measured between adjacent crests or between adjacent troughs. Fora longitudinal wave, wavelength is the distance between adjacent com-pressions or rarefactions. Notice in Figure 6 that when wavelength isshorter, crests are closer together. They must occur more frequently.

Increasing the frequency of a wave decreases its wavelength.

Wave SpeedRecall that the speed of an object equals distance divided by time. Tocalculate a swimmer’s speed, for example, you can measure the lengthof one lap in a pool and the time it takes to swim one lap. This is likemeasuring the wavelength (one lap) and period (time to swim one lap)of the swimmer’s motion. In much the same way, you can calculate thespeed of a wave by dividing its wavelength by its period. You can alsocalculate wave speed by multiplying wavelength by frequency.

Speed of Waves

Speed � Wavelength � Frequency

When the wavelength is in meters and the frequency is in hertz, theunits for speed are meters per second. If you know any two of thevalues in this formula, you can solve for the third value.

What is wavelength?

Short wavelength

B

Long wavelength

Rest position

A

Comparing Frequency and Wave Speed

Materials3-m rope, tape measure, stopwatch

Procedure1. Tie one end of the rope to a chair. Shake the

other end to send waves down the rope.

2. With a partner, measure the time it takes yourhand to move back and forth ten times. Then,measure and record the distance from yourhand to the chair and the time it takes a wavecrest to travel this distance.

3. Repeat Step 2, but this time shake the ropemore rapidly. Record your data.

Analyze and Conclude1. Calculating What was the frequency of the

waves in Steps 2 and 3? (Hint: Divide 10 wavesby the time it took to make them.)

2. Calculating What was the wave speed inSteps 2 and 3? (Hint: Divide distance by time.)

3. Drawing Conclusions How was wave speedaffected by increasing the frequency?

Figure 6 Wavelength can bemeasured from any point on awave to the same point on thenext cycle of the wave. A Thewavelength of a transverse waveequals the distance from crest tocrest or from trough to trough. B. The wavelength of this wave ishalf the wavelength of the wavein A. Inferring Which wave has agreater frequency?

WavelengthBuild Science SkillsMeasuring Have students use a smallruler to measure the wavelength of eachwave in Figure 6 and verify that thewavelength measured does not dependon which two corresponding points areused. Visual, Logical

Wave Speed

Comparing Frequency and Wave Speed

ObjectiveAfter completing this activity, studentswill be able to • distinguish between wave frequency

and wave speed.

Skills Focus Measuring, Comparingand Contrasting

Prep Time 10 minutes


Teaching Tips• Have students practice before doing

timed trials.• Using a longer rope gives more clear-

cut results.

Expected Outcome Neither theamplitude nor the frequency affects thespeed of propagation of a wave.

Analyze and Conclude1. Answers will depend on student data.Answers of one to five waves per secondare reasonable.2. Answers will depend on student databut should be expressed in units of m/s.3. The wave frequency had no effect onthe wave speed. Logical

For EnrichmentStudents can time the speed of waves invarious other solid media, such as wiresand monofilament fishing line, andattempt to determine the properties ofmaterials that affect the speed of wavetransmission. Kinesthetic

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Customize for Inclusion Students

Visually ImpairedTie a weight (such as a washer or fishingweight) to a string, and attach the string to apendulum clamp on a ring stand so that ithangs vertically and is free to swing. Allowstudents to touch the pendulum and make itswing. By holding their hand in the right

place, they can feel the weight touch theirhand as it completes each cycle. Students can compare the periods and frequencies ofpendulums with different lengths (lengths of 1 m, 25 cm, and 6.25 cm should producefrequencies of about 0.5 Hz, 1 Hz, and 2 Hz,respectively).

Answer to . . .

Figure 6 The frequency of B is greaterthan the frequency of A.

Wavelength is thedistance between one

point on a wave and the same pointon the next cycle of the wave.


506 Chapter 17

The speed of a wave can change if it enters a new medium or ifvariables such as pressure and temperature change. However, for manykinds of waves, the speed of the waves is roughly constant for a rangeof different frequencies. If you assume that waves are travelingat a constant speed, then wavelength is inversely proportional tofrequency. What does this mean for two waves with different frequen-cies? The wave with the lower frequency has a longer wavelength.

Speed of Mechanical WavesOne end of a rope is vibrated to produce a wave with awavelength of 0.25 meters. The frequency of the wave is 3.0 hertz. What is the speed of the wave?

Read and UnderstandWhat information are you given?

Wavelength = 0.25 m

Frequency = 3.0 Hz

Plan and SolveWhat unknown are you trying to calculate?

Speed = ?

What formula contains the given quantities and the unknown?

Speed � Wavelength � Frequency

Replace each variable with its known value.

(Hint: 1Hz � )

Speed � 0.25 m � 3.0 Hz

� 0.25 m � 3.0

Speed � 0.75 m/s

Look Back and CheckIs your answer reasonable?

Because the frequency is 3.0 hertz, the wave should travel a distance of 3 wavelengths in 1 second. This distance is 0.75 meters, which agrees with the calculated speed of 0.75 m/s.

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1. A wave on a rope has awavelength of 2.0 m and afrequency of 2.0 Hz. What is the speed of the wave?

2. A motorboat is tied to a dockwith its motor running. Thespinning propeller makes asurface wave in the water with afrequency of 4 Hz and awavelength of 0.1 m. What isthe speed of the wave?

3. What is the speed of a wave in aspring if it has a wavelength of10 cm and a period of 0.2 s?(Hint: Use the equation

Speed � .)

4. What is the wavelength of anearthquake wave if it has aspeed of 5 km/s and a frequencyof 10 Hz?

WavelengthPeriod

For: Links on wave properties


Web Code: ccn-2172

506 Chapter 17

Solutions1. The speed is 2.0 m � 2.0 Hz �4.0 m/s.2. The speed is 0.1 m � 4 Hz � 0.4 m/s.3. The speed is 10 cm/0.2 s � 50 cm/s.4. The wavelength is (5 km/s)/10 Hz �0.5 km.Logical

For Extra HelpHave students write the equation requiredto solve each problem first. Then, checkthat they recognize what they know anddon’t know in the equation. Logical

Direct students to the Math Skills in theSkills and Reference Handbook at theend of the student text for additionalhelp.

Additional Problems1. The waves in a pool have a wavelengthof 0.20 m and a frequency of 2.8 Hz.What is the speed of these waves? (0.56 m/s)2. A student moves the end of a softspring back and forth to make waves.The waves travel at 1.8 m/s and have awavelength of 1.2 m. What is thefrequency of these waves? (1.5 Hz)Logical, Portfolio

Build Science SkillsInferring Physics includes manyrelationships similar to the wave equation.Students need to develop a sense of howeach variable relates to the others. Ask, If the speed of a wave decreases, butthe frequency stays the same, whathappens to the wavelength? (The wave-length must decrease.)Logical

Students are sometimes unable todistinguish between the transversemotion of the medium and motion ofthe wave and think that waves with ahigher frequency will move faster. Inmany mediums, mechanical wavesmove at approximately the same speedfor a wide range of frequencies. Toreinforce this idea, have students shake the end of the rope at severalfrequencies. They can confirm that thisdoesn’t affect the wave speed. Logical

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Download a worksheet on waveproperties for students to complete,and find additional teacher supportfrom NSTA SciLinks.


LowamplitudeHigh

amplitude

Rest position

A B


Reviewing Concepts1. How is the vibration of the source related

to a wave’s frequency?

2. How is wavelength related to frequencyfor waves moving at a constant speed?

3. How is the energy of a wave related to its amplitude?

4. Describe two ways you could measure thewavelength of a longitudinal wave.

5. Describe how you measure the amplitude of atransverse wave.

Critical Thinking6. Applying Concepts If a wave’s period

doubles, how does the wave’s frequency change? (Hint: Period � )1

Frequency

7. Designing Experiments Describe anexperiment to measure the frequency of alongitudinal wave in a spring.

8. Predicting If you double the frequency of awave, what is the effect on its wavelength(assuming speed does not change)?

9. A wave on a rope has a frequency of3.3 Hz and a wavelength of 1.2 m.What is the speed of the wave?

10. A spring toy vibrates at 2 Hz toproduce a wave. What is the period ofthe wave?

AmplitudeIf you drop a pebble into a pond, the wave is not very high. If you doa “cannonball” jump into the water, you know the wave will be muchhigher. These two waves have different amplitudes. The amplitude(AM pluh tood) of a wave is the maximum displacement of themedium from its rest position.

Figure 7 shows the amplitudes of two transverse waves in a rope.The amplitude of a transverse wave is the distance from the rest posi-tion to a crest or a trough. It takes more energy to produce a wave withhigher crests and deeper troughs. The more energy a wave has,the greater is its amplitude.

How do you measure the amplitude of a longitudinal wave? In thiscase, the amplitude is the maximum displacement of a point from itsrest position. The more energy the wave has, the more the mediumwill be compressed or displaced.

Figure 7 The more energy awave has, the greater is itsamplitude. A The amplitude of atransverse wave equals thedistance to the highest pointabove the rest position. B Thiswave’s amplitude is one half theamplitude of the wave in A. Applying Concepts Whichwave has more energy?

For: Activity on tsunamis

Visit: PHSchool.com

Web Code: ccc-2172

AmplitudeUse VisualsFigure 7 The two waves shown havedifferent amplitudes, but the samewavelength and frequency. Ask, If youwere shaking the ropes, how wouldwhat you would feel in Figure 7Adiffer from what you feel in Figure 7B?(“My hand would shake harder in A thanB.”) Explain that wave A delivers moreenergy than wave B. Visual, Logical

Build Reading LiteracyOutline Refer to page 156D inChapter 6, which provides theguidelines for an outline.

Have students outline the section,leaving room for notes. Then, theyshould scan through each heading andtry to find the main idea. Logical

ASSESSEvaluate UnderstandingHave students create cards of vocabularyterms and definitions, and then work inpairs to match them.

ReteachUse Figure 7 to review the section’s keyconcepts. Ask students to describe thefrequency and wavelength of the waves.

Solutions 9. The speed is 1.2 m � 3.3 Hz �

4.0 m/s.10. The period is 1/2 Hz � 0.5 s.

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 17.2.

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Answer to . . .

Figure 7 The wave in A

5. The amplitude equals the distance from therest position to a crest or trough.6. The frequency is one half as great.7. Put a ribbon on the spring. Set up amechanical device to vibrate the spring backand forth in periodic motion. Measure thetime it takes for the ribbon to vibrate back andforth 20 times. Divide the measured time into20 cycles to calculate the frequency in Hz.Repeat the procedure for several trials andaverage the results.8. Doubling the frequency will halve thewavelength.


1. A wave’s frequency equals the frequency ofthe vibrating source. 2. Wavelength is inversely proportional tofrequency for waves moving at a given speed. 3. The more energy a wave has, the greater isits amplitude. 4. The wavelength could be found bymeasuring the distance between adjacentcompressions or adjacent rarefactions. Thewavelength also could be calculated bymeasuring the wave speed and frequency.

PPLS

Find links to additional activitiesand have students monitorphenomena that affect Earth and its residents.


508 Chapter 17

17.3 Behavior of Waves

Reading StrategyIdentifying Main Ideas Copy and expandthe table below. As you read, write the mainidea of each topic.

Key ConceptsHow does reflectionchange a wave?

What causes the refractionof a wave when it enters anew medium?

What factors affect theamount of diffraction ofa wave?

What are two types ofinterference?

What wavelengths willproduce a standing wave?

Vocabulary◆ reflection◆ refraction◆ diffraction◆ interference◆ constructive

interference◆ destructive

interference◆ standing wave◆ node◆ antinode

Have you ever noticed bright lines like those shown in Figure 8dancing on the bottom of a pool? These lines are produced when lightshines through waves on the surface of the water. The lines don’t seemto have a pattern because there are so many waves interacting. Imaginefollowing just one of these waves. What will happen when it strikes theside of the pool? When it encounters another wave or an obstacle likea person? As the waves crisscross back and forth, many interactions canoccur, including reflection, refraction, diffraction, and interference.

ReflectionThe next time you are in a pool, try to observe ripplesas they hit the side of the pool. Reflection occurswhen a wave bounces off a surface that it cannot passthrough. The reflection of a wave is like the reflec-tion of a ball thrown at a wall. The ball cannot gothrough the wall, so it bounces back.

If you send a transverse wave down a ropeattached to a wall, the wave reflects when it hits thewall. Reflection does not change the speed orfrequency of a wave, but the wave can be flippedupside down. If reflection occurs at a fixed bound-ary, then the reflected wave will be upside downcompared to the original wave.

