Chapter 17 Mechanical Waves and Sound. 17.1 Mechanical Waves Mechanical Waves and Sound 17.1 Mechanical Waves

Chapter 17Chapter 17Mechanical Waves and Mechanical Waves and

SoundSound

17.1 Mechanical Waves17.1 Mechanical Waves Mechanical Waves and SoundMechanical Waves and Sound

17.1 Mechanical Waves17.1 Mechanical Waves


What are Mechanical Waves?What are Mechanical Waves?

A mechanical wave is a disturbance in A mechanical wave is a disturbance in matter that carries energy from one place matter that carries energy from one place to another. to another.

Mechanical waves require a medium, or Mechanical waves require a medium, or matter, to travel through. matter, to travel through.


A mechanical wave is created when a A mechanical wave is created when a source of energy causes a vibration to source of energy causes a vibration to travel through a medium. travel through a medium.

A vibration is a back-and-forth motion. A vibration is a back-and-forth motion.


Types of Mechanical WavesTypes of Mechanical Waves

Mechanical waves are classified by the Mechanical waves are classified by the way they move through a medium. way they move through a medium.

The three main types of mechanical waves The three main types of mechanical waves are are transverse (or transverse (or shearshear) waves ) waves longitudinal (or longitudinal (or compressionalcompressional) waves) waves surface wavessurface waves


Transverse WavesTransverse Waves

A transverse waves is a wave that causes A transverse waves is a wave that causes the medium vibrate at right angles to the the medium vibrate at right angles to the direction in which the wave travels. direction in which the wave travels.

Each point on the wave vibrates up and Each point on the wave vibrates up and down between a maximum and minimum down between a maximum and minimum height. height.

The highest point of the wave is the crest. The highest point of the wave is the crest. The lowest point is the trough. The lowest point is the trough.

17.1 Mechanical Waves17.1 Mechanical Waves Mechanical Waves and SoundMechanical Waves and SoundFigure 2




Longitudinal WavesLongitudinal Waves

A longitudinal wave is a wave in which the A longitudinal wave is a wave in which the vibration of the medium is parallel to the vibration of the medium is parallel to the direction the wave travels. direction the wave travels.

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

An area where the particles are spread out An area where the particles are spread out is called a rarefaction. is called a rarefaction.


As compressions and rarefactions travel As compressions and rarefactions travel along the wave, particles vibrate back and along the wave, particles vibrate back and forth around the rest position. forth around the rest position.





compression rarefaction


Surface WavesSurface Waves

A surface wave is a wave that travels A surface wave is a wave that travels along a surface separating two media. along a surface separating two media.

Surface waves have characteristics of Surface waves have characteristics of both transverse and longitudinal waves.both transverse and longitudinal waves.


Ocean waves are an example of a surface Ocean waves are an example of a surface wave. wave. When the wave crest passes a point, When the wave crest passes a point,

the particles move up. the particles move up. When the trough passes, the particles When the trough passes, the particles

move down. move down.


These two motions cause particles to These two motions cause particles to move in a circle. move in a circle.

Most waves do Most waves do notnot transport matter as the transport matter as the energy moves. energy moves.






17.2 Properties of Mechanical Waves17.2 Properties of Mechanical Waves Mechanical Waves and SoundMechanical Waves and Sound

17.2 Properties of Mechanical 17.2 Properties of Mechanical WavesWaves


Frequency and PeriodFrequency and Period

Any motion that repeats at regular Any motion that repeats at regular intervals is called periodic motion. intervals is called periodic motion.

The time required for one cycle, a The time required for one cycle, a complete motion that returns to its starting complete motion that returns to its starting point, is called the period (T). point, is called the period (T).

For surface waves, the period is the time For surface waves, the period is the time between two successive crests. between two successive crests.


Periodic motion has a frequency, which is Periodic motion has a frequency, which is the number of complete cycles in a given the number of complete cycles in a given time. time.

For a wave, the frequency (f) is the For a wave, the frequency (f) is the number of wave cycles that pass a point in number of wave cycles that pass a point in a given time. a given time.

Frequency and period are inversely Frequency and period are inversely related. related. ((f=1/f=1/T).T).


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

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


WavelengthWavelength

The distance between a point on one The distance between a point on one wave and the same point on the next cycle wave and the same point on the next cycle of the wave is wavelength. of the wave is wavelength.

For a transverse wave, wavelength is For a transverse wave, wavelength is measured between adjacent crests or measured between adjacent crests or troughs. troughs.

For a longitudinal wave, wavelength is For a longitudinal wave, wavelength is distance between adjacent rarefactions or distance between adjacent rarefactions or compressions. compressions.


Increasing the frequency of a wave Increasing the frequency of a wave decreases its wavelength. decreases its wavelength.

Wavelength is represented by Greek letter Wavelength is represented by Greek letter lambda, (λ). lambda, (λ).


