Adv Physics

1
Adv Physics

• Chapter 14
• Sections 3 and 5

2
Doppler Effect

• How does the speed of the source effect your
  results?
• http//www.upscale.utoronto.ca/GeneralInterest/Har
  rison/Flash/ClassMechanics/DopplerWaveFronts/Doppl
  erWaveFronts.swf

3
Doppler Effect

• What does the wave pattern look like when you are
  traveling just below the speed of the wave?
• The waves compress in front of you

4
Doppler Effect

• What does the wave pattern look like when you are
  traveling at the speed of the wave?
• The waves in front of you compress into a single
  wave front
• In terms of sound you have created a region with
  a dramatic pressure change
• - called the sound barrier

5
Doppler Effect

• What does the wave pattern look like if you are
  traveling faster than the speed of the wave?
• You out run the waves and a cone-shaped pattern
  is formed
• -called a shock wave

6
Shock Wave

• commons.wikimedia.org/wiki/FileSonic_boom.svg
• Along the edges of the cone a large pressure
  ridge is formed
• When the edge of the cone reaches the ear of a
  listener a loud boom is heard
• -called a sonic boom

7
Sonic Boom

• Common misconception that sonic boom is heard
  when a plane breaks the sound barrier
• Sonic boom is heard whenever a listener comes in
  contact with the edge of the shock wave

8
Shock Wave

• Is possible for a shock wave to break windows and
  cause damage to structures
• Pilots are instructed to fly supersonically at
  high altitudes and away from populated areas

9
Intensity

• How a wave spreads its power over an area
• -power divided by the cross-sectional area
  perpendicular to the direction of wave motion
• I P/A where I intensity
• P power
• A cross sectional area
• I W/m2

10
Sample Problem

• Suppose 12 x 10-5 W of power pass perpendicularly
  through 2 surfaces having areas of 4 m2 and 12
  m2. Find the sound intensity at each surface.

11
Range of Intensities

• Threshold of hearing smallest intensity the
  human ear can detect
• I0 12 x 10-5 W/m2 at 1000 Hz
• Threshold of pain intensity that is so large
  that it causes the human ear actually pain, 1
  W/m2

12
Sample Problem

• During a fireworks display a rocket explodes high
  in the air. Sound spreads out uniformly. When
  the sound reaches a listener 640 m away the sound
  has an intensity of 0.1 W/m2. What is the
  intensity detected by a listener 160 m away?

13
Intensity Level

• Measure of the intensity of sound relative to a
  reference intensity using the logarithmic scale
• ß 10 log (I /I0)
• where ß intensity level
• I intensity
• I0 reference intensity (usually
  threshold
• of hearing)
• ß decibels, dB

14
Notes about Decibels

• Threshold of hearing is 0 dB
• Threshold of pain is 120 dB
• 0 dB doesnt mean no sound it means the sound
  has the same intensity as the threshold of
  hearing
• Doubling the intensity doesnt double the
  loudness (takes 10 dB increase to do that)
• 1 dB change is the smallest change in loudness
  detectable

15
Decibels

• http//www.nidcd.nih.gov/staticresources/health/ed
  ucation/decibel/decible4.swf
• Sound levels of 90 dB and above will damage
  receptor nerves in the ear resulting in a loss of
  hearing

16
Sample Problem

• Two people talk simultaneously. If the intensity
  level is 60 dB when either one speaks alone, what
  is the intensity level when both speak at once?

17
Sample Problem

• When a 100 dB sound wave comes through an open
  window of area 0.5 m2, how much acoustic energy
  passes through the window in a 10 min interval?