Chapter 17: Chapter 17: Mechanical Mechanical

Waves and SoundWaves and SoundJennie L. BordersJennie L. Borders

Section 17.1 – Section 17.1 – Mechanical WavesMechanical Waves A A mechanical wavemechanical wave is a disturbance in is a disturbance in

matter that carries matter that carries energyenergy from one from one place to another.place to another.

Mechanical wavesMechanical waves require require mattermatter to to travel through.travel through.

The The materialmaterial through which a wave through which a wave travels is called a travels is called a mediummedium..

A mechanical wave is created when a A mechanical wave is created when a source of source of energyenergy causes a causes a vibrationvibration to to travel through a travel through a mediummedium..

Types of Mechanical Types of Mechanical WavesWaves

The three main types of mechanical The three main types of mechanical waves are waves are transverse waves, transverse waves, longitudinal waves, and surface longitudinal waves, and surface waveswaves..

A A pointpoint on the wave vibrates up and on the wave vibrates up and down between a down between a maximummaximum and and minimumminimum height. height.

Transverse WavesTransverse Waves

A A transverse wavetransverse wave is a wave that is a wave that causes the medium to vibrate at causes the medium to vibrate at right anglesright angles to the to the directiondirection in which in which the wave travels.the wave travels.

Transverse WavesTransverse Waves

The The highesthighest point of the wave above point of the wave above the rest position is the the rest position is the crestcrest. .

The The lowestlowest point below the rest point below the rest position is the position is the troughtrough..

Longitudinal WavesLongitudinal Waves A A longitudinal wavelongitudinal wave is a wave in which is a wave in which

the vibration of the medium is the vibration of the medium is parallelparallel to the to the directiondirection the wave travels. the wave travels.

An area where the particles in a An area where the particles in a medium are spaced medium are spaced close togetherclose together is is called a called a compressioncompression..

An area where the particles in a An area where the particles in a medium are medium are spread outspread out is called a is called a rarefactionrarefaction..

Surface WavesSurface Waves

A A surface wavesurface wave is a wave that travels is a wave that travels along a surface separating along a surface separating two two mediamedia..

The motion of the particles is in a The motion of the particles is in a circlecircle..

Section 17.1 AssessmentSection 17.1 Assessment

List the three main types of List the three main types of mechanical waves.mechanical waves.

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

Section 17.2 – Properties of Section 17.2 – Properties of Mechanical WavesMechanical Waves

Any Any motionmotion that repeats at regular that repeats at regular time time intervalsintervals is called is called periodicperiodic motion.motion.

The time required for The time required for one cycleone cycle, a , a complete motion that returns to its complete motion that returns to its starting point, is called the starting point, is called the periodperiod..

FrequencyFrequency Any periodic motion has a Any periodic motion has a

frequencyfrequency, which is the number of , which is the number of complete cyclescomplete cycles in a given time. in a given time.

FrequencyFrequency is measured in cycles per is measured in cycles per second, or second, or hertz (Hz)hertz (Hz)..

A wave’s frequency equals the A wave’s frequency equals the frequency of the frequency of the vibrating sourcevibrating source producing the waves.producing the waves.

WavelengthWavelength

WavelengthWavelength is the distance between is the distance between a point on one wave and the a point on one wave and the samesame point on the next cycle of the wave.point on the next cycle of the wave.

IncreasingIncreasing the frequency of a wave the frequency of a wave decreasesdecreases the wavelength. the wavelength.

Wave SpeedWave Speed

Formula for the speed of wavesFormula for the speed of waves

Speed = Wavelength x FrequencySpeed = Wavelength x Frequencyv = v = x x

Speed = m/sSpeed = m/sWavelength = mWavelength = mFrequency = 1/s or HzFrequency = 1/s or Hz

Sample ProblemSample Problem

One end of a rope is vibrated to One end of a rope is vibrated to produce a wave with a wavelength of produce a wave with a wavelength of 0.25m. The frequency of the wave is 0.25m. The frequency of the wave is 3.0Hz. What is the speed of the wave?3.0Hz. What is the speed of the wave?

v = v = x x v = ?v = ? v = v = x x = 0.25m= 0.25m v = 0.25m x 3.0(1/s)v = 0.25m x 3.0(1/s)

= 3.0Hz= 3.0Hz v = 0.75m/sv = 0.75m/s

Practice ProblemsPractice Problems

A wave on a rope has a wavelength A wave on a rope has a wavelength of 2.0m and a frequency of 2.0Hz. of 2.0m and a frequency of 2.0Hz. What is the speed of the wave?What is the speed of the wave?

