Physics 108 lecture 13: Wave Phenomena

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Physics 108 lecture 13 Wave Phenomena

• Doppler EffectThe Doppler effect describes a
  change in frequency (pitch) of sound waves due to
  a moving source or moving observer. Example
  train approaches with high pitched whistle,
  passes by, and pitch decreases.
•  
• Source moves toward observer ________away from
  observer________
• Observer moves toward source _______away from
  source _________ 

Source http//hyperphysics.phy-astr.gsu.edu/hbase
/sound/imgsou/dopp2.gif
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• Doppler Effect in Light
• Red Shift - light from objects receding (moving
  away) from us is shifted to the red side of the
  spectrum
• Blue Shift - light from objects approaching
  (moving toward) us is shifted to the blue side of
  the spectrum

Doppler Effect Equations Stationary
observer Stationary source  
getting closer - moving away
moving away - getting closer
vo observer velocity fo observed frequency vs
source velocity fs source frequency v speed
of sound
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Example (Doppler Effect) A storm is formulating
with winds of up to 150km/hr. A Doppler radar
device is monitoring the storm by sending out a
35MHz signal? What frequency will bounce back to
the station if the storm winds are A)
approaching? B) receding ?
Given                    Path                  
Want           Conversions/Equations
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Sound Intensity

• Intensity power divided by area
• Units W/m2
• Inverse Square Law

Decibels measure intensity relative to the
minimum intensity we can hear. The decibel is a
__________ scale. Our hearing works on this
scale. 10 dB increase ? increase by factor of 10
in intensity 20 dB increase ? increase by factor
of 100 in intensity 30 dB increase ? increase by
factor of 1000 in intensity and so on Decibel
Equation
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Example (Inverse Square Law) You are at a party
next to stereo speakers at a distance of 0.3m
with an intensity 4.0x10-2 W/m2. What distance
do you need to move to in order to reduce the
intensity by a factor of 100?
Given                    Path                
Want           Conversions/Equations
Example (Decibels) How many decibels was the
music at each location above?
Given                    Path              
Want         Conversions/Equations