Figure 8 The ripples visible onthe bottom of the pool arecaused by light shining throughsurface waves.

Reflection

Refraction

Interference

Standing waves

Topic Main Idea

b. ?

a. ?

d. ?

e. ?

Diffraction c. ?

508 Chapter 17

FOCUS

Objectives17.3.1 Describe how reflection,

refraction, diffraction, andinterference affect waves.

17.3.2 State a rule that explainsrefraction of a wave as it passesfrom one medium to another.

17.3.3 Identify factors that affect theamount of refraction, diffraction,or interference.

17.3.4 Distinguish betweenconstructive and destructiveinterference and explain howstanding waves form.

Build VocabularyConcept Map Have students build aconcept map using the terms in thissection. Students should write Behaviorof Waves in an oval at the top andconnect it with linking words to ovalscontaining vocabulary terms.

Reading Strategya. A wave reflected at a fixed boundary is inverted but has the same speed andfrequency. b. Refraction occurs becauseone side of a wave front moves moreslowly than the other side. c. The larger the wavelength, the more a wave diffracts. d. It can be constructiveor destructive. e. It forms only formultiples of one-half wavelength.

INSTRUCT

Reflection

Water-Wave ReflectionsPurpose Students will observe surfacewave reflections.

Materials clear bowl, water, overhead projector

Procedure Fill the bowl with waterand place it on the overhead projector.Make gentle waves with a finger.

Expected Outcome Surface wavescan be observed reflecting off the side ofthe bowl. Visual

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Section 17.3


Math Support, Section 17.3• Transparencies, Section 17.3

Technology• Interactive Textbook, Section 17.3• Presentation Pro CD-ROM, Section 17.3• Go Online, NSTA SciLinks, Diffraction and

interference

Section Resources

Figure 10 As an ocean waveapproaches the shore at an angle, thewave bends, or refracts, because oneside of each wave front slows downbefore the other side does.

Figure 9 A lawnmower turnswhen it is pushed at an anglefrom the grass onto the gravel.Relating Cause and EffectExplain why the lawnmowerstraightens out after both wheelsare on the gravel.

RefractionRefraction is the bending of a wave as it enters a newmedium at an angle. Imagine pushing a lawnmower fromgrass onto gravel, as shown in Figure 9. The direction ofthe lawnmower changes because one wheel enters thegravel before the other one does. The wheel on the gravelslows down, but the other wheel is still moving at a fasterspeed on the grass. The speed difference between the twowheels causes the lawnmower to change direction.Refraction changes the direction of a wave in much thesame way. When a wave enters a medium at anangle, refraction occurs because one side of the wavemoves more slowly than the other side.

Figure 10 shows the refraction of an ocean wave as itflows into a shallow area. The shallower water can be con-sidered a new medium. The lines on the photograph showthe changing direction of the wave. These lines, calledwave fronts, are parallel to the crests of the wave.

Notice that the wave fronts approach the shore at an angle. The leftside of each wave enters shallower water before the right side does. Asthe left side of the wave slows down, the wave bends toward the left.

If a wave front is parallel to the shoreline, the wave enters the shal-lower water all at once. The wave will slow down but it will not changedirection. Refraction of the wave occurs only when the two sides of awave travel at different speeds.

What is refraction?

GravelGrass

Directionchanges.

Left wheel isstill rollingfaster on grass.

Mower pivotsbecause the right wheel moves more slowly when it reaches gravel.


Build Reading LiteracyActive Comprehension Refer to page 498D in this chapter, whichprovides the guidelines for activecomprehension.

Read the introductory paragraph on p. 508. Ask, What more would you liketo know about how waves interact? orWhat about wave behavior interestsyou? You will need to make connectionsfor the students between waves and theirlives. For example, students may haveobserved ripples in a drink sitting on topof a loudspeaker. Write down several ofthe students’ responses. Have studentsread the section. While reading, havethem consider the questions that theyhad about the material. Have studentsdiscuss the section content, making sure that each question raised at thebeginning is answered or that studentsknow where to look for the answer. Verbal

Refraction

Some students may think that a wavealways changes direction as it passesthrough a boundary. Refraction is alwaysaccompanied by a change in wavelengthand speed. However, the direction of a wave does not always change. Forexample, although water wavesapproaching a beach perpendicular to the beach do not change direction,they do slow down as the wave frontsget closer together. Verbal, Visual

Build Science SkillsPosing Questions Have students lookat Figure 10. Ask, How could you testthe assertion that water waves movemore slowly in more shallow water?(Possible answer: Set up a wave tank andmeasure wave speed at different depths,making sure to hold all other variablesconstant.)Logical

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Customize for English Language Learners

Increase Word ExposureThis section contains several new andpotentially confusing vocabulary word pairs,such as reflection and refraction or constructiveand destructive. While students read the

section in class, post the vocabulary terms onthe board or wall. Carefully define each wordwhen it first appears. Then, ask for studenthelp in distinguishing between the similar-sounding words.

Answer to . . .

Figure 9 Once both wheels are onthe gravel, they move at the samespeed, so the lawnmower no longerchanges direction.

Refraction is the bendingof a wave as it enters a

new medium at an angle.


510 Chapter 17

DiffractionDiffraction (dih FRAK shun) is the bending of a wave as it movesaround an obstacle or passes through a narrow opening. Figure 11Ashows how water waves spread out as they pass through a narrowopening. The pattern produced is very similar to the circular ripplesyou see when a pebble is tossed into a pond. Diffraction also occurswhen waves bend around an obstacle, as shown in Figure 11B.

A wave diffracts more if its wavelength is large compared tothe size of an opening or obstacle. If the wavelength is small com-pared to the opening or obstacle, the wave bends very little. The largerthe wavelength is compared to the size to the opening or obstacle, themore the wave diffracts.

Interference If two balls collide, they cannot continue on their original paths as ifthey had never met. But waves can occupy the same region of spaceand then continue on. Interference occurs when two or more wavesoverlap and combine together. Two types of interference areconstructive interference and destructive interference. The displace-ments of waves combine to increase amplitude in constructiveinterference and to decrease amplitude in destructive interference.

What is diffraction?

Figure 11 A Mechanical waves,like the water waves shown here,diffract as they move past anobstacle or through an opening.A This wave diffracts, or spreadsout, after it passes through anarrow opening. B Diffractionalso occurs when a waveencounters an obstacle.

For: Links on diffraction andinterference


Web Code: ccn-2173

A B

510 Chapter 17

DiffractionUse VisualsFigure 11 Emphasize to students thatboth pictures are showing the samephenomena, diffraction. Point out thesimilarities between the two images.Ask, What would the diffractionpattern look like if one of the barrierswere removed in Figure 11A? (Thewaves would still spread out behind theother barrier. This would look like half ofthe diffraction pattern in Figure 11B.)Visual, Logical

Interference

Students sometimes make an analogybetween pulses and particles travelingtoward each other and assume thatwhen two pulses meet in the center of a long spring, they bounce or reflectas if they were solid objects. Use a long, soft spring and a jump rope todemonstrate two differently sized andshaped pulses approaching each otherso that students can see that they passthrough each other.Verbal

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The First Known Seismoscope Earthquakestravel through Earth as longitudinal waves,transverse waves, and surface waves. Surfacewaves are the most destructive and are thewaves that people feel during an earthquake.The world’s first earthquake wave detector wasinvented in 132 A.D. by Zhang Heng, a scientistin the Han Dynasty in China. It was sensitiveenough to detect small surface waves. The

device had eight dragons arranged in a circle.Each dragon’s mouth held a brass ball. Whenan earthquake wave passed, a brass ball wouldfall, indicating the direction of the wave. Oneday a ball fell, indicating that an earthquakehad occurred, although no one had felt anearthquake. A few days later, couriers arrived,reporting an earthquake in Lung-Hsi, about 640 km away.

Facts and Figures

Download a worksheet ondiffraction and interference forstudents to complete, and findadditional teacher support fromNSTA SciLinks.

Figure 12 Two waves with equalfrequencies travel in oppositedirections. The motions aregraphed here to make it easier tosee how the waves combine. A When a crest meets a crest, theresult is a wave with an increasedamplitude. B When a crest meetsa trough, the result is a wave witha reduced amplitude. Making Generalizations Howis the amplitude of wave 3related to the amplitudes ofwaves 1 and 2?

Constructive Interference Imagine a child being pushed ona swing by her mother. If the mother times her pushes correctly, shewill push on the swing just as the child starts to move forward. Thenthe mother’s effort is maximized and the child gets a boost to gohigher. In the same way, the amplitudes of two waves can add together.Constructive interference occurs when two or more waves combineto produce a wave with a larger displacement.

What happens if you and a friend send waves with equal frequen-cies toward each other on a jump rope? Figure 12A shows howconstructive interference produces a wave with an increased amplitude.The crests of waves 1 and 2 combine to make a higher crest in wave 3.At the point where two troughs meet, wave 3 has a lower trough.

Destructive Interference What happens if the mother hasbad timing while pushing on the swing? Instead of working to boosther daughter upward, some of her effort is wasted, and the girl will notswing as high. In much the same way, destructive interference canreduce the amplitude of a wave. Destructive interference occurswhen two or more waves combine to produce a wave with a smallerdisplacements.

In Figure 12B, two waves with the same frequency meet, but thistime the crest of wave 1 meets the trough of wave 2. The resulting wave3 has a crest at this point, but it is lower than the crest of wave 1.Destructive interference produces a wave with a reduced amplitude.


Wave 1

Destructive Interference

210

�1�2

� Wave 210

�1

� Wave 3

3210

�1�2�3

Wave 1

Constructive Interference

210

�1�2

� Wave 210

�1

� Wave 3

3210

�1�2�3

A B

For: Activity on interference

Visit: PHSchool.com

Web Code: ccp-2173

Use VisualsFigure 12 Have students examine thefigure carefully. Note that the two wavesare moving in opposite directions. InFigure 12A, the crests line up with crestsand the troughs line up with troughs. In Figure 12B, the crests line up withtroughs and the troughs with crests. Ask, In Figure 12A, what would wave 3look like if wave 1 and wave 2 had thesame amplitude? (The amplitude of wave 3 would be twice that of wave 1 or 2.)In Figure 12B, what would wave 3 looklike if wave 1 and wave 2 had the sameamplitude? (Wave 3 would have zeroamplitude.) Point out to students thateven in the case of two waves with thesame amplitude, the waves do notentirely cancel. After they pass througheach other, they continue on in theiroriginal form.Visual, Logical

Build Science SkillsUsing Tables and Graphs Givestudents dimensions for the horizontalaxis of the grid in Figure 12, such as eachincrement equals 10 cm. Ask students todetermine the wavelength of waves 1and 2 in Figures 12A and 12B. (Thewavelength of each of the waves equals 60 cm.) Ask, Do waves 1 and 2 in Figure 12B have the same wavelength?(Yes) If each graph grid represents atime period of 10 seconds, what is theperiod and frequency of wave 1 inFigure 12A? (Period � 6 s; frequency �1/6 Hz)

Logical

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Answer to . . .

Figure 12 The amplitude of wave 3equals the sum of the amplitudes ofwaves 1 and 2 for constructive interfer-ence. For destructive interference, it’s thedifference of the amplitudes.

Diffraction is thebending of a wave

when it encounters an obstacle or a narrow opening.

IPLS

For: Activity on interferenceVisit: PHSchool.comWeb Code: ccp-2173

Students can interact withsimulations of constructive anddestructive interference online.


512 Chapter 17


Reviewing Concepts1. How is a wave changed by reflection?

2. What causes refraction when a waveenters a medium at an angle?

3. What determines how much a wavediffracts when it encounters an openingor an obstacle?

4. List the types of interference.

5. At what wavelengths can a standingwave form in an elastic cord?

Critical Thinking6. Comparing and Contrasting How does

the frequency of a reflected wave compare tothe frequency of the incoming wave?

7. Comparing and Contrasting How arediffraction and refraction similar? How arethey different?

8. Applying Concepts What is the amplitudeof the wave that results when two identicalwaves interfere constructively?

Standing WavesIf you tie one end of a rope to a chair and shake the other end,waves travel up the rope, reflect off the chair, and travel backdown the rope. Interference occurs as the incoming wavespass through the reflected waves.At certain frequencies, inter-ference between a wave and its reflection can produce astanding wave. A standing wave is a wave that appears to stayin one place—it does not seem to move through the medium.

