Chapter 17 Mechanical Waves

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Chapter 17Mechanical Waves Sound
WHY IS THIS???
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17.1 Mechanical Waves
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What types of waves can you identify in a wave
pool _at_ at amusement park?

• In a wave pool, energy is being carried across
  the pool in the form of waves.
• The sounds of laughter and talking in the pool
  are also being carried by waves. 

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Mechanical Waves

• Mechanical wave- a disturbance in matter that
  carries energy from one place to another
• You can see the effects of a waves energy in the
  wave pool when it lifts people in the water. 
• Mechanical waves require matter to travel
  through. 

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Medium

• Medium- material through which a wave travels
• solids, liquids, and gases can all act as
  mediums
• Waves travel through a rope when you shake one
  end of it. Rope medium

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How are mechanical waves created?

• Vibration- a repeating back-and-forth motion
• A mechanical wave is created when a source of
  energy causes a vibration to travel through a
  medium. 

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3 Types of Mechanical Waves

• There are three (3) main types of mechanical
  waves
• Transverse
• Longitudinal
• Surface

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Transverse Waves

• Transverse wave- wave that causes the medium to
  vibrate at right angles to the direction in which
  the wave travels
• These waves carry energy from left to right in a
  direction perpendicular to the up and down motion
  of the medium
• Example shaking one end of a rope up and down 

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Parts of a Transverse Wave

• Crest- the highest point of the wave above the
  rest position
• Trough- the lowest below the rest position 

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Parts of a Transverse Wave

• Identify points B, D, F, H as either crests or
  troughs.

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QUESTION

• How does the direction of a transverse wave
  compare with the direction of the medium? 
• Answer They are opposite/perpendicular! 

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Longitudinal Waves

• Longitudinal wave- wave in which the vibration of
  the medium is parallel to the direction the wave
  travels
• Example Sound
• Wave in spring moving BACK and FORTH (not up and
  down)
• P-Waves - longitudinal waves produced by
  earthquakes

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Parts of a Longitudinal Wave

• Compression- the area where the particles in a
  medium are spaced close together
• Rarefaction- the area where the particles in a
  medium are spread out

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Parts of a Longitudinal Wave

• Label at least one compression and one
  rarefaction on the longitudinal wave below

Compression
Rarefaction
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Question

• How does the direction of a longitudinal wave
  compare with the direction of the medium? 
• Answer They are the same/parallel! 

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Surface Waves

• Surface wave- wave that travels along a surface
  separating two media
• Example- ocean waves
• They occur at the surface between water and air
• A bobber floating in the waves will travel in a
  circle because the motion from these surface
  waves is both up-and-down and back-and-forth like
  in the transverse and longitudinal waves
• Most waves do not move matter from one place to
  another

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Most waves do not move matter from one place to
anotherhowever

• When the waves approach the shore they behave
  differently
• What causes a wave to break?
• As a wave enters shallow water the bottom of the
  wave has too much friction acting on it from the
  seafloor, while the top of the wave continues at
  its original speed
• As a result, the wave carries the medium and
  anything in it toward the shore

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17.2 Properties of Mechanical Waves
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Surfing

• Question How do surfers know when the next wave
  is coming? 
• Answer They can count the time between crests,
  and the next crest will usually follow this
  pattern. 
• They can do this because waves follow periodic
  motion. 

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Periodic Motion

• Periodic motion- any motion that repeats at
  regular time intervals
• Many things display periodic motion

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The surfers were timing the PERIOD

• Period- time required for one cycle, a complete
  motion that returns to its starting point
• For an ocean wave, the period is the time between
  2 successive crests

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Frequency

• To find how many crests pass in a given time, you
  need to know the frequency  
• Frequency- number of complete cycles in a given
  time
• The frequency of a wave is how many wave cycles
  pass a point in a given time
• Frequency is measured in cycles per second, or
  hertz  (Hz)
• A waves frequency equals the frequency of the
  vibrating source producing the wave 

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Period Frequency
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Wavelength

• Wavelength- distance between a point on one wave
  and the same point on the next cycle of the
  waves.
• For a transverse wave, the wavelength is measured
  between adjacent crests or between adjacent
  troughs
• For longitudinal waves, the wavelength is the
  distance between the center of a compression to
  the center of the next compression

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How to measure wavelength
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Wavelength Frequency

• Question When wavelengths are shorter, the
  crests are closer together, is the frequency 
  higher or lower? 
• Answer HIGHER! 
• Increasing the frequency of a wave decreases its
  wavelength! 