FrequencyFrequency


FrequencyFrequency WavelengthWavelength


Wave SpeedWave Speed

The wave speed is calculated by The wave speed is calculated by dividing the wavelength by the period dividing the wavelength by the period

v v = λ/T= λ/T by multiplying the wavelength by by multiplying the wavelength by

frequency. frequency.

v v = = f f λλ


The speed of a wave can change if The speed of a wave can change if it enters a new medium or it enters a new medium or variables such as pressure and variables such as pressure and

temperature change. temperature change.

17.2 Properties of Mechanical Waves17.2 Properties of Mechanical Waves Mechanical Waves and SoundMechanical Waves and SoundSection 17.2





AmplitudeAmplitude

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

The amplitude of a transverse wave is the The amplitude of a transverse wave is the distance from the rest position to the crest or distance from the rest position to the crest or trough. trough.


For a longitudinal wave, the amplitude is For a longitudinal wave, the amplitude is the maximum displacement of a point from the maximum displacement of a point from its rest position. its rest position.

The more energy a wave has, the greater The more energy a wave has, the greater is its amplitude. is its amplitude.


AmplitudeAmplitude ofof Transverse WavesTransverse Waves

17.3 Behavior of Waves17.3 Behavior of Waves Mechanical Waves and SoundMechanical Waves and Sound

17.3 Behavior of Waves17.3 Behavior of Waves

As waves crisscross back and forth, many As waves crisscross back and forth, many interactions can occur, including reflection, interactions can occur, including reflection, refraction, diffraction, and interference. refraction, diffraction, and interference.


ReflectionReflection

Reflection occurs when a wave bounces Reflection occurs when a wave bounces off a surface that it cannot pass through. off a surface that it cannot pass through.

Reflection does not change the speed or Reflection does not change the speed or frequency of a wave, but the wave can be frequency of a wave, but the wave can be inverted. inverted. If reflection occurs at a fixed boundary, If reflection occurs at a fixed boundary,

the wave will be upside down (inverted) the wave will be upside down (inverted) compared to the original wave. compared to the original wave.



RefractionRefraction

Refraction is the bending of a wave as it Refraction is the bending of a wave as it enters a new medium at an angle. enters a new medium at an angle.

Refraction changes the direction of a Refraction changes the direction of a wave. wave.

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



Figure 10 shows how an ocean wave Figure 10 shows how an ocean wave bends or refracts as it approaches the bends or refracts as it approaches the shore at an angle because one side of shore at an angle because one side of each wave slows down before the other each wave slows down before the other side. side.

Refraction of the wave occurs only when Refraction of the wave occurs only when the two sides of a wave travel at different the two sides of a wave travel at different speeds.speeds.


Frequency cannot change; it depends on the frequency of the source.


DiffractionDiffraction

Diffraction is the bending of a wave as it Diffraction is the bending of a wave as it moves around an obstacle or passes moves around an obstacle or passes through a narrow opening. through a narrow opening.

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

If the wavelength is small compared to the If the wavelength is small compared to the opening, the wave bends very little. opening, the wave bends very little.



InterferenceInterference

When two objects collide, they cannot When two objects collide, they cannot continue on their original path. continue on their original path.

But waves can occupy the same region of But waves can occupy the same region of space and then continue on. space and then continue on.

Interference occurs when two or more Interference occurs when two or more waves overlap and combine together. waves overlap and combine together.


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

The displacements of waves combine to The displacements of waves combine to increase amplitude in constructive increase amplitude in constructive interference and to decrease amplitude in interference and to decrease amplitude in destructive interference. destructive interference.


Constructive InterferenceConstructive Interference

Constructive interference occurs when two Constructive interference occurs when two or more waves combine to produce a or more waves combine to produce a wave with a larger displacement. wave with a larger displacement.

Figure 12A shows how constructive Figure 12A shows how constructive interference produces a wave with an interference produces a wave with an increased amplitude.increased amplitude.


17.3 Behavior of Waves17.3 Behavior of Waves Mechanical Waves and SoundMechanical Waves and SoundFigure 12


Destructive InterferenceDestructive Interference

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

Destructive interference produces a wave Destructive interference produces a wave with a reduced amplitude.with a reduced amplitude.




Standing WavesStanding Waves

At certain frequencies, interference At certain frequencies, interference between a wave and its reflection can between a wave and its reflection can produce a standing wave.produce a standing wave.

A standing wave is a wave that appears to A standing wave is a wave that appears to stay in one place – it does not seem to stay in one place – it does not seem to move through the medium. move through the medium.


A node is a point on a standing wave that A node is a point on a standing wave that has no displacement from the rest has no displacement from the rest position. position.

At the nodes, there is complete destructive At the nodes, there is complete destructive interference between the incoming and interference between the incoming and reflected waves. reflected waves.


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

A standing wave forms only if half a A standing wave forms only if half a wavelength fits exactly into the length of a wavelength fits exactly into the length of a vibrating cord.vibrating cord.