You can observe a standing wave if you pluck a guitarstring or any elastic cord. Only certain points on the wave,called nodes, are stationary. A node is a point on a standingwave that has no displacement from the rest position. At thenodes, there is complete destructive interference betweenthe incoming and reflected waves. An antinode is a pointwhere a crest or trough occurs midway between two nodes.

Why does a standing wave happen only at particularfrequencies? A standing wave forms only if half awavelength or a multiple of half a wavelength fits exactlyinto the length of a vibrating cord. In Figure 13A, the

wavelength equals the length of the cord. In Figure 13B, the wave-length is halved. You can adjust the wavelength by changing thefrequency of the waves. Once you find a frequency that produces astanding wave, doubling or tripling the frequency will also produce astanding wave.

Figure 13 These photos showstanding waves for two differentfrequencies. A One wavelengthequals the length of the cord. B Two wavelengths equal thelength of the cord. Interpreting Photos In whichphoto do the waves have alonger wavelength?

Antinode Antinode

Node

One wavelength

AntinodeNode

Two wavelengths

Explain a Sequence Imagine you are float-ing in a wave pool. The crest of one wave hitsyou from the left just as the crest of anotherhits you from the right. The two waves areotherwise identical. A friend takes a series offive photos starting when the crests hit you.Write a paragraph describing the photos.

A

B

512 Chapter 17

Standing Waves

Standing WavesPurpose Students will observedifferent standing waves.

Materials long, soft, heavy rope, suchas a jump rope


Procedure Tie one end of the rope to a chair or other firm support. Holdthe other end of the rope so that it issuspended in air. Start with the ropehanging in an arc. Make standing waves by shaking the rope at differentfrequencies. Ask students to estimate the wavelength as you increase thefrequency and produce more nodes. (As the frequency increases, thewavelength decreases.)

Expected Outcome The length of therope must be an integral number of halfwavelengths for a standing wave to occur.Kinesthetic, Visual

ASSESSEvaluate UnderstandingHave students write three review questionsfor this section.

ReteachUse Figures 9–13 as examples thatillustrate the key concepts of the section.

Students will need to first decide how thetime between photographs compareswith the period of the wave. If the fivephotographs occur during one half periodof the wave, the sequence will show theperson starting at a maximum crest andmoving downward to a minimum trough.No circular motion should result becausethe lateral motions caused by the twowaves should cancel.

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 17.3.

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4. Destructive, constructive5. A standing wave forms only if half a wave-length or a multiple of half a wavelength fitsexactly into the length of the vibrating object.6. The frequencies are equal. Reflection doesnot change the frequency of a wave.7. Diffraction and refraction both involve thebending of waves. Refraction occurs when awave enters a new medium at an angle, whilediffraction occurs when a wave encounters anobstacle or a narrow opening. 8. The amplitude of the resulting wave is doublethe amplitude of the two interfering waves.


1. The reflected wave is upside down comparedto the original wave.2. Refraction occurs because the entire wavedoes not enter the new medium at the sametime. One side of a wave front entering the newmedium undergoes a speed change before therest of the wave front does, resulting in thechange in direction.3. The larger the wavelength of a wavecompared to the size of the obstacle oropening, the more the wave diffracts.

Answer to . . .

Figure 13 The upper photograph haswaves with a longer wavelength.

Are Regulations Needed toProtect Whales from NoisePollution?

Regulations Are Needed to ReduceNoise Pollution From Large Ships Whales use their songs in ways that affect theirsurvival—eating, mating, and avoiding predators.Studies often focus on the effects of noise from asingle ship, but in routes taken by ocean freighters,noise from many ships combines to produce ahigher volume. Ocean freighters often travel nearwhale migration routes, so even noise that affectswhales at a distance of 20 kilometers may have animpact on whale survival. If regulations are delayeduntil research can prove that noise pollution affectswhales, it may be too late to help the whales. Manykinds of whales are on the endangered species list,so it is important to err on the side of safety.

Regulations Are Not Needed toReduce Noise Pollution FromLarge Ships Whale songs can be lengthy and are oftenrepeated, so the effect of noise from ships is limitedbecause ships quickly move out of an area. Onestudy showed that whales changed the rhythm andtempo of their songs in response to noise fromlarge ships, but there was no evidence that thecommunication was less effective. Also, it isexpensive to modify ship propellers to reduce low-frequency noise. If less-developed countries cannotafford to modify ships, regulations will not beeffective in reducing ocean noise levels.

1. Defining the Issue In your own words,describe the major issue that needs to beresolved about ocean noise pollution.

2. Analyzing the Viewpoints List threearguments for those who think regulationsshould require large ships to reduce noisepollution. List three arguments for those whothink regulations are not necessary.

3. Forming Your Opinion Explain whichargument you find most convincing.

Researchers have known for decades that humpback whales singcomplicated songs. Their songs can be as long as 30 minutes, and awhale may repeat the song for two or more hours. Songs can be heard atdistances of hundreds of kilometers. There is evidence that whales usevariations in the songs to tell other whales about the location of food andpredators. Only the male humpbacks sing, which has led some researchersto think that songs are also used to attract a mate.

The whale songs may be threatened by noise pollution. In the past50 years, ocean noise has increased due to human activity. Goods aretransported across the ocean in larger ships than ever before. Large shipsuse bigger engines. They produce low-frequency noise by stirring up airbubbles with their propellers. Unfortunately, whales also use low-frequencysound in their songs, perhaps because these sounds carry farther than high-frequency sounds in the ocean. Propeller noise from large ships is loudenough to interfere with whale songs at a distance of 20 kilometers.

The Viewpoints

Research and Decide

For: More on this issue

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Are Regulations Needed to Protect Whales fromNoise Pollution?BackgroundIn the United States, whales fall under theMarine Mammal Protection Act of 1972(MMPA). This act makes it illegal for anyperson residing in the United States tokill, hunt, injure, or harass any species ofmarine mammal. The act includes noisepollution. Ocean noise may result frommilitary testing, sonar, assembling anddismantling of drilling rigs, seismic test-ing, marine commerce, and proposedexperiments using 195-dB pulses tomeasure the temper-ature in the NorthAtlantic. Low-frequency sonar produces235-dB pulses. Oil tankers continuouslyproduce low-frequency sounds of 177 dBat 500 Hz in all the shipping lanes of theworld. Seismic oil-exploration pulses are210 dB. For comparison, the call of a graywhale is 185 dB.

Since light does not penetrate very far inocean water, whales use sound to findtheir way around, locate food, andunderstand their environment. Soundtravels five times faster in water than inair and is transmitted more efficiently.Some sounds can travel for hundreds ofkilometers.

Whales are thought to hear in the rangeof 40 Hz to 150 kHz, depending on thespecies. However, the upper and lowerlimits are inferred for many whales.Research into large whale hearing islimited by the large size and behaviorsof these animals.

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Answers1. The issue is: Should regulations be passed tolimit noise from large ships? 2. For Regulation: Whales depend on communi-cation for breeding and locating food resources.Noise that affects whales at a short distance maystill have an impact on behavior. Because manyspecies are endangered, it is wise to err on the safeside of the issue. Against Regulation: Whale songs

can be lengthy and are often repeated, so thereare several opportunities for a message to getthrough. Whales may modify songs in response tonoise pollution, but there’s no evidence it makescommunication less effective. Reducing noise oflarge ships may not be feasible in less developedcountries, which makes regulations ineffective. 3. Students should support their decision byreferring to the arguments in Question 2.

Have students further research theissues related to this topic.


514 Chapter 17

17.4 Sound and Hearing

Reading StrategyUsing Prior Knowledge Copy the webdiagram below. Before you read, addproperties you already know about. Then adddetails about each property as you read.

Key ConceptsWhat properties explainthe behavior of sound?

How is ultrasound used?

How does frequency ofsound change for amoving source?

What are the functions ofthe three main regions ofthe ear?

How is sound recorded?

How do musicalinstruments vary pitch?

Vocabulary◆ sound waves◆ intensity◆ decibel◆ loudness◆ pitch◆ sonar◆ Doppler effect◆ resonance

Take a moment to listen. Even in a quiet room you can usually hearmany different sounds. You might hear someone opening a book,people talking in the hall, cars and trucks driving outside, and maybeeven an airplane flying overhead. You can identify sounds withoutseeing them because sound waves carry information to your ears.

Properties of Sound WavesSound waves are longitudinal waves—compressions and rarefactionsthat travel through a medium. Have you ever stopped to question whysounds can hurt your ears? Why there is a delay before you hear anecho down a long, empty hallway at school? Many behaviors ofsound can be explained using a few properties—speed, intensity andloudness, and frequency and pitch.

Speed Why is there a delay when you hear an echo? It takes time forsound to travel from place to place. In dry air at 20°C, the speed ofsound is 342 meters per second. That’s more than ten times faster thanyour speed in a car on a highway!

Figure 14 shows how the speed of sound varies in different media.In general, sound waves travel fastest in solids, slower in liquids, andslowest in gases. This is partly due to the fact that particles in a solidtend to be closer together than particles in a liquid or a gas. The speedof sound depends on many factors, including the density of themedium and how elastic the medium is.

Medium (at 1 atm)

Dry air, 0�C

Dry air, 20�C

Fresh water, 0�C

Fresh water, 30�C

Salt water, 0�C

Salt water, 30�C

Lead, 25�C

Cast iron, 25�C

Aluminum, 25�C

Borosilicate glass, 25�C

Speed (m/s)

331

342

1401

1509

1449

1546

1210

4480

5000

5170

Speed of Sound

b. ?

Properties ofSound Waves

Speed

a. ?

Figure 14 The speed of soundis shown here for a varietyof materials.Making Generalizations Howdoes temperature affect thespeed of sound?

514 Chapter 17

FOCUS

Objectives17.4.1 Describe the properties of

sound waves and explainhow sound is produced andreproduced.

17.4.2 Describe how sound wavesbehave in applications such asultrasound and music.

17.4.3 Explain how relative motiondetermines the frequency ofsound an observer hears.

17.4.4 Analyze the functions of themain regions of the human ear.

Build VocabularyParaphrase Some terms in this section,such as intensity, decibel, and resonance,may be unfamiliar to students. Havestudents create a definition in their ownwords for these or other difficult words.

Reading Strategya. Intensity and loudness b. Frequencyand pitch

INSTRUCT

Properties of Sound WavesUse Community ResourcesInvite members of the school band or orchestra or a community band or orchestra to demonstrate musicalinstruments for the class. Allow studentsto pose questions to the musicians, suchas “How is the pitch of your instrumentchanged?” or “How do you increase the volume of sound coming from your instrument?”Musical, Visual

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Reading Focus

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Section 17.4

Print• Laboratory Manual, Investigations 17A

and 17B• Reading and Study Workbook With

Math Support, Section 17.4• Transparencies, Section 17.4

Technology• Interactive Textbook, Section 17.4• Presentation Pro CD-ROM, Section 17.4• Go Online, Science News, Sound

Section Resources

Intensity and Loudness Intensity isthe rate at which a wave’s energy flows througha given area. Sound intensity depends on boththe wave’s amplitude and the distance from thesound source. If someone whispers in your ear,the sound intensity may be greater than whensomeone shouts at you from the other end ofa field.

Sound intensity levels are measured inunits called decibels. The decibel (dB) is a unitthat compares the intensity of different sounds.The decibel scale is based on powers of ten. Forevery 10-decibel increase, the sound intensityincreases tenfold. Figure 15 shows the intensitylevels of some common sounds. A 0-decibelsound can just barely be heard. A 20-decibel sound has 100 times moreenergy per second than a 0-decibel sound. A 30-decibel sound delivers1000 times more energy per second than a 0-decibel sound.

Unlike intensity, loudness is subjective—it is subject to a person’sinterpretation. Loudness is a physical response to the intensity ofsound, modified by physical factors. The loudness you hear depends,of course, on sound intensity. As intensity increases, loudness increases.But loudness also depends on factors such as the health of your earsand how your brain interprets the information in sound waves.

Frequency and Pitch Try plucking a stretched rubber band.Then, stretch the rubber band farther and pluck again. You should beable to see the vibration become faster as you hear the sound frequencybecome higher. The frequency of a sound wave depends on how fastthe source of the sound is vibrating.

The size of a musical instrument tells you something about the fre-quencies it can produce. The trumpet in Figure 16 can produce higherfrequencies than the French horn. Both instruments produce differ-ent frequencies by changing the length of tubing through which airmoves. The air in the tubing forms a standing wave. The longer thetubing, the longer is the wavelength of the standing wave, and the loweris the frequency of the note produced.