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Which has the highest frequency? _______Which
has the lowest frequency? _______ Which has the
shortest wavelength? _______ Which has the
longest wavelength? _______
A
B
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Wave Speed

• To determine how fast a wave is traveling,
  remember how we find speed distance/time
• In waves, distance is like wavelength, and time
  is period. 
• So we can solve for the speed of a wave using the
  following formula 
• Speed WAVELENGTH  x FREQUENCY

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Solving for Wave Speed

• The units for wave speed are the same as the
  units for speed we covered earlier
• meters per second (m/s)
• If we keep speed constant, wavelength is
  inversely proportional to frequency
• What does this mean if you have 2 waves with
  different frequencies?
• The wave with the lower frequency has a longer
  wavelength
• Refer to Math Skills on page 506 for practice 

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AmplitudeEnergy

• Consider the difference in the waves you would
  see if you dropped a pebble into the water
  compared to a cannonball. 
• Which waves would be higher?
• The cannonball

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Amplitude Energy

• Amplitude- the maximum displacement of the medium
  from its rest position
• Considered the height of the wave
• The more energy a wave has, the greater its
  amplitude  

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17.3 Behavior of Waves

• What do you think of when you hear the word
  reflection?

Nowrelate that thought to waves!!!
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Wave Reflection

• Reflection- occurs when a wave bounces off a
  surface that it cannot pass through, ex echo
• The reflection of a wave is like the reflection
  of a ball thrown at a wall. The ball cannot go
  through the wall so it bounces back. 
• If you send a transverse wave down a rope
  attached to a wall, the wave reflects when it
  hits the wall

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Wave Reflection
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Wave Reflection

• Reflection does not change the speed or frequency
  of a wave, but the wave can be flipped upside
  down. 
• If the reflection occurs at a fixed boundary
  (wall), then the reflected wave will be
  upside-down compared to the original wave (see
  picture on previous slide)

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Wave Refraction

• Refraction- the bending of a wave as it enters a
  new medium at an angle
• As an ocean wave approaches the shore at an
  angle, the wave bends, or refracts toward shore
  because one side of each wave front hits before
  the other side does as it gets into shallow
  water. 
• Refraction of the wave occurs only when the two
  sides of a wave travel at different speeds. 

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Wave Refraction

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

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Wave Diffraction

• Diffraction- the bending of a wave as it moves
  around an obstacle or passes through a narrow
  opening.
• The pattern is very similar to the circular
  ripples you see when a pebble is tossed into a
  pond. 
• A wave diffracts more if its wavelength is large
  compared to the size of an opening or obstacle  

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

• The pattern is very similar to the circular
  ripples you see when a pebble is tossed into a
  pond. 

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Interference

• Interference occurs when two or more waves
  overlap and combine together
• There are two types of interference
• constructive interference
• destructive interference 

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Interference (2 Types)
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Constructive Interference

• Constructive interference- occurs when two or
  more waves combine to produce a wave with a
  larger displacement (taller), ex crest meets
  crest

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Destructive Interference

• Destructive interference- occurs when two or more
  waves combine to produce a wave with a smaller
  displacement, ex crest meets trough

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Standing Waves

• Standing waves- wave that appears to stay in one
  place it does not seem to move through the
  medium
• Interference from reflected wave
• You can observe one if you pluck a guitar
• Only certain points are stationary- called
  nodes. 

