Pearson Prentice Hall Physical Science: Concepts in Action

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Pearson Prentice Hall Physical Science Concepts
in Action

• Chapter 17
• Mechanical Waves and Sound

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17.1 Mechanical Waves

• Objectives
• 1. Explain what causes mechanical waves
• 2. Name and describe the three main types of
  mechanical waves

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What Causes Mechanical Waves

• Def A wave is a disturbance that carries energy
  through space or matter
• Def What a wave travels through is the medium
  (sound-air, earthquake-ground)
• Def Waves that require a medium to travel are
  called mechanical waves (majority of waves)
• Mechanical waves carry energy from one place to
  another by using matter (a medium)
• Mechanical waves are started with a vibration

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Types of Mechanical Waves

• The three main types of mechanical waves are
  transverse waves, longitudinal waves and surface
  waves
• Particles in a medium can vibrate up and down or
  back and forth as a wave moves by
• Def If the particles move up and down they will
  move perpendicular to the direction of the
  wave-called a transverse wave

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• Transverse waves take the shape of sine curves
  (looks like an s on its side)
• Def High points are crests
• Def Low points are troughs
• Def Difference between high and low is called
  amplitude
• Bigger amplitudes mean more energy
• Def in a longitudinal wave the medium moves in
  the same direction as the wave

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• Def In longitudinal waves the bunched area is a
  compression (think of a slinky)
• Def The spaced out areas are called rarefactions
  
• Def Amplitude on a longitudinal wave is maximum
  deviation from normal density or pressure
• At the boundary between two mediums (on the ocean
  for example) surface waves develop
• Def Surface waves are combinations of both types
  of waves
• The particles in the medium of a surface wave
  move back and forth and up and down resulting in
  a circle, but they end up where they started

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17.2 Properties of Mechanical Waves

• Objectives
• 1. Explain what determines the frequency of a
  wave
• 2. Solve problems for frequency, wavelength and
  speed
• 3. Describe how amplitude and energy are related

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Frequency

• Def periodic motion is any motion that repeats
  at regular time intervals
• Def Frequency is how many waves pass by in a
  given time
• Frequency 1/period 1 / T f
• It is measure in hertz (Hz) which is 1/s where s
  is seconds
• Humans can hear 20Hz to 20000Hz
• A waves frequency equals the frequency of the
  vibrating source producing the wave

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Problems involving f, ? (wavelength) speed

• Def Wavelengths of waves are measured from one
  crest to the next (crest to crest OR trough to
  trough) or from one compression to the next
• It is represented by the Greek letter lambda
  whose symbol is ?
• The period of a wave is how long it takes for a
  complete wave to go by a spot, symbol is T
• Increasing the frequency (f) of a wave decreases
  the wavelength (?)

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• Recall that speed distance / time
• For waves we can use wavelength for distance (in
  meters, m) and period for time (s for seconds)
• Speed wavelength X frequency
• v ? f
• The speed of a wave depends on what it travels
  through (medium)
• If the medium particles are closer together the
  energy from the wave can make vibrations easier
• So waves travels best in solids, then liquids,
  and worse in air

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Amplitude and Energy

• Def amplitude is the difference between crest
  and the rest position or point of origin
• Def the rest position or point of origin is an
  imaginary line through the middle of the wave
  that separates the crest from the trough
• The more energy a wave has, the greater its
  amplitude

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17.3 Behavior of Waves

• Objectives
• 1. Explain reflection and refraction and how they
  affect waves
• 2. Identify several factors that affect the
  amount of wave diffraction
• 3. Discuss two types of interference
• 4. Explain what a standing wave is and detail the
  wavelengths that produce it

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Reflection and Refraction

• Def Reflection occurs when a wave meets a
  boundary and bounces off
• The wave cannot pass through the surface
• Reflection does not change the speed or frequency
  of a wave, but the wave can be flipped upside
  down
• Def Refraction is the bending of a wave as it
  travels through different mediums
• When a wave enters a medium at an angle,
  refraction occurs because one side of the wave
  moves more slowly than the other side (ex pencil
  in water)

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Diffraction and Interference

• Def Diffraction is the bending of a wave around
  an obstacle
• A wave diffracts more if its wavelength is large
  compared to the size of an opening or obstacle
• The two types of interference are constructive
  interference and destructive interference

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• The combo of two or more waves at the same place
  at the same time causes interference
• Def When two crests meet the interference is
  constructive-the wave becomes more energetic-the
  amplitude increases
• Def When a crest and a trough meet they cancel
  each other and decrease the amplitude
  destructive

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Standing Waves

• Interference may cause standing waves-
• Def Standing waves appear not to move along the
  medium
• Has areas of destructive interference where there
  is no vibration called nodes
• Def a node is a point on a standing wave that
  has no displacement from the rest position

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• Areas of maximum interference called antinodes
• Def an antinode is a point where a crest
  trough meet midway between 2 nodes
• A standing wave forms only if half a wavelength
  or a multiple of half a wavelength fits exactly
  into the length of a vibrating cord

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17.4 Sound and Hearing

• Objectives
• 1. Describe the properties of sound waves and
  explain how sound is produced and reproduced
• 2. Describe how sound waves behave in
  applications such as ultrasound and music
• 3. Explain how relative motion determines the
  frequency of sound as the observer hears

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Properties of Sound Waves

• Sound waves are longitudinal waves
• They have compressions are rarefactions
• Many behaviors can be explained by the properties
  of speed, intensity, loudness, frequency and
  pitch
• At 20C in dry air, the speed of sound is 342 m/s
• Sound waves travel fastest in solids, slower in
  liquids and slowest in gases

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• Def intensity is the rate at which a waves
  energy flow through a given area
• The decibel (dB) compares the intensity of
  different sounds
• Def loudness is a physical response to the
  intensity of sound modified by physical factors
• As intensity increases, loudness increases
• Loudness also depends on the health of your ears
  and how your brain interpret sounds
• Def pitch is the frequency of a sound as you
  perceive it

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How Sound Waves Behave Relative Motion

• Ultrasound is used in a variety of applications,
  including sonar and ultrasound imaging
• Def Sonar is a technique for determining the
  distance to an object under water
• The pitch of a sound is determined by the
  frequency
• Higher pitch means faster frequency
• As the source of the waves moves it changes the
  frequency (this is the Doppler Effect)
• As it moves toward you the pitch rises and away
  from you the pitch lowers Doppler Effect

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• For us to hear, the outer ear gathers focuses
  sound into the middle ear where the vibrations
  are received and amplified
• The inner ear uses nerve endings to sense
  vibrations and send signals to the brain
• Sound is recorded by converting sound waves into
  electronic signals that can be processed and
  stored
• Sound is reproduced by converting electronic
  signals back to sound waves

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• Most musical instruments vary pitch by changing
  the frequency of standing waves
• Def resonance is the response of a standing wave
  to another wave of the same frequency
• Musical instruments often use resonance to
  amplify sound