Pitch is the frequency of a sound as you perceive it. Pitch doesdepend upon a wave’s frequency. High-frequency sounds have a highpitch, and low-frequency sounds have a low pitch. But pitch, likeloudness, also depends on other factors such as your age and thehealth of your ears.

What is loudness?

Sound

Threshold of human hearing

Whisper

Normal conversation

Street noise

Inside a bus

Operating heavy machinery

Rock concert (in audience)

Threshold of pain

Jet plane (taking off)

Intensity Level (decibels)

Sound Intensity Level

0

15–20

40–50

60–70

90–100

80–120

110–120

120

120–160

Figure 15 Lengthy exposure tosounds more intense than 90 decibels can cause hearingdamage. Analyzing Data Whichsounds are potentially dangerous?

Figure 16 The French horn canproduce lower notes than thetrumpet because it can make alonger tube for a standing wave.


Trumpet

French Horn


Hearing Impaired Depending on their degree of hearing loss,hearing-impaired students may have somedifficulty with many of the concepts in thissection. Since sound originates from vibratingobjects, touch can substitute for hearing insome cases. Allow students to feel the

vibrations of musical instruments as they arebeing played. Encourage them to describedifferences in what they feel as high and lowpitches are played. Students can also “see”vibrations if you strike a tuning fork, thenquickly place the end of it in a pan of water.The ripples in the water show the vibrations.

Customize for Inclusion Students

Build Science Skills

Observing

Purpose Students use tuning forks to model vibrations and sound waves.

Materials set of tuning forks of similarconstruction


Procedure Instruct students in theproper way to strike the tuning forks.Emphasize that hitting the forks on a deskor other hard surface can damage themand also make it harder to hear the tone.Have students strike several differenttuning forks and compare their pitches.Ask, What is the relationship betweenthe length of the tuning fork and itspitch? (The longer tuning forks producelower pitches.) Have students place thebase of a vibrating tuning fork against a surface that can act as a soundboard,such as a window pane, desk, or thinboard. Ask, What happened when the vibrating tuning fork was placedagainst the soundboard? (The sound got louder.)

Expected Outcome Students willobserve that tuning forks of differentlengths produce different pitches andthat soundboards amplify sound.Visual, Kinesthetic

L2

Answer to . . .

Figure 14 From the data given, the speed of sound increases withincreasing temperature.

Figure 15 Riding in a bus, operatingheavy machinery, attending a rockconcert, and listening to a jet taking offare sounds that may damage hearing.

Loudness is a physicalresponse to the intensity

of sound, modified by physical factors.


516 Chapter 17

UltrasoundMost people hear sounds between 20 hertz and 20,000 hertz.Infrasound is sound at frequencies lower than most peoplecan hear, and ultrasound is sound at frequencies higher thanmost people hear. Ultrasound is used in a variety ofapplications, including sonar and ultrasound imaging.

Sonar is a technique for determining the distance to anobject under water. Sonar stands for sound navigation andranging. The distance to the object is calculated using thespeed of sound in water and the time that the sound wavetakes to reach an object and the echo takes to return.

Ultrasound imaging is an important medical technique. Figure 17shows an image of the heart made by sending ultrasound pulses intoa patient. A pulse is a very short burst of a wave. Each ultrasound pulseis short—about of a second—so that it doesn’t interfere with thereflected pulse. Computer software uses the reflected pulses to make adetailed map of structures and organs inside the body.

The Doppler EffectPerhaps you have heard the pitch of a siren change as it passed you.This is the Doppler effect—a change in sound frequency caused bymotion of the sound source, motion of the listener, or both. TheDoppler effect was discovered by the Austrian scientist ChristianDoppler (1803–1853).

As a source of sound approaches, an observer hears a higherfrequency. When the sound source moves away, the observer hears alower frequency. Figure 18 shows a single frequency emitted by theambulance siren. As the ambulance moves toward observer B, the wavefronts bunch together. Observer B hears a higher frequency than thefrequency of the source. For observer A, however, the wave fronts arespread out, and the frequency is lower than the source frequency.

18000

Figure 17 Ultrasound can be usedto make images of the heart, whichhelp doctors diagnose disease.

Observer A(decreasedfrequency)

Observer B(increasedfrequency)

Figure 18 Observer A hears alower-pitch sound than observerB because the wave fronts arefarther apart for observer A.Inferring What can you inferabout the pitch the ambulancedriver hears?

For:Activity on sonar and marine mammals

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Web Code: ccc-2174

516 Chapter 17

UltrasoundBuild Reading LiteracySQ3R Refer to page 530D in Chapter 18, which provides theguidelines for SQ3R (Study, Question,Read, Recite, Review).

Teach this independent study skill as awhole-class exercise. Direct students tosurvey Section 17.4 and list the headings:Speed, Intensity and Loudness, Fre-quency, Pitch, and so on. As they survey,have students write one question for eachheading, such as “What is one commonapplication for ultrasound?” Then, havestudents write answers to the questionsas they read the section. After reading,have students recite the questions andanswers, explaining that vocalizing inyour own words helps you retain whatyou learned. Auditory, Group

The Doppler EffectUse VisualsFigure 18 Point out that the ambulancewas in a different place when eachcircular wave was emitted. Reproducethis diagram on the board and numberthe waves 1 through 6, starting with thelargest wave. Ask, In the figure, wherewas the ambulance when wave 1 wasemitted? (At the center of that wave)Where was the ambulance when theother waves were emitted? (At thecenter of each wave, moving to the right)Visual, Logical

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Diffraction of Sound All sound waves arediffracted when they pass by an obstacle orthrough an opening. The human voice has arange of approximately 70 to 400 Hz. This cor-responds to wavelengths between 4.9 m and0.86 m. Recall that the amount of diffraction

depends on the wavelength of the wavecompared to the size of the opening or obstacle.Because sound waves have wavelengths approx-imately the same size as the width of a doorwayor a window opening, the sound of a person’svoice easily diffracts from room to room.

Facts and Figures

Find links to additional activitiesand have students monitorphenomena that affect Earth and its residents.


Hearing and the EarCan you feel sound waves with your hand at this very moment? Probablyyou can’t. But suppose you hold a balloon. Then your hand can feelsounds because the balloon membrane vibrates. Just like the balloon,your ear has a membrane that vibrates when sound waves strike it.

Your ear is a complex system that consists of three main regions—the outer ear, the middle ear, and the inner ear—as shown in Figure 19.

The outer ear gathers and focuses sound into the middle ear,which receives and amplifies the vibrations. The inner ear uses nerveendings to sense vibrations and send signals to the brain.

For: Articles on sound

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Web Code: cce-2174

Hammer

Anvil

Stirrup

Auditorynerve

Cochlea

Eardrum

Earcanal

OuterEar

MiddleEar

InnerEar

Figure 19

The Anatomy of the Ear

Outer Ear The part of the earyou can see funnels sound wavesdown the ear canal, a tunnelabout 2.5 cm long. There, soundwaves strike the eardrum, atightly stretched membranebetween the outer and middleear. The eardrum vibrates at thesame frequency as the soundwaves striking it.

Middle Ear The middle earcontains three tiny bones—thehammer, the anvil, and the stirrup.When the eardrum vibrates, thehammer vibrates at the samefrequency. The hammer strikes theanvil, which in turn moves thestirrup back and forth. The threebones act as a lever system toamplify the motion of the eardrum.

Inner Ear Vibrations from thestirrup travel into the cochlea, aspiral-shaped canal filled withfluid. The inside of the cochlea islined with thousands of nerve cellswith tiny hair-like projections. Asthe fluid in the cochlea vibrates,the projections sway back andforth and send electrical impulsesto the brain.

Hearing and the EarUse VisualsFigure 19 This figure contains a lot ofinformation and is worth some extratime. Point out important features of thediagram for students. The outer ear ismade up of the visible, sound-collectingpart (the pinna) and the ear canal. Thepinna gathers and reflects sound wavesdown the canal. Ask, How do soundsget to the eardrum? (Through the air inthe ear canal) What is the stirrup? (Oneof the small bones in the middle ear)What does the stirrup do? (It transfersvibrations into the cochlea.) What is thefunction of the auditory nerves? (Totransmit nerve impulses to the brain)Visual, Logical

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Answer to . . .

Figure 18 The frequency for thedriver is unchanged because the driverhas no motion relative to the siren.

Science News provides studentswith current information on sound.


518 Chapter 17

How Sound Is ReproducedSound has been reproduced in many ways, from old-fashioned recordsto modern digital technologies. But no matter how sound is recordedor stored, in the end it must be converted back into sound waves byloudspeakers. Sound is recorded by converting sound wavesinto electronic signals that can be processed and stored. Sound isreproduced by converting electronic signals back into sound waves.

A modern speaker produces sound waves in much the same waythat a drum does. A drum skin vibrates up and down like a trampoline.As the drum vibrates, it sends a series of compressions and rarefac-tions through the air across the room. They carry energy to your earsin the form of sound waves.

1900

1885 The wax-coated cylinderis introduced asan improvementon Edison’s firstdesign.

Sound RecordingRecording technology has come a longway since Thomas Edison made the firstsound recording of the human voice athis New Jersey laboratory in 1877.

1900 Danishinventor ValdemarPoulsen unveils amagnetic recordingdevice, called theTelegraphone.

Mouthpiece Themouthpiece recordssounds as scratches onthe tinfoil.

PHONOGRAPH

Handcrank

Metal cylinderThis is wrapped

with a layer of tinfoil.

1887 German-bornAmerican EmileBerliner invents theGramophone, whichstores sounds asgrooves in the waxsurface of a flat disc.

GRAMOPHONE

Funnel Thefunnel is usedfor recordingand playback.

Steelneedle

Horn The hornchannels soundsfrom an irondiaphragm.

REEL-TO-REELRECORDER

As wire recording gives way tomagnetic tape, portable reel-to-reel recorders, like this 1950smodel, become very popular.

1877Americaninventor ThomasEdison recordshis voice on atinfoil cylinderphonograph.

1875

1928 Tape with amagnetic coating isdeveloped inGermany. Furtherrefinements and a 1930s taperecorder called theMagnetophonemake reel-to-reelrecording popular.

1925

518 Chapter 17

How Sound Is Reproduced

Students sometimes think that soundwaves can push a dust particle awayfrom a speaker or blow out a candleflame placed in front of a speaker.However, sound waves would actuallymake the dust particle or candle flamevibrate back and forth in a directionparallel to the direction in which thewave is moving.Verbal

Sound RecordingEdison, who was slightly deaf, discov-ered he could feel the vibrations in atelephone speaker with his finger. If aloudspeaker is available, have studentsgently touch the surface of the speakergrille as music is being played. Theyshould easily be able to detect thevibrations. Ask, What is causing thevibrations? (Sound waves striking thegrille) Students can also hold a book in front of their face, sing a note orspeak loudly, and feel the vibrations in the book. Ask, What is causing thevibrations of the book? (Sound waves in air striking the book) Edison alsonoticed that a paper telegraph tape,when pulled through a telegraphmachine at high speed, made soundsthat resembled a human voice. He putthese two ideas together and designedthe first phonograph.

Over the years since, inventors haveconstructed many other types of record-ing devices. Today, there are severaldifferent digital audio compressiontechnologies available in addition toMP3. Only time will tell if one becomesthe preferred standard.Logical, Kinesthetic

Students should compare and contrastthe new technology with a previoustechnology. To be persuasive, their writingmust use facts to support benefits claimedfor the new technology.Logical

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Sound Reproduction The first audiorecordings were made by purely mechanicalaction. The singer or instrumentalist sang orplayed into a funnel-shaped tube that leddirectly to a cork diaphragm. The diaphragmwas attached through a series of levers to aneedle that would carve a groove into a waxcylinder or disk. Since there was no way toduplicate a recording, the orchestra or

performer would be required to play thecomposition over and over for hours as up to 10 individual recordings were made at a time.Ensembles of musicians made recordings in asmall room. One wall of the room was theopening of the huge recording tube. In aninteresting historical twist, several contemporarymusicians have started to use this oldtechnology to record their compositions.

Facts and Figures

In a speaker, an electronic signal causes a magnet to vibrate. Themagnet is attached to a membrane. The vibrating membrane sendssound waves through the air. Larger-diameter speakers, like a large bassdrum, are better at reproducing lower frequencies of sound. Smaller-diameter speakers, like a small bongo drum, are better for reproducinghigher frequencies of sound.

