Physics 211 lecture 28: Sound Waves

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Physics 211 lecture 28 Sound Waves

• Sound Waves - mechanical longitudinal waves
•  Sound waves come from periodic pressure
  variations moving along in a substance.

Sound Speed   Sound speed IN AIR at room
temperature (20 ?C) is _____________   Sound
speed equation (IN AIR only)
Note speed ? as density ? and
speed ? as elasticity (stiffness) ?
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Wave Equation for Sound
Recall
For transverse, we now have longitudinal
Max longitudinal displacement
Or in terms of pressure
Derivation in book
Where
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Sound Intensity
Intensity power (or energy transfer rate)
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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Doppler Effect
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
Note Storm is like observer moving toward storm.
Then, it bounces back signals with same
frequency it observed.
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Example Ch17 3Flowerpot 20m up falls towards
1.75m tall person. Find max time can wait before
shouting from top if person below needs 0.3s to
move.
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• Example Ch17 16Cu bar is at 99.5 of
  Y13N/m2. 500Hz sound wave is then transmitted.
  
• Find displacement amplitude required to break bar
• Find max speed of Cu atoms at breaking.
• Find sound intensity in bar.

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Example Ch17 34Firework explodes 100m up.
Observer directly under explosion hears average
intensity of 0.07W/m2 for 0.2s. a) Find total
sound energy of explosion b) Find decibels
measured by observer
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Example Ch17 38Fetus ventricular wall moves
in simple harmonic motion with amplitude 1.8mm at
115 beats per minute. Detector on mother
procudes sound at 2x106Hz which travels through
tissue at 1.5km/s. Find a) Max linear speed of
heart wall b) Max frequency arriving at wall of
heart c) Max frequency of reflected sound
detected