17.4 Sound and Hearing17.4 Sound and Hearing Mechanical Waves and SoundMechanical Waves and Sound

17.4 Sound and Hearing17.4 Sound and Hearing


Properties of Sound WavesProperties of Sound Waves

Sound waves are longitudinal waves – Sound waves are longitudinal waves – compressions and rarefactions that travel compressions and rarefactions that travel through a medium. through a medium.

Many behaviors of sound can be Many behaviors of sound can be explained using a few properties explained using a few properties

– – speedspeed

– – intensity and loudnessintensity and loudness

– – frequency and pitchfrequency and pitch


SpeedSpeed

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

In general, sound waves travel fastest in In general, sound waves travel fastest in solids, slower in liquids, and slowest in solids, slower in liquids, and slowest in gases. gases.

The speed of sound depends on many The speed of sound depends on many factors, including the density of the factors, including the density of the medium and how elastic the medium is. medium and how elastic the medium is.





Intensity and LoudnessIntensity and Loudness

Intensity is the rate at which a wave’s Intensity is the rate at which a wave’s energy flows through a given area. energy flows through a given area.

Sound intensity depends on both the Sound intensity depends on both the wave’s amplitude and the distance from wave’s amplitude and the distance from the sound source. the sound source.

Sound intensity levels are measured in Sound intensity levels are measured in units called decibels. units called decibels.


A decibel is one tenth of a bel (B). A decibel is one tenth of a bel (B). Devised by engineers of Bell Telephone to Devised by engineers of Bell Telephone to

quantify the reduction in audio level over a 1 quantify the reduction in audio level over a 1 mile length of standard phone cable, the bel was mile length of standard phone cable, the bel was originally called the transmission unit or TU, but originally called the transmission unit or TU, but was renamed in 1923/24 in honor of Alexander was renamed in 1923/24 in honor of Alexander Graham Bell. Graham Bell.

In many situations, however, the bel proved In many situations, however, the bel proved inconveniently large, so the decibel has become inconveniently large, so the decibel has become more common.more common.


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

The decibel scale is a log scale, for every The decibel scale is a log scale, for every 10-dB increase, the intensity increases 10-dB increase, the intensity increases tenfold. tenfold.

A 0-dB sound is barely audible. A 0-dB sound is barely audible. Lengthy exposure to more than 90 dB can Lengthy exposure to more than 90 dB can

cause hearing damage. cause hearing damage.



Unlike intensity, loudness is subjective – it Unlike intensity, loudness is subjective – it is subject to a person’s interpretation. is subject to a person’s interpretation.

Loudness is a physical response to the Loudness is a physical response to the intensity of sound, modified by physical intensity of sound, modified by physical factors. factors.


Frequency and PitchFrequency and Pitch

The frequency of a sound wave depends The frequency of a sound wave depends on how fast the sound source is vibrating. on how fast the sound source is vibrating.

The size of a musical instrument tells The size of a musical instrument tells something about the frequency it can something about the frequency it can produce. produce.

The longer the tubing, the longer is the The longer the tubing, the longer is the wavelength of the standing wave, and the wavelength of the standing wave, and the lower the frequency produced. lower the frequency produced.


Pitch is the frequency of a sound as you Pitch is the frequency of a sound as you perceive it. perceive it.

Pitch does depend upon a wave’s Pitch does depend upon a wave’s frequency. frequency.

High frequency sounds have a high pitch, High frequency sounds have a high pitch, and low frequency sounds have a low and low frequency sounds have a low pitch. pitch.


UltrasoundUltrasound

Most people can hear sounds between 20 Most people can hear sounds between 20 Hz and 20,000 Hz. Hz and 20,000 Hz.

Infrasound is sound at frequencies lower Infrasound is sound at frequencies lower than the average person can hear. than the average person can hear.

Ultrasound is at frequencies higher than Ultrasound is at frequencies higher than most can hear. most can hear.


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

SONAR (sound navigation and ranging) is SONAR (sound navigation and ranging) is a technique for determining the distance to a technique for determining the distance to an object under water. an object under water.


Depth = ½ t X1500m/s



The Doppler EffectThe Doppler Effect

The Doppler effect is a change in frequency The Doppler effect is a change in frequency caused by motion of the source, the receiver, or caused by motion of the source, the receiver, or both. both.

As a source approaches, an observer receives As a source approaches, an observer receives a higher frequency because the waves are a higher frequency because the waves are bunched together producing a higher pitch. bunched together producing a higher pitch.


When the source moves away, the observer When the source moves away, the observer receives a lower frequency because the waves receives a lower frequency because the waves spread out producing a lower pitch.spread out producing a lower pitch.


The Doppler EffectThe Doppler Effect


Resonance is the response of a standing Resonance is the response of a standing wave to another of the same frequency.wave to another of the same frequency.

Resonance can produce a dramatic Resonance can produce a dramatic increase in amplitudeincrease in amplitude


Tacoma Narrows collapseTacoma Narrows collapse

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Chapter 17 Mechanical Waves and Sound. 17.1 Mechanical Waves Mechanical Waves and Sound 17.1 Mechanical Waves