When a singer sings into a microphone, the reverse process hap-pens. Sound waves from the singer’s voice vibrate a membrane insidethe microphone. The membrane causes a magnet to vibrate, which pro-duces an electronic signal in the microphone wires. The energy ofsound waves has been converted into an electronic signal that can beprocessed and stored.


1950

1962 Philips, based in theNetherlands, demonstrates itsfirst compact audio cassette. It issold the following year withdictation machines. A stereoversion is introduced in 1967.

1948 Columbiaintroduces thefirst 12-inch vinylrecords. Relativelycheap to make,these allow for alonger playingtime.

1982 The firstdigital audio compact discs (CDs)are sold in Japan.

VINYL LONG PLAYING

CASSETTE TAPERECORDER

1975

1998 MP3 technologybecomes popular. Thisformat allows music to becompressed, stored, andtransferred digitally overcomputer networks.

2000

MP3PLAYER

Audio cassettes, acompact version ofreel-to-reel magnetictapes, are soonin demand.

Writing to PersuadeResearch one type of recordingtechnology. Write a productreview as if you lived at thetime it was invented. Persuadepeople that this technology ismuch better than previouslyavailable technologies.

Integrate Social StudiesThomas Edison is generally credited as the inventor of the phonograph, which he publicly demonstrated in 1877. He did not pursue the invention at first.Alexander Graham Bell, his cousin Chester Bell, and C. S. Tainter madeimprovements to Edison’s design toimprove the sound quality. They calledtheir device the graphophone. The Bellsand Tainter offered to combine theirpatents and designs with Edison’s work,but Edison rejected the offer. Instead, hedeveloped an improved machine similarto the graphophone. Edison’s new systemused thicker, reusable wax cylinders and ajewel-tipped recording stylus that did notneed to be replaced after every recording.Emile Berliner further improved thephonograph by using a flat disk and asystem for duplicating records from a steelmaster. These various phonographs werevery popular, and after 1895, hundreds ofcompanies appeared, yet quickly failed.Only a few companies survived into thetwentieth century. Have students researchlater technological developments thataffected phonographs and record players.Logical

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Bass stringsLong, thickstrings producelower pitches.The bassstrings crossover the mid-range stringsto use spaceefficiently.

Treble stringsShort, thinstrings producehigher pitches.

Cast iron frame This rigid frame is strong enough

to allow huge force to beexerted on the strings.

Key

Damper

Pins for tuning

Soundboard Made froma large, thin piece ofwood (usually spruce),the soundboardresonates and amplifiesthe sound from thevibrating wires.

Grand pianoPianos can be upright or, like thisgrand, horizontal. A grand pianomakes a rich and powerful sound.

Bridge Each string passes over acurved piece of wood called a

bridge, which transmits the string’svibrations to the soundboard.

Bassbridge

Felt-tipped hammer

Interconnectedlevers

Damper lever

Damper Wire string

Keyboard actionAs a key is pressed down,the section beyond thepivot moves up. Thisactivates a series of leversthat flick a hammerupward to strike thestring and make it vibrate.The damper is released,which allows the string tocontinue vibrating. Pivot

The PianoThe piano is a highly versatile musical instrument.Usually there are 88 separate keys, which producea range of notes and volume. The sounds aremade by wire strings that vibrate against asoundboard inside the piano. Interpreting Photos What causes the strings in a piano to vibrate?

Key

520 Chapter 17

520 Chapter 17

The Piano Several keyboard instruments predatedthe piano. The most popular was theharpsichord. When a key is pressed on aharpsichord, a corresponding string isplucked. Consequently, it is difficult tocontrol the loudness of the instrument. Ina piano, however, the string is struck by afelt-covered hammer. Pressing the keywith more force causes the string to bestruck harder, producing a louder sound.Because of this, these instruments werecalled fortepianos (literally “loud-soft”)for their ability to change dynamics. Asthe instrument evolved, the name wasshortened to piano.

When a piano key is pressed, twothings happen. The hammer strikes thestring and rebounds, while the damperpad moves away. Therefore, the stringwill continue to vibrate as long as thekey is held down.

The use of cast-iron frames allowsheavier strings to be strung with greatertension. Greater tension in the stringproduces a better, cleaner sound andallows the string to vibrate much longer.

Interpreting Photos When a pianokey is struck, a series of levers causes the hammer to strike a string. Thevibration of the string is amplified by the soundboard.Visual, Musical

For EnrichmentThe piano and its family of instrumentsprobably originated with the hammereddulcimer, which is played by striking thestrings with small “hammers.” Interestedstudents can research the origin ofplucked and struck stringed instrumentssuch as dulcimers and harps. Verbal, Portfolio

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Reviewing Concepts1. List five properties used to explain the

behavior of sound waves.

2. Name two uses for ultrasound.

3. What is the Doppler effect?

4. What are the ear’s three main regions?Describe the function of each region.

5. How is sound recorded?

6. How does a musical instrument producenotes at different pitches?

Critical Thinking7. Applying Concepts If workers in a distant

stone quarry are blasting, why can you feelthe explosion in your feet before you hear it?

8. Comparing and Contrasting How doesthe intensity of a 40-decibel sound compare to the intensity of a 20-decibel sound?

9. Applying Concepts How could a bat usereflections of sound waves to determinedistance to an insect?

MusicMusical instruments can produce a wide variety of sounds. Ina wind instrument, such as a flute or trumpet, holes are closedusing fingers or valves. This changes the length of the columnof air in which a standing sound wave is produced. For somestringed instruments, such as a violin, musicians change thelength of the strings by pressing down with their fingers.For other instruments, such as a piano, they use a fixed set ofstrings of different lengths. Most musical instruments varypitch by changing the frequency of standing waves.

Musical instruments often use resonance to amplifysound. Resonance (REZ uh nuhns) is the response of a stand-ing wave to another wave of the same frequency. Think of achild being pushed on a swing. If the pushes are timed at theright frequency, the child can swing higher and higher. In thesame way, one wave can “push” another wave to a higheramplitude. Resonance can produce a dramatic increase in amplitude.A piano, for example, amplifies sound with a soundboard. The sound-board resonates in response to the vibrating strings.

Once sound waves leave an instrument, they can take several routesto a listener. In a large concert hall, interference with reflected soundwaves can be a problem. Theaters such as the one in Figure 20 aredesigned with reflecting panels and sound-absorbing tiles. These arelocated with great care to prevent “dead spots” where the volume isreduced by destructive interference of reflected sound waves.


Frames of Reference Review what youlearned about combining velocities inSection 11.2. Then explain why the Dopplereffect depends on the velocity of the soundsource in the observer’s frame of reference.

Figure 20 The Central MichiganUniversity Music Building, likemany concert halls, was designedby acoustic engineers. Sound-absorbing tiles (on the sidesand rear) reduce unwantedreflections. The curved reflectingpanels above the stage helpgather and direct sound wavestoward the audience.

MusicIntegrate Language ArtsThe folk musician Greg Brown has setseveral of William Blake’s poems tomusic. Many other performers havefound musical inspiration in poetry.Choose a recording, such as GregBrown’s setting of Blake’s poem “TheChimney Sweeper,” and allow studentsto listen. Ask, How did the performeruse rhythm or pitch to enhance themeaning of the poem? (Possibleanswers: A simple melody can heightenthe drama of powerful images, such asthose in “The Chimney Sweeper.” Therhythm of the music can enhance thesense of rhythm in the poem.)Verbal, Musical

ASSESSEvaluate UnderstandingHave students outline and summarizethe section. Then, divide the class intogroups and have students exchangepapers and edit each other’s outlines.Exchange the papers again, and havestudents edit the edits. Then, return thepapers to the original owners to rewrite.

ReteachWrite the vocabulary words for thissection. Have students review thesection and create definitions for thevocabulary words in their own words.

Choosing a particular frame of referencedoes not change the sound heard by theobserver. In physics, one often chooses aframe of reference that makes it easier tounderstand or solve a problem. With theDoppler effect, one simple choice is touse the observer’s frame of reference.The shift in frequency can then bedetermined from the speed of the soundsource in this frame of reference.

If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 17.4.

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5. Sound is recorded by converting soundwaves into other forms of energy. 6. By changing the frequency of standing waves7. The speed of sound in solids is generally fasterthan the speed of sound in air, so the soundwave reaches your feet first through the ground.8. The intensity of a 40-dB sound is 100 timesgreater than the intensity of a 20-dB sound.9. A bat can determine distance based on theamount of time it takes for ultrasound wavesthe bat emits to reach the insect and bounceback to the bat’s ears.


1. Speed, intensity, loudness, frequency, pitch2. Medical imaging, sonar3. The Doppler effect occurs when an observerhears a higher frequency as a sound sourceapproaches. When a sound source movesaway, the observer hears a lower frequency. 4. The outer ear gathers and focuses sound intothe middle ear, which amplifies the vibrations.The inner ear has nerve endings that sensevibrations and transmit signals to the brain.


In the modern world, there is plenty of unwantednoise from jet aircraft engines, constructionequipment, and factory machinery. Traditionally,the way to deal with noise has been to puta barrier, such as an ear plug or ear muffler,between the source of the noise and your ears.This type of ear protection has the drawback,however, that it blocks out most sound, not

just unwanted noise. That makes communi-cation with co-workers difficult. Now, new

technology is available that selectivelyremoves unwanted noise, but trans-mits the sounds you need to hear.

Imagine eliminating all unwanted noise and onlyhearing the sounds you need to hear. That is the goal of noise-cancellation technology.

Now Hear This

Noise-cancellingheadset This

headset removesnoise but allowscommunication

with co-workers.

Sound ControlAirport workers, jet pilots,rock musicians, and somefactory workers all have aneed for noise reduction.

522 Chapter 17

522 Chapter 17

Now Hear This BackgroundThere are many ways to reduce noise.Barriers are the most common. The ideaof creating “anti-noise” was firstdeveloped in the 1970s. Because noise ischaotic and unpredictable, the only wayto cancel a noise is by continuouslymonitoring the noise and generating an appropriate anti-noise signal. Onlyrecently has technology becomeavailable that allows real-time, activenoise cancellation. An active noise-cancellation system uses a high-speeddigital signal processor to sample thenoise thousands of times a second. Itthen is able to create the anti-noisewave. Applications for noise-cancellationtechnology include active mufflers thatreduce exhaust noise; quiet zones inautomobiles, airplanes, trucks, andlocomotives; and active headsets thatprovide better hearing protection whilestill allowing communication.

Build Science Skills

Using Models

Purpose Students observe destructive interference of sound waves.

Materials tuning fork hammer, 2 tuning forks with the same frequency(about 400 Hz), stethoscope

Safety Make sure that students handlethe tuning forks carefully.

Procedure One student carefully strikesthe tuning forks with the tuning forkhammer and holds the tuning forks about 10 cm apart. A second studentlistens with the stethoscope’s listeningend placed about 20 cm from the tuning forks. The listener moves thestethoscope’s end to find a node wherethe volume is greatly reduced. This mayrequire several trials. Ask, What is causingthe sound to become inaudible incertain places? (Destructive interferencebetween the two sound waves) Ask, How isthis different from noise-cancellationtechnology? (In noise-cancellationtechnology, the signal is much morecomplicated.)

Expected Outcome Students will findregions where the sound is not audible. Musical, Logical

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� Research in the library or on the Internetproducts that use noise cancellation, such ascar stereos that reduce road noise, mobilephones, stereo headphones,or headsets for customerservice operators. Write aparagraph explaining howone of these products works.

� Take a Discovery ChannelVideo Field Trip bywatching “Noise!”

Going Further

Video Field Trip

Car racingNoise-cancellation technology has numerousapplications. It is used in the army, on buildingsites, and in noisy sports like car racing.

Unwantednoise wave

Wall of earcup cuts outsome noise.

Music orspeech can be channeledelectronicallythrough a wire.

Destructive InterferenceThe anti-noise wave is

generated in such a waythat its peaks coincide

exactly with the troughs ofthe noise wave, and vice

versa. All that is left of bothwaves is a faint hiss.

Noise-cancellation technologyIn noise-cancellation technology, a headset is fitted with a tiny microphone. This microphone is linked to an electronic device that can analyze the amplitudeand frequency of sounds. When an undesirable noiseis picked up by the microphone, a tiny speakergenerates an anti-noise wave. The two waves canceleach other in destructive interference.

Desirable soundspass through tothe ear.

Microphone

The anti-noise generator createsan anti-noise wave.