A wave in the water has a frequency A wave in the water has a frequency of 4Hz and a wavelength of 0.1m. of 4Hz and a wavelength of 0.1m. What is the speed of the wave?What is the speed of the wave?

v = v = x x v = 2.0m x 2.0(1/s) = 4.0m/s

v = x v = 0.1m x 4(1/s) = 0.4m/s

Practice ProblemsPractice Problems

What is the wavelength of an What is the wavelength of an earthquake wave if it has a speed of earthquake wave if it has a speed of 5km/s and a frequency of 10Hz?5km/s and a frequency of 10Hz?

A wave travels at 1.8m/s and have a A wave travels at 1.8m/s and have a wavelength of 1.2m. What is the wavelength of 1.2m. What is the frequency of this wave?frequency of this wave?

v = x = v/ = 5km/s / 10(1/s) = 0.5km

v = x = v/ = 1.8m/s / 1.2m = 1.5Hz

SpeedSpeed If you assume that waves are If you assume that waves are

traveling at a traveling at a constant speedconstant speed, then , then wavelength is wavelength is inversely proportionalinversely proportional to frequency.to frequency.

If wavelength If wavelength increasesincreases, then , then frequency frequency decreasesdecreases..

If wavelength If wavelength decreasesdecreases, then , then frequency frequency increasesincreases..

AmplitudeAmplitude

The The amplitudeamplitude of a wave is the of a wave is the maximum maximum displacement of the displacement of the medium from its rest position.medium from its rest position.

The more The more energyenergy a wave has, the a wave has, the greater is its greater is its amplitudeamplitude..

Section 17.2 AssessmentSection 17.2 Assessment

How is wavelength related to How is wavelength related to frequency for waves moving at a frequency for waves moving at a constant speed?constant speed?

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

If you double the frequency of a If you double the frequency of a wave, what is the effect on its wave, what is the effect on its wavelength?wavelength?

Section 17.2 AssessmentSection 17.2 Assessment

A wave on a rope has a frequency of A wave on a rope has a frequency of 3.3Hz and a wavelength of 1.2m. 3.3Hz and a wavelength of 1.2m. What is the speed of the wave?What is the speed of the wave?

v = x v = 1.2m x 3.3(1/s) = 4.0m/s

Section 17.3 – Behavior Section 17.3 – Behavior of Wavesof Waves

ReflectionReflection occurs when a wave occurs when a wave bouncesbounces off a surface that is cannot off a surface that is cannot pass through.pass through.

ReflectionReflection does not change the does not change the speed or frequencyspeed or frequency of a wave, but of a wave, but the wave can be flipped upside the wave can be flipped upside down.down.

RefractionRefraction

RefractionRefraction is the is the bendingbending of a wave of a wave as it enters a new medium at an as it enters a new medium at an angleangle..

When a wave enters a medium at an When a wave enters a medium at an angleangle, refraction occurs because one , refraction occurs because one side of the wave moves more side of the wave moves more slowlyslowly than the other side.than the other side.

DiffractionDiffraction

DiffractionDiffraction is the is the bendingbending of a wave of a wave as it moves around an as it moves around an obstacleobstacle or or passes through a passes through a narrow openingnarrow opening..

A wave diffracts A wave diffracts moremore if its if its wavelength is wavelength is largelarge compared to the compared to the size of an opening or obstacle.size of an opening or obstacle.

InterferenceInterference

InterferenceInterference occurs when two or occurs when two or more waves overlap and more waves overlap and combinecombine together.together.

Two types of interference are Two types of interference are constructive interference and constructive interference and destructive interferencedestructive interference..

Constructive Constructive InterferenceInterference

Constructive InterferenceConstructive Interference occurs occurs when two or more waves combine to when two or more waves combine to produce a wave with a produce a wave with a larger larger displacementdisplacement..