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17.4 Sound Hearing
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Sound Waves

• QUESTION What type of waves are sound waves?
• Hint compressions and rarefactions travel
  through a medium in sound waves 
• ANSWER Longitudinal Waves

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Sound Waves
Here's what sound waves look like. The caption
reads, "A visible pattern of sound waves. This
new technique of studying sound demonstrates the
focusing effect of an acoustical lens on sound
waves issuing from the horn at extreme left.
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Properties of Sound

• Some properties of sound
• Speed
• Intensity
• Loudness
• Frequency
• Pitch

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Speed of a Wave

• It takes time for sound to travel
• travels at 342 m/s

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Speed of Sound Varies

• Refer to Figure 14 on page 514 for the speeds of
  sound in different media!
• Generally, sound waves travel fastest in solids,
  slower in liquids, and slowest in gases
• This is partly due to the spacing of the
  particles in each phase 

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Intensity

• Intensity- rate at which a waves energy flows
  through a given area
• Sound intensity depends on both waves amplitude
  and the distance from the sound source
• Measured in decibel (dB)
• See Figure 15 Sound Intensity Level
• Whisper 15-20 dB
• Rock Concert 110-120 dB
• Jet Plane (taking off) 120-160 dB

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• Intensity depends on waves amplitude (energy)
  and distance from the sound source
• If someone whispers in your ear, the sound
  intensity may be greater than when someone shouts
  at you from the other end of a field

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Loudness

• Loudness- physical response to the intensity of
  sound, modified by physical factors
• Subjective, open to interpretation 
• As intensity increases, loudness increases,
  however
• Loudness also depends on factors such as the
  health of your ears and how your brain interprets
  the information in sound waves!

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Frequency of a Sound Wave

• Vibration becomes faster as sound frequency
  becomes higher
• The frequency of a sound wave depends on how fast
  the source of the sound is vibrating
• In musical instruments, the size of the tubing
  through which air moves produces different
  frequencies
• Longer tubing longer wavelength lower
  frequency

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Pitch of a Sound Wave

• frequency of a sound as you perceive it
• High-frequency sounds have a high pitch
• Low-frequency sounds have a low pitch
• BUT pitch, like loudness, depends on other
  factors such as your age and health of your ears!

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Ultrasound

• Most people hear sounds between 20 Hz and 20,000
  Hz.
• Infrasound sound at frequencies lower than most
  people hear
• Ultrasound sound at frequencies higher than most
  people hear
• Ultrasound is used in a variety of applications,
  sonar ultrasound imaging

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Sonar

• Sonar a technique for determining the distance
  to an object under water
• Stands for SOund NAvigation and Ranging
• Distance calculated by using the speed of sound
  in water and the time that the sound wave takes
  to reach and object and the echo takes to return

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Sonar
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Doppler Effect

• Doppler Effect - a change in sound frequency
  caused by motion of the sound source, motion of
  the listener, or both
• As a source of sound approaches, an observer
  hears a higher frequency
• When the sound source moves away, the observer
  hears a lower frequency
• This is due to the sound waves being close
  together and spread out 

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Doppler Effect
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Hearing the Ear

• You are able to pick up slight vibrations in the
  air from sound because your ear has a membrane
  that vibrates when a sound wave strikes it.
• The outer ear gathers, and focuses sound into the
  middle ear like a funnel
• The middle ear receives and amplifies the
  vibrations
• The inner ear uses nerve endings to sense
  vibrations and send signals to the brain to
  decode 

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Hearing the Ear
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Recording Sound

• Sound is recorded by converting sound waves into
  electronic signals that can be processed and
  stored
• Sound is reproduced by converting electronic
  signals back into sound waves
• Larger-diameter speakers are better at
  reproducing lower frequencies of sound (like a
  bass drum)
• Smaller-diameter speakers are better for
  reproducing higher frequencies of sound (like a
  small bongo drum) 

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Speakers
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Music

• Most instruments vary pitch by changing the
  frequency of standing waves
• Resonance- the response of a standing wave to
  another wave of the same frequency
• Think of a child being pushed on a swingif the
  pushes are timed at the right frequency, the
  child can swing higher and higher.
• In the same way, one wave can push another wave
  to a higher amplitude

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Resonance