Anti-noisewave

Noisewave

Faint hiss

Eardrum


Going FurtherStudents should write about how a particular product uses noise-cancellation technology. Briefexplanations for two devices follow.• Car stereos A microphone in the car

picks up sound in the car. The noise-cancellation system subtracts musicfrom the stereo and uses the remainingsound to create an anti-noise wave. Thestereo system speakers then combinethe anti-noise waves with the music.

• Customer service headsets Often,many operators answer phones in alarge, noisy room. To avoid confusingcross-conversations, special headsetshelp both the operator and thecustomer. The headsets use noise-cancellation technology that separatesspeech, which varies rapidly, frombackground noise, which has slowervariations. The headsets have to belight because operators wear them for hours at a time.Logical


After students have viewed the Video Field Trip, ask them the following questions: How can loudsounds be harmful? (Over time, loud sounds candamage the ability to hear.) What units are used tocompare the intensity levels of sound? (Decibels)How are sounds transferred through air? (Studentanswers may include waves, mechanical waves, orvibrations of the air.)

Video Field Trip

Noise!


Mechanical Wavesand Sound

C H A P T E R

498 Chapter 17

Ocean waves, like all mechanical waves, �carry energy through a medium.

How do science concepts apply to yourworld? Here are some questions you’ll beable to answer after you read this chapter.

■ Why does a wave topple over on itself when it approaches the shore? (Section 17.1)

■ How does a surfer know when the next wave is coming? (Section 17.2)

■ Why does a gymnast on a trampoline time herjumps to match the movement of the trampoline surface? (Section 17.3)

■ How much faster is a sound wave than a car traveling on the highway? (Section 17.4)

■ How can headphones reduce noise without interfering with sounds you want to hear?(page 522)

498 Chapter 17

ASSESS PRIORKNOWLEDGE Use the Chapter Pretest below to assessstudents’ prior knowledge. As needed,review these Science Concepts andMath Skills with students.

Review Science ConceptsSection 17.1 Waves carry energy, soreview the concept of energy andenergy conversions with students.

Section 17.2 Remind students of theunit of time (seconds, s). Review what itmeans to say something is moving.Remind students how to measure andcalculate speed (distance/time).

Section 17.3 Review velocity anddisplacement, which help with under-standing interference and refraction.Remind students that energy is theability to do work and that waves carry energy.

Section 17.4 Since the speed of sounddepends on temperature and density,review how density and temperature arerelated to solids, liquids, and gases.

Review Math SkillsFormulas and Equations, Percentsand Decimals Students will need tomanipulate equations to solve for theunknown. They also may need practiceworking with decimal fractions. Reviewwhat is meant by powers of ten.

Direct students to Math Skills in theSkills and Reference Handbook at theend of the student text.

PHYSICS

Chapter 17

Chapter Pretest

1. What is energy? (Ability to do work)2. What is mechanical energy? (Energy dueto the motion or position of an object) 3. True or False: Displacements in oppositedirections add together. (False. Displacementsin opposite directions subtract.)4. How is speed calculated? (Distance/time)5. What are the standard units of distanceand time? (Meters and seconds)

6. Which is longer, 0.25 m or 25 m? (25 m)7. What happens to the spacing of theparticles within a solid or liquid as thetemperature increases? (The particles movefarther apart.)8. What happens to the speed of theparticles in a solid, liquid, or gas as thetemperature is increased? (The particles movefaster.)

9. If a car takes 2 hrs to travel 100 km, whatis its average speed? (b)

a. 25 km/hb. 50 km/hc. 75 km/hd. 100 km/h

PHYSICS

17.1 Mechanical Waves




Chapter Preview


How Does a Disturbance Produce Waves?Procedure1. Fill a clear plastic container and a dropper

pipet with water.

2. Observe the surface of the water by lookingdown at an angle into the container. Use thedropper pipet to release a drop of water froma height of about 3 cm above the surface ofthe water.

3. Repeat Step 2 with a drop released fromeach of these heights: 10, 20, 50, and 70 cm.

Think About It1. Observing Which drop produced the

highest waves?

2. Making Generalizations In general, howwould you expect the distance a drop fallsto affect the wave it produces? Explainyour answer.

3. Formulating Hypotheses Why does thedistance a drop falls affect the height of thewaves it produces? Explain your answer.

Video Field Trip

Noise!How Does a DisturbanceProduce Waves?

Purpose In this activity, students begin to understand that the height or amplitude of a wave depends on the wave’s energy.

Skills Focus Observing, Comparingand Contrasting

Prep Time 15 Minutes

Materials dropper pipet; wide, flat-bottomed container; meter stick

Advance Prep Distribute emptycontainers and fill them from a gallonjug once they are in place at the labstations.


Teaching Tips• Instruct students to look at the surface

of the water diagonally, rather thanhorizontally or vertically.

Expected Outcome The height(amplitude) of the wave will increase as the height from which the water isdropped increases.

Think About It1. The drop that fell from 70 cmproduced the highest waves. 2. The greater the distance a drop falls,the more kinetic energy it will have, andthe larger the wave it will produce. 3. The greater the height from which a drop falls, the more energy it hasbecause it accelerated to a greater speedthan a drop that fell a shorter distance.Therefore, it transfers more energy tothe surface of the water, producinghigher waves. Visual, Logical

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Encourage students to view the Video Field Trip “Noise!”

ENGAGE/EXPLORE

Video Field Trip

Noise!

0498_hsps09te_Ch17.qxp 3/6/07 1:58 PM Page 499

Sound is produced when a vibrating source causes amedium to vibrate. In this lab, you will investigatehow the vibrating source affects characteristics of thesound produced.

Problem What determines the frequencyand amplitude of the sound produced by avibrating object?

Materials

Skills Observing, Inferring, DrawingConclusions, Controlling Variables

• meter stick• two cardboard

tubes • scissors or scalpel• two rubber bands

• wax paper• balloon• small mirror• transparent tape• flashlight

ProcedurePart A: Investigating How LengthAffects Pitch

1. Hold one end of a meter stick down firmly on atable so that 20 centimeters of the meter stickextends past the edge of the table. Pluck theend of the meter stick that extends past thetable to produce a vibration and a sound asshown. Observe the vibration and sound ofthe meter stick.

2. Repeat Step 1, but this time allow 40 centimetersof the meter stick to extend past the edge of thetable. Observe and record how the length of thevibrating part of the meter stick affects the pitch.

3. Repeat Step 1, but this time allow 60 centimetersof the meter stick to extend past the edge ofthe table. Record your observations.

4. Investigate the relationship between lengthand frequency for a vibrating column of air, asyou did with the vibrating meter stick. Make akazoo by cutting a hole in the middle of one ofthe cardboard tubes. Make the hole approx-imately 1 centimeter in diameter. Use a rubberband to fasten the piece of wax paper over oneend of the tube. CAUTION Be careful whencutting with sharp instruments; always cut awayfrom yourself and away from nearby people.

5. Make a second kazoo by cutting the secondtube 10 centimeters shorter than the first tube.Using the short tube, repeat Step 4.

6. Hold the shorter kazoo in front of your mouthand hum into the open end, keeping your pitchsteady. Repeat this action with the longerkazoo, making sure to hum exactly as you didbefore. Observe and record how the length ofthe kazoo affects the pitch of the sound.

Investigating Sound Waves

524 Chapter 17

524 Chapter 17

Investigating Sound WavesObjectiveAfter completing this activity, studentswill be able to • define sound as a mechanical

vibration whose frequency dependson the length of the vibrating object.

• state that pitch depends on thefrequency and that loudness dependson the amplitude of the sound wave.

This lab can help dispel themisconception that high-pitched soundsare louder than low-pitched sounds.

Skills Focus Observing, Inferring,Drawing Conclusions

Prep Time 15–30 minutes

Advance Preparation To save classtime, cut the wax paper into 10-cmsquare sheets and cut 1-cm holes in thecardboard tubes. Use a thumbtack as atemplate for the 1-cm holes and use asharp scalpel, razor blade, or electric drillto cut the holes. If small plastic mirrorsare unavailable, you can substitutesequins or small pieces of aluminum foil.


Teaching Tips• Remind students that they must hum,

and not sing, into the kazoo.• Have half the class start Part A while

the other half starts Part B. This willrequire fewer flashlights and metersticks. It will also allow the students tohear the differences in pitch becausefewer students are humming at thesame time.

Questioning StrategiesAsk students: What is the source ofenergy that caused the meter stick tovibrate? (The mechanical energy ormuscle power used to pluck the meterstick) What happened to the soundwaves when you changed the lengthof the vibrating object? (The pitchchanged.) How does the pitch affectthe frequency of the reflected light’svibration? (The higher the pitch, thegreater the frequency is.) How doesloudness affect the amplitude of thereflected light’s vibration? (The greaterthe loudness, the greater the amplitude is.)

L2



Part B: Investigating How FrequencyAffects Pitch and How AmplitudeAffects Loudness

7. Cut the neck off of the balloon. Replacethe wax paper on the longer kazoo withthe cut-open balloon. Wrap the rubberband several times around the end of thecardboard tube. The rubber band shouldhold the balloon tightly stretched overthe end of the tube. Use tape to attachthe small mirror onto the balloon on theend of the tube.

8. Have a classmate shine a flashlight on themirror as shown while you hum into thekazoo. Your classmate should position theflashlight so that a spot of light is reflectedon the wall. It may be necessary to darkenthe room. Observe how the spot of lightmoves when you hum into the kazoo. Make a note of your position and the position andangle of the kazoo and the flashlight.

9. Without changing how loudly you hum, useyour voice to raise the pitch of your humming.Observe and record how the movement of thespot of light differs from your observations inStep 8. Make sure you do not change yourdistance from the wall or the angle at whichthe light from the flashlight strikes the mirrorattached to the kazoo.

10. Repeat Step 9, but this time hum at a lowerpitch than you did in Step 8.

11. Repeat Steps 9 and 10, but this time vary theloudness of your humming while keeping thepitch constant.

Analyze and Conclude1. Observing What happened to the frequency

of the meter stick’s vibration when you madethe overhanging part longer?

2. Inferring How did the frequency of the meterstick’s vibration affect the pitch of its sound?

3. Inferring How did the kazoo’s length affectits pitch?

4. Analyzing Data When you changed thepitch of your humming, how did it affectthe frequency of vibration of the mirror?

5. Analyzing Data How is the amplitude of thekazoo’s vibration related to its loudness?

6. Controlling Variables Explain why it wasimportant to keep loudness constant whenyou changed the pitch of your humming inStep 9.

Design an experiment toinvestigate what variables

affect the pitch and loudness of vibrating strings.Use an instrument such as a guitar or violin.After your teacher approves your plan, carryout your experiment.

Go Further

Expected Outcome The pitchbecomes lower as the length of thevibrating part of the meter stick and theair column inside the kazoo arelengthened. An increase in the sound’sloudness increases the amplitude of thevibration of the reflected light. Anincrease of the sound’s pitch increasesthe frequency of the vibration of thereflected light.

Go Further

The frequency and amplitude of avibrating string are easily varied byadjusting the length of the vibratingportion of the string, or the force withwhich it is plucked, respectively.Kinesthetic, Logical


Analyze and Conclude1. The frequency was reduced.2. The pitch became lower as the frequency was reduced.3. The longer kazoo had the lower pitch.4. Humming at a higher pitch increased thefrequency of the mirror’s vibration.

