Waves and Sound

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Waves and Sound

• Honors Physics
• Chapter 14

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Definition of a wave

• A disturbance that propagates from 1 place to
  another.
• Characterized by a large transfer of energy
  without a large transfer of medium

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Types of waves

• Mechanical waves require a medium (air, water,
  ropes) to travel
• Electromagnetic waves do not require a medium to
  travel (light, radio)
• Matter waves produced by electrons and particles

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Mechanical Waves

• Transverse the displacement of the individual
  particles is perpendicular to the direction of
  propagation.

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Mechanical Waves

• Longitudinal The displacement of the individual
  particles is parallel to the direction of
  propagation.

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Mechanical Waves

• Surface waves the displacement of individual
  particles is circular (result of both transverse
  and longitudinal motion)

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Waves in motion

• Longitudinal and Transverse Wave Motion

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Properties of waves

1. Amplitude (A) the maximum displacement from
   equilibrium position, measured in meters.
2. Wavelength ( ?) the distance between 2 particles
   that are in phase with each other, measured in
   meters.

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Wave diagram
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Properties of waves

• 3. Frequency (f) number of complete waves that
  pass a point in one second, measured in 1/seconds
  or Hertz (Hz)
• 4. Period (T) the time it takes for one complete
  wave to pass a given point, measured in seconds.

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Properties of waves

• Velocity of propagation (v) horizontal speed of
  a point on a wave as it propagates, measured in
  m/s.

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Relationships/equations

• T 1/ f
• or f 1/T
• v f ?

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Phase

• In-phase when waves are synchronized (crest
  meets crest)
• Out-of-phase waves are not synchronized
• Opposite phase (180º out-of-phase) crest meets
  trough

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Reflections

• Fixed End
• Explanation
• Animation
• Open End
• Explanation
• Animation
• Between different mediums scroll down (What do
  you notice about the phases? Transmitted?
  Reflected?)

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Principle of Superposition

• Occurs when two waves travel through the same
  medium at the same time.
• Each wave affects the medium independently.
• The displacement of the medium is the algebraic
  sum of the displacements.
• Animation (scroll down)

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Interference

• Constructive occurs when wave displacements are
  in the same direction (in-phase)
• Destructive occurs when wave displacement are in
  different directions (out-of-phase)
• animation

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What is Sound?

• Longitudinal
• Mechanical
• Rarefaction low air pressure
• Compression high air pressure
• module
• animation

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Speed of sound

• Depends on
• Temperature
• V air 331m/s (0.6 m/s/ºC)T
• Density/kind of medium
• Gases ltliquidsltsolids
• Chart of speeds

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Speed of sound
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Pitch

• How we perceive variations in frequency
• Audible range 20-20,000 Hz (listen)
• Most sensitive to 1,000 to 5,000Hz
• Loudness can distort our perception of pitch
  (listen tape)
• module

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Doppler Effect

• Variation in the frequency of sound due to the
  relative motion of the sound source or the
  listener.
• Animation1
• Picture of a sonic boom
• Video of sonic boom
• Space shuttle breaking sound barrier

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Doppler Effect Results

• As an moving sound source approaches a listener
  the frequency (pitch) increase.
• As a moving sound source passes by a listener the
  frequency (pitch) decreases.
• Same effect if sound source is stationary and
  listener is moving.

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Calculating Frequency change

• f f (v vR / v vs)
• f new frequency
• f original frequency of source sound
• v velocity of sound
• vR velocity of receiver
• vs velocity of source

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Loudness

• How we perceive variations in amplitude and
  intensity.
• Module
• In general, sound waves of higher intensity sound
  louder but we are not equally sensitive to all
  frequencies.

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Sound Intensity

• The amount of energy that passes through a given
  area in a given time.
• IP/A
• Ppower (watts)
• A area (m2)
• I intensity (W / m2)
• Directly proportional to the square of the
  amplitude.
• Inversely proportional to the square of the
  distance from the source.

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Relative Intensity

• Logarithmic scale used to indicate the intensity
  level of a sound.
• Measured in decibels or dB
• ß 10 log (I / Io)
• Io 110-12 W/m2 (intensity of the faintest sound
  that can be heard)
• I intensity of sound in W/m2

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How loud is a decibel?

• Threshold of hearing( Io ) 0dB (air pressure
  210-5 Pa)
• Threshold of pain 120 dB (air pressure 20 Pa)
• We perceive a 10 dB increase as twice as loud.
• Every 20 decibels air pressure increases 10 times
• Table of sound levels

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Resonance

• Causing the vibration of an object by the
  influence of another vibrating body.
• Must match the natural frequency of vibration of
  the object
• Whole-number multiple of the natural frequency
  work too.
• DEMO (wood blocks)

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Standing waves

• Caused by the interference of reflected waves
  with incident waves from the source.
• Nodes pts of no displacement
• Antinodes pts of maximum displacement
• Animation
• applet

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Vibrating Columns of Air

• Column will emit a sound when the air inside
  achieves resonance.
• DEMO (cardboard moose call)
• Frequency of vibration depend on
• Length of column
• Type of column
• Open end
• Closed end

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Fundamentals and Harmonics

• Fundamental Frequency lowest frequency of
  vibration
• Harmonics whole number multiples of the
  fundamental
• Note the fundamental frequency is the 1st
  harmonic.
• Animation

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Closed (at one end) pipe

• The standing wave created has
• Node at closed-end
• Antinode at open-end

A
N
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Calculations for closed pipe

• Fundamental frequency
• f1 v /4L (Llength of air column)
• Harmonics
• fn nf1 (n1,3,5,)
• ? 4L/n
• Note only ODD harmonics are produced
• Animation

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Open pipe

• The standing wave created has
• Antinode at both ends

A
A
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Calculations for open pipe

• Fundamental frequency
• f1 v /2L (Llength of air column)
• Harmonics
• fn nf1 (n1,2,3,)
• ? 2L/n
• Note ALL harmonics are produced
• animation

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Frequency

• Light
• Sound (listen)
• BACK