Destructive InterferenceDestructive Interference

Destructive interferenceDestructive interference occurs occurs when two or more waves combine to when two or more waves combine to produce a wave with a produce a wave with a smaller smaller displacementdisplacement..

Standing WavesStanding Waves A A standing wavestanding wave is a wave that is a wave that

appears to appears to staystay in one place. in one place. A A nodenode is a point on a standing wave is a point on a standing wave

that has that has no displacementno displacement from the from the rest position.rest position.

An An antinodeantinode is a point where a is a point where a crest crest or troughor trough occurs midway between occurs midway between two nodes.two nodes.

Section 17.3 AssessmentSection 17.3 Assessment How is a wave changed by reflection?How is a wave changed by reflection? What causes refraction when a wave What causes refraction when a wave

enters a medium at an angle?enters a medium at an angle? What determines how much a wave What determines how much a wave

diffracts when it encounters an opening diffracts when it encounters an opening or an obstacle?or an obstacle?

How does the frequency of a reflected How does the frequency of a reflected wave compare with the frequency of wave compare with the frequency of the incoming wave?the incoming wave?

What is the amplitude of a wave that What is the amplitude of a wave that results when two identical waves results when two identical waves interfere constructively?interfere constructively?

Section 17.4 – Sound and Section 17.4 – Sound and HearingHearing

Sound waves are Sound waves are longitudinal waveslongitudinal waves that travel through a that travel through a mediummedium..

Many behaviors of sound can be Many behaviors of sound can be explained using a few properties – explained using a few properties – speed, intensity and loudness, and speed, intensity and loudness, and frequency and pitchfrequency and pitch..

SpeedSpeed

In dry air at 20In dry air at 20ooC, the speed of sound C, the speed of sound is is 342 m/s342 m/s..

In general, sound waves travel In general, sound waves travel fastest in fastest in solidssolids, slower in , slower in liquidsliquids, , and slowest in and slowest in gasesgases. .

This is due to the fact that particles This is due to the fact that particles in a solid tend to be in a solid tend to be closer togethercloser together than particles in a liquid or a gas. than particles in a liquid or a gas.

Intensity and LoudnessIntensity and Loudness

IntensityIntensity is the rate at which a wave’s is the rate at which a wave’s energyenergy flows through a given area. flows through a given area.

Sound intensity depends on both the Sound intensity depends on both the wave’s wave’s amplitudeamplitude and the and the distancedistance from the sound source. from the sound source.

The The decidel (dB)decidel (dB) is a unit that is a unit that compares the intensity of different compares the intensity of different sounds.sounds.

LoudnessLoudness is a physical response to is a physical response to the intensity of sound.the intensity of sound.

Frequency and PitchFrequency and Pitch

PitchPitch is the is the frequencyfrequency of a sound as of a sound as you perceive it. you perceive it.

High-frequency sounds have a High-frequency sounds have a high high pitchpitch, and low-frequency sounds , and low-frequency sounds have a have a low pitchlow pitch..

UltrasoundUltrasound

UltrasoundUltrasound is sound at frequencies is sound at frequencies higherhigher than most people hear. than most people hear.

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

SonarSonar

SonarSonar is a technique for determining is a technique for determining the distance to an object the distance to an object underwaterunderwater..

Sonar stands for Sonar stands for sound navigation sound navigation and rangingand ranging..

Doppler EffectDoppler Effect

The The Doppler effectDoppler effect is a change in is a change in sound sound frequencyfrequency caused by caused by motionmotion of the sound source, motion of the of the sound source, motion of the listener, or both.listener, or both.

As a source of sound As a source of sound approachesapproaches, an , an observer hears a observer hears a higher frequencyhigher frequency. . When the sound source moves When the sound source moves awayaway, , the observer hears a the observer hears a lower lower frequencyfrequency..

Section 17.4 AssessmentSection 17.4 Assessment

List five properties used to explain List five properties used to explain the behavior of sound waves.the behavior of sound waves.

Names two uses for ultrasound.Names two uses for ultrasound. What is the Doppler effect?What is the Doppler effect? If workers in a distant stone quarry If workers in a distant stone quarry

are blasting, why can you feel the are blasting, why can you feel the explosion in your feet before you explosion in your feet before you hear it?hear it?