5. Increasing the loudness increased theamplitude of the kazoo’s vibration.6. Keeping the loudness constant ensured thatany change in the frequency of the mirror’svibration was due to the change in pitch.Logical


498A Chapter 17

Planning Guide

SECTION OBJECTIVES STANDARDS ACTIVITIES and LABSNATIONAL STATE

A-1, A-2, B-6

B-6

A-1, A-2, B-6,C-6, E-2, F-1, F-2F-5, G-1

A-1, A-2, B-6

17.2 Properties of Mechanical Waves,pp. 504–507

1 block or 2 periods

17.2.1 Define frequency, period, wavelength, andwave speed and describe these propertiesfor different kinds of waves.

17.2.2 Solve equations relating wave speed towavelength and frequency or period.

17.2.3 Describe how to measure amplitude andrelate amplitude to the energy of a wave.

17.3 Behavior of Waves, pp. 508–512


17.3.1 Describe how reflection, refraction,diffraction, and interference affect waves.

17.3.2 State a rule that explains refraction of awave as it passes from one medium toanother.

17.3.3 Identify factors that affect the amount ofrefraction, diffraction, or interference.

17.3.4 Distinguish between constructive anddestructive interference and explain howstanding waves form.

17.4 Sound and Hearing, pp. 514–521


17.4.1 Describe the properties of sound wavesand explain how sound is produced and reproduced.

17.4.2 Describe how sound waves behave inapplications such as ultrasound andmusic.

17.4.3 Explain how relative motion determinesthe frequency of sound an observerhears.

17.4.4 Analyze the functions of the main regionsof the human ear.

17.1 Mechanical Waves, pp. 500–503


17.1.1 Define mechanical waves and relatewaves to energy.

17.1.2 Describe transverse, longitudinal, andsurface waves and discuss how they are produced.

17.1.3 Identify examples of transverse andlongitudinal waves.

17.1.4 Analyze the motion of a medium as eachkind of mechanical wave passes through it.

SE Quick Lab: Comparing Frequency and Wave Speed, p. 505 L2

TE Teacher Demo: Water-Wave Reflections, p. 508

TE Teacher Demo: Standing Waves,p. 512 L2

L2

SE Exploration Lab: Investigating Sound Waves, pp. 524–525

TE Build Science Skills: Observing,p. 515

LM Investigation 17A: Comparing the Speed of Sound

LM Investigation 17B: Comparing Sound Conduction L1

L2

L2

L2

SE Inquiry Activity: How Does aDisturbance Produce Waves? p. 499

SE Quick Lab: Observing Waves in a Medium, p. 502

TE Teacher Demo: Wave Dance,p. 501 L2

L2

L2

Easy Planner Teacher Express

Mechanical Waves and Sound 498B

Quantities for each group

STUDENT EDITION

Inquiry Activity, p. 499dropper pipet; wide, flat-bottomed container; meter stick

Quick Lab, p. 502large, clear container; foodcoloring; ruler; droppers(optional)

Quick Lab, p. 5053-m rope, tape measure,stopwatch

Exploration Lab, pp. 524–525meter stick, 2 cardboard tubes,scissors or scalpel, 2 rubberbands, wax paper, balloon,small mirror, transparent tape, flashlight

TEACHER’S EDITION

Teacher Demo, p. 50110 chairs, 10 students, space to move around

Teacher Demo, p. 508clear bowl, water, overheadprojector

Teacher Demo, p. 512long, soft, heavy rope, such as a jump rope

Build Science Skills, p. 515set of tuning forks of similarconstruction

Build Science Skills, p. 522tuning fork hammer, 2 tuningforks with the same frequency(about 400 Hz), stethoscope

Materials for Activities and Labs

Chapter Assessment

CHAPTER ASSESSMENT

SE Chapter Assessment, pp. 527–528

CUT Chapter 17 Test A, BCTB Chapter 17iT Chapter 17PHSchool.com GOWeb Code: cca-2170

STANDARDIZED TEST PREP

SE Chapter 17, p. 529TP Diagnose and Prescribe

Go online for these Internet resources.

Web Code: cch-2173Web Code: cca-2170

Web Code: cce-2174

Web Code: ccn-2171Web Code: ccn-2172Web Code: ccn-2173

Interactive Textbook withassessment at PHSchool.com

Ability Levels Components

For students who need additional help

For all students

For students who need to be challengedL3

L2

L1 SE Student EditionTE Teacher’s EditionLM Laboratory ManualPLM Probeware Lab

Manual

RSW Reading & StudyWorkbook

MSPS Math Skills &Problem SolvingWorkbook

CUT Chapter & Unit TestsCTB Computer Test BankTP Test Prep ResourcesDC Discovery Channel

Videotapes & DVDs

T TransparenciesiT Interactive TextbookP Presentation Pro

CD-ROMGO Internet Resources

RESOURCES SECTION

PRINT and TECHNOLOGY ASSESSMENT

RSW Section 17.1

T Chapter 17 Pretest

Section 17.1

P Chapter 17 Pretest

Section 17.1

GO Vibrations

and waves L2

L2

L2

L2

L2

L1 SE Section 17.1Assessment, p. 503

iT Section 17.1

RSW Section 17.2

RSW Math Skill

MSPS Section 17.2

T Section 17.2

P Section 17.2

GO Wave properties L2

L2

L2

L2

L2


iT Section 17.2

RSW Section 17.3

T Section 17.3

P Section 17.3

GO Diffraction

and interference L2

L2

L2


iT Section 17.3

RSW Section 17.4

DC Noise!

T Section 17.4

P Section 17.4

GO Sound L2

L2

L2

L2


iT Section 17.4

498C Chapter 17

Before you teach

Big IdeasLater chapters will build on the basics of mechanicalwaves, so it’s important that students get the wave types,properties, and behaviors under their belts. For wavestraveling at a given speed, it can be shown that wavelengthis inversely proportional to frequency. This is easilyderived from the equation distance equals velocity timestime because “distance” is equal to wavelength, and “time”for the passage of one wave is its period, which is inverselyproportional to frequency. The most important idea isthat when an object moves, the object itself moves frompoint A to point B. A wave, however, is different. A wave is the disturbance in a medium. Although the mediummoves somewhat as the wave travels, what moves frompoint A to point B in this case is the disturbance.

Matter and Energy A mechanical wave is definedas a disturbance in matter that carries energy from oneplace to another. It takes energy to create any kind ofwave, but a mechanical wave, unlike an electromagneticwave, cannot travel without a medium.

Forces and Motion The behavior of a wave—whether this behavior is refraction, reflection, diffraction,or interference—depends on the medium through whichit travels, as well as on the wave type. Sound waves, forexample, can be manipulated in various ways, or evencancelled. Discussing technological applications thatstudents are probably familiar with may be a good way to generate interest. Be as concrete and visual as possible,since the subject of electromagnetic waves builds on thisinformation, and is far less easily grasped.

From the AuthorSophia YancopoulosManhattan College

Physics Refresher

Mechanical Waves 17.1

Electromagnetic waves are notmechanical waves. For manyyears, physicists tried todevelop a mechanical under-standing of electromagneticwaves, searching without suc-cess for proof of the “ether,” amedium for these waves.Physicists now know that elec-tromagnetic waves do notrequire a medium.

For a medium to transmit amechanical wave, the mediummust be elastic. For example,when a gas is compressed in acylinder with a piston, the pis-ton springs back when released. Gases are elastic under compression, so they will transmit compression waves.Gases have almost no tensile strength, so a gas will not transmita transverse wave. A steel bar will transmit both longitudinal andtransverse waves.

Properties of Mechanical Waves 17.2

The period of a wave, measured in seconds, equals . Insymbols, the wave equation can be written as v � l� f, where vis the wave speed, l is the wavelength, and f is the frequency.Amplitude, the maximum distance the medium is displaced fromits rest position, is a measure of wave energy. As amplitudeincreases, the wave energy increases. The relationship is not linear. For most waves, energy is proportional to (amplitude)2.

Behavior of Waves 17.3

Reflection and refraction occur at the boundary between twomediums. Part or all of the wave is reflected. Part of the wave maypass through, but its speed and direction may change. Thereflected wave always has the same speed and frequency as theoriginal wave. The refracted wave will have the same frequencybut different speed and wavelength. Refraction is a behavior ofall waves. Earthquake waves are refracted as they pass throughdifferent layers inside of Earth. By studying the patterns andkinds of earthquake waves received around the world, geologistsare able to infer the interior structure of Earth.

1Frequency

Students may mistakenlythink that waves are partof the medium that travelwith the waves. However,waves carry energythrough a medium, butthe medium usually hasno net movement. For a strategy to overcomethis misconception, seeAddress Misconceptionson page 502.

When a wave moves past an obstacle or passes through a narrow opening, it diffracts, or bends in such a way as to recre-ate the form (in frequency and amplitude) that the wave hadbefore encountering the obstacle or opening. Diffraction, asshown below, depends on how the size of the opening comparesto the wavelength. The larger the opening, the greater the amount of signal that can pass through it unimpeded. Thesmaller the opening, the more the wave has to bend to recon-struct its original form.

Sound and Hearing 17.4

The three aspects of any soundare its source, the transferenceof energy from this source, andthe detection of the sound. Thetransference of sound energytakes the form of longitudinalwaves. Sound waves are longitu-dinal waves in matter. Fluids,such as air and water, can onlytransmit longitudinal waves. Ifthe frequency of a sound wave isabove the range of human hear-ing, it is called ultrasound. If thefrequency of a sound wave isbelow the range of human hear-ing, it is called infrasound.

The unit of intensity level, the decibel or dB, was named afterAlexander Graham Bell. Intensity can also be expressed in unitsof watts per square meter. Here are some intensities of soundsexpressed in W/m2.

Build Reading Literacy

Active Comprehension

Engaging Interest in a TopicStrategy Stimulate students’ interest in a topic prior toreading. As with the KWL strategy, students generate questionsbased on their curiosity. Interest is thus translated into apurpose for reading. Looking for answers to the questionsduring reading helps keep students engaged in what they arereading. Before students begin reading, choose an openingparagraph from one of the sections in Chapter 17, for example,the first paragraph on p. 500.

Example1. Have a student read the opening paragraph.2. Ask the group, “What more would you like to know about

?” (for example, mechanical waves). Make a list ofstudent responses.3. Tell students to read the remainder of the section, keepingthe questions in mind as they read.4. After reading, you may discuss the extent to which eachquestion was answered by the text. Ask students also to commenton any new information they learned that was surprising.5. Have students work in small groups, applying the activecomprehension strategy to the reading of each section ofthe chapter.

See p. 509 for a script on how to use the active comprehensionstrategy with students. For additional Build Reading Literacystrategies, see pp. 503, 507, and 516.

Some students think thatsound waves can push adust particle away from a speaker or blow out acandle flame. Generally,there is no net transportof matter by a wave. Fora strategy to overcomethis misconception, seeAddress Misconceptionson page 518.

Mechanical Waves and Sound 498D

Diffraction at a large opening Diffraction at a small opening

A B

Wave fronts Wave fronts

BarrierBarrier

Sound Intensity Level (dB) Intensity (W/m2)

Threshold of hearing 0 1 � 10�12

Whisper 20 1 � 10�10

Normal conversation 65 3 � 10�6

Street noise 70 1 � 10�5

Rock concert 120 1

Threshold of pain 120 1

For: Teaching methods for mechanical waves and soundsVisit: www.SciLinks.org/PDLinksWeb Code: ccn-1799

526 Chapter 17

CHAPTER

17 Study Guide17.1 Mechanical Waves

Key Concepts

• A wave is created when a source of energy causesa vibration to move through a medium.

• The three main types of mechanical waves aretransverse, longitudinal, and surface waves.

Vocabulary

mechanical wave, p. 500; medium, p. 500;crest, p. 501; trough, p. 501; transverse wave, p. 501;compression, p. 502; rarefaction, p. 502;longitudinal wave, p. 502; surface wave, p. 503


Key Concepts

• A wave’s frequency equals the frequency of thevibrating source producing the wave.

• For waves traveling at a constant speed,wavelength is inversely proportional to frequency.

• As energy of a wave increases, amplitude increases.

Vocabulary

periodic motion, p. 504; period, p. 504; frequency,p. 504; hertz, p. 504; wavelength, p. 505;amplitude, p. 507


Key Concepts

• Reflection does not change the speed or frequency ofa wave, but the wave can be flipped upside down.

• Refraction occurs because one side of a wavemoves more slowly than the other side.

• A wave diffracts more if its wavelength is largecompared to the size of an opening or obstacle.

• Interference can be constructive or destructive.

• A standing wave forms only if the length of avibrating cord is a multiple of one half wavelength.

Vocabulary

reflection, p. 508; refraction, p. 509; diffraction,p. 510; interference, p. 510; constructive interference,p. 511; destructive interference, p. 511; standingwave, p. 512; node, p. 512; antinode, p. 512


Key Concepts

• Many behaviors of sound can be explained using afew properties—speed, intensity and loudness, andfrequency and pitch.

• Ultrasound is used in a variety of applications,including sonar and ultrasound imaging.

• As a source of sound approaches, an observerhears a higher frequency. When the sound sourcemoves away, the observer hears a lower frequency.

• The outer ear gathers and focuses sound into themiddle ear, which receives and amplifies thevibrations. The inner ear uses nerve endings tosense vibrations and send signals to the brain.

• Sound is recorded by converting sound waves intoelectronic signals that can be processed andstored. Sound is reproduced by convertingelectronic signals back into sound waves.

• Most instruments vary pitch by changing thefrequency of standing waves.

Vocabularysound waves, p. 514; intensity, p. 515; decibel, p. 515;loudness, p. 515; pitch, p. 515; sonar, p. 516; Dopplereffect, p. 516; resonance, p. 521

Web Diagram Copy the web diagram below onto asheet of paper. Use information from the chapter tocomplete the diagram.

Thinking Visually

Interference

c. ?a. ?

StandingWaves

b. ?

Behavior ofWaves

526 Chapter 17

Study Guide

Study TipOrganize New InformationThis chapter contains many new words and much new information. Tell students to create outlines, charts,flashcards, time lines, and concept maps to help them visualize relation-ships. Have them create a vocabulary listwith definitions for all of the vocabularyterms in their own words.

Thinking Visuallya. Reflectionb. Refractionc. Diffraction

Assessment

If your class subscribes to the Interactive Textbook, your stu-dents can go online to access an inter-active version of the Student Editionand a self-test.

Reviewing Content1. b 2. a 3. c4. b 5. d 6. c7. b 8. a 9. c

10. b

Understanding Concepts11. A wave in a spring and a P wave arelongitudinal waves. Both waves havecompressions and rarefactions.12. A surface wave combines themotion of a transverse wave,(perpendicular to the direction of travel)and the motion of a longitudinal wave(back-and-forth parallel to the directionof travel).13. See Wave B on the diagram below.14. See Wave C on the diagram below.

Chapter 17

Print• Chapter and Unit Tests, Chapter 17

Test A and Test B• Test Prep Resources, Chapter 17

Technology• Computer Test Bank, Chapter Test 17• Interactive Textbook, Chapter 17• Go Online, PHSchool.com, Chapter 17

Chapter Resources

Distance (m)

Wave B

Wave C

Dis

pla

cem

ent

(m)

00 2 4 6 8

1234

-4-3-2-1


CHAPTER

17 Assessment

Choose the letter that best answers the question orcompletes the statement.

1. Which of the following is NOT true aboutmechanical waves?

a. They carry energy.b. They transfer matter.c. They can be longitudinal. d. They require a medium.

2. In a transverse wave, the medium vibrates a. at right angles to the wave direction.b. in the same direction as the wave.c. in a direction opposite that of the wave.d. at a 45° angle to the wave direction.

3. The height of a wave crest is called a. wavelength. b. frequency.c. amplitude. d. energy.

4. For waves moving at a constant speed, ifwavelength is doubled, then frequency is

a. doubled. b. halved.c. unchanged. d. quadrupled.

5. When a wave is reflected, its speed a. increases. b. increases or decreases.c. decreases. d. is unchanged.

6. When a wave bends around an obstacle, it is called

a. reflection. b. refraction.c. diffraction. d. interference.

7. When two waves interfere, the displacementwhere two troughs meet is

a. positive. b. negative.c. zero. d. a crest.

8. A large speaker is better than a small speaker forproducing sounds with

a. low frequency. b. high frequency.c. low intensity. d. high intensity.

9. The highest-frequency sound human ears canusually hear is about

a. 20 Hz. b. 10,000 Hz.c. 20,000 Hz. d. 30,000 Hz.

10. Sonar can make use ofa. the Doppler effect. b. ultrasound.c. infrasound. d. resonance.

Reviewing Content

11. Name two kinds of longitudinal waves andexplain how you know they are longitudinal.

12. How are some surface waves similar to bothtransverse and longitudinal waves?

Copy the diagram below on a separate piece of paperand use it to answer Questions 13–15.

13. On your diagram, draw wave B with the samewavelength as wave A, but twice the amplitude.

14. On your diagram, draw wave C with the sameamplitude as wave A, but twice the wavelength.

15. How does the frequency of wave C comparewith the frequency of wave A, assuming theytravel at the same speed?

16. What causes refraction of a wave as it enters anew medium at an angle?

17. Regardless of the direction of waves far from anisland, waves close to the island move towardthe shore on all sides. Explain.

18. Why does a node in a standing wave have zero displacement?

19. How is intensity different from loudness?

20. Explain why a fire engine’s siren sounds lower inpitch after the fire engine passes you.

21. What is the function of the eardrum?

22. What are the names of the three small bones inthe middle ear, and what is their purpose?

23. Why are the materials used in construction of aconcert hall important?

Understanding Concepts

1

–1

–2

2

Distance(m)

Dis

pla

cem

ent

(m)

Wave A

1 3 42

Interactive Textbook withassessment at PHSchool.com

Assessment (continued)

15. The frequency of Wave C is half thefrequency of Wave A.16. A wave refracts because one side ofthe wave front moves faster than theother side.17. As waves move into shallower watersurrounding the island, they refract orbend toward the land because theshallower water acts as a new medium.18. At a node, there is completedestructive interference at all times, sothe displacement is zero.19. Intensity is the rate at which awave’s energy flows through a givenarea. Loudness is a physical response tothe intensity of sound. 20. As the siren moves away, each wavefront produced by the siren is fartherfrom the previous wave front than if thesiren were standing still. The actualfrequency does not change but thefrequency or pitch heard by theobserver is lower.21. The eardrum is located between themiddle ear and outer ear. Sound wavescause it to vibrate and convert theenergy of the sound waves intomechanical energy.22. The bones of the inner ear are thehammer, the anvil, and the stirrup. Thevibration of the eardrum causes thehammer to vibrate. It strikes the anvil,which causes the stirrup to move backand forth. Together the three bonesamplify the motion of the eardrum.23. Reflection and interference canaffect sound quality. Materials should beused that absorb sound to minimizereflection. The overall design of theinterior of the concert hall can minimizethe dead spots where destructiveinterference can occur.


Homework GuideSection

17.117.217.317.4

Questions1–2, 11–15, 24, 363–4, 16–17, 25, 30–335–8, 18, 269–10, 19–23, 27–29, 34–35

PPLS


CHAPTER

17 Assessment (continued)

24. Making Generalizations A friend says that allmechanical waves must lose energy as theymove through a medium because some energy islost due to friction. Explain why this is true.

25. Forming Operational Definitions Two soundwaves with different frequencies travel through asteel rod at the same speed. Explain by giving anoperational definition of wave speed.

26. Relating Cause and Effect Can two wavestraveling in the same direction form a standingwave? Explain why or why not.

27. Inferring The buzzing sound of a mosquitoflying around your head has a higher pitch thanthe buzz of a bumblebee. What can you inferabout the mosquito’s wings compared to thebee’s wings?

28. Relating Cause and Effect At the same time,you hear a kitten’s purr and a clap of thunder.The kitten’s purr is louder. Explain how thisis possible.

29. Applying Concepts How much greater is theintensity of a 25-dB sound than a 15-dB sound?What would be the intensity in dB for a soundthat is 100 times louder than the 25-dB sound?

Use the illustration below to answer Questions 30–32.

30. Interpreting Diagrams What is thewavelength of the ocean wave shown in the figure?

Math Skills

Critical Thinking 31. Evaluating Expressions Three completewavelengths pass a fixed point once every 2 seconds. What is the frequency of the wave?

32. Calculating What is the speed of the oceanwave in Question 31?

33. Calculating A deep-water wave has awavelength of 10.0 meters. If it travels at 3.9 m/s, what is the wave’s period?

34. Applying Concepts Some gardeners protecttheir gardens from animals by using sound thatanimals find irritating. Explain how the gardenerscan tolerate the sound, while the animals cannot.

35. Formulating Hypotheses Generate ahypothesis about how the frequency of noisemade by a machine changes as the machineoperates at higher speeds.

36. Writing in Science Write a paragraphdescribing the motion of a lily pad as several wavesfrom a boat pass by. Explain why the motionwould be the same or different if the lily pad werefloating free instead of attached by roots to thepond floor. (Hint: Before you write, draw a series ofdiagrams to show the motion.)

Using Models Try blowing across the top of a bottleto produce a tone. Experiment to see how addingwater of different heights affects the pitch. See if youcan arrange a series of eight bottles to produce amusical scale. Learn to play a simple tune on yourmusical bottles. Summarize your results in a computerpresentation that explains the relationship betweenthe height of the air column and the pitch produced.

Performance-Based Assessment

Concepts in Action

Ocean wave

0.69 m

528 Chapter 17

For: Self-grading assessment

Visit: PHSchool.com

Web Code: cca-2170

528 Chapter 17

Critical Thinking24. The energy of a mechanical wavecauses the medium to vibrate. Asvibrating particles in the mediumcollide, most of the energy can bepassed on to the next particle, but fluidfriction will cause some energy to beconverted into thermal energy. 25. The speed of a wave in a medium ismeasured under a particular set ofconditions. The speed can change if theconditions change or if the density ofthe medium changes. Otherwise, thespeed in the medium is a constant.26. To form a standing wave, two wavesmust pass through each other andinterfere in a regular way. This cannothappen if the waves are traveling in thesame direction. 27. The mosquito’s wings must bevibrating faster than the bee’s. Thehigher-frequency vibration produces ahigher-frequency sound.28. Intensity depends upon the distancefrom the source. If the kitten is close andthe thunder far away, the kitten’s purrcould have a greater intensity.29. The intensity of the 25-dB sound is10 times greater. A sound 100 timeslouder than a 25-dB sound has anintensity of 45 dB.

Math Skills30. The wavelength is 0.69 m.31. The frequency is 3 cycles/2 s �1.5 Hz.32. Speed � frequency � wavelength �1.5 Hz � 0.69 m � 1.0 m/s33. Period � wavelength/speed �(10.0 m)/(3.9 m/s) � 2.6 s

Concepts in Action34. The device uses ultrasoundfrequencies that are above the range ofhuman hearing, but are in a frequencyrange that animals can hear.35. As a machine operates at higherspeed, the vibrational frequencyincreases. The pitch of the noise shouldincrease as well.36. Students should describe theperiodic circular motion of the lily pad. Ifthe waves are not breaking waves, thenthe lily pad should have no net motiontoward shore, regardless of whether theroots are attached to the pond floor.

Chapter 17

Performance-Based AssessmentEncourage students to critique presentations in small groups before making whole-classpresentations. The frequency of the tone increasesas the water level (depth) in a bottle increases.Students may be curious to try different bottleshapes to see if this affects the tone produced mosteasily. Students will discover that to play a scale,the series of bottles needs to be arranged in orderof lowest water depth to greatest water depth.

Your students can independentlytest their knowledge of the chapterand print out their test results foryour files.


Standardized Test Prep

Choose the letter that best answers the question orcompletes the statement.

1. Which of the following is required to transmitenergy using mechanical waves?(A) medium (B) crest(C) trough (D) reflection(E) period

2. How are wavelength and frequency related for awave moving at constant speed?(A) Wavelength equals frequency.(B) Wavelength is greater than frequency.(C) Frequency is greater than wavelength.(D) Wavelength is proportional to frequency.(E) Wavelength is inversely proportional

to frequency.

3. Which of the following statements is generallytrue for the musical instrument (panpipes) below?

(A) The longest pipe has the highest pitch.(B) The shortest pipe has the highest pitch.(C) The pipe in the middle has the

highest pitch.(D) The pitch cannot be changed. (E) Any of the pipes could produce the

highest pitch.

4. Which conditions best describe what happenswhen a transverse wave is reflected? (A) Some of a wave does not pass through a

boundary or surface.(B) A wave enters a new medium at an angle.(C) A wave becomes longitudinal. (D) A wave combines with another wave.(E) A wave interferes with another wave.

5. Which of the following variables can affect thewavelength of a sound wave?

I. the medium II. the frequency of the waveIII. the amplitude of the wave

(A) I only (B) II only(C) I and II only (D) II and III only(E) I, II, and III

6. Ocean waves flow past a concrete support for abridge. The support is 30 meters wide and it sitsin very deep water. Which of the following isLEAST likely to occur? (A) The waves will reflect.(B) The waves will diffract. (C) The waves will lose some energy.(D) The waves will continue past the support.(E) The waves will refract.

Test-Taking Tip

Controlling VariablesSome test questions ask which variables must becontrolled in an experiment. It helps to start bywriting an equation for the variables you aregiven. For the question below, write an equationthat includes wavelength and frequency.

Speed � Wavelength � FrequencyThis equation gives you one variable to control:the wave speed. Next, think of any conditionsthat might affect this variable. You know thatwave speed changes in a different medium, sothe medium must be controlled. Other condi-tions that are often controlled include conditionsin the lab, such as temperature and pressure, orquantities of matter, such as mass or volume.

Which variables must be held constant in anexperiment to determine how the frequency ofa sound wave affects wavelength?

I. time II. wave speedIII. temperature

(A) I only(B) II only(C) I and II only(D) II and III only(E) I, II, and III

(Answer: D)

Standardized Test Prep1. A 2. E 3. B4. A 5. A 6. E



section 17.1 17.1 mechanical waves -...

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