Chapter 12: Section 1
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Describe the motion of sound waves
Explain how frequency and pitch are related
Discuss why the speed of sound is different
depending on the state of matter
Explain the Doppler effect
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Sound waves begin with a vibrating object
When the prongs of a tuning fork vibrate, they set
the air molecules near them in motion
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At the front of the movement the air molecules are
forced closer together
 This region of high density and pressure is called
compression
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As the prongs move back, the molecules spread
apart, and the density and pressure become
lower than normal.
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This region of is called a rarefaction
Compressions and rarefactions spread out in all
directions, vibrating the air molecules back and
forth with simple harmonic motion
Sound waves are longitudinal because, the
vibrations of air molecules are parallel to the
direction of the wave motion
The average human ear can only hear, sound
waves with frequencies between 20 and 20,000 Hz
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Called audible sound waves
Although waves are often shown to travel in one
dimension, sound waves are actually three
dimensional
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The frequency of an audible sound wave
determines how high or low we perceive
sound to be
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This is known as pitch
As frequency of a sound wave increase, the
pitch rises
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Sound waves can travel through solids,
liquids, and gases
Because waves consist of particle vibrations, a
wave’s speed depends on how quickly one
particle can transfer its motion to another
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Since the particles in a solid are so close together,
sound waves generally travel faster though solids than
through gases
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The speed of sound in a gas also depends on
the temperature of the medium.
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As temperature rises, the particles in a gas collide
more frequently and the disturbance can spread
faster
In solids and liquids, the particles are close
enough together that the difference due to
temperature is less noticeable
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If you stand on the street while an ambulance
speeds by with its siren on, you will notice the
pitch of the siren changes
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The pitch will be higher as the ambulance
approaches and lower as it moves away
As you learned earlier, pitch depends on
frequency
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So how can the siren change pitch if its frequency is not
changing?
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When the ambulance is moving, there is
relative motion between the ambulance and
the stationary observer
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This relative motion affects the way the sound
waves are perceived
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Although the frequency of the siren remains
constant, the waves reach an observer in front
of the ambulance more often than they would
if the ambulance were stationary
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Even though the speed of sound in the air does not
change, the frequency heard by the observer is
greater, and thus the pitch is higher
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For the same reason, the waves reach an
observer behind the ambulance less often than
they would if the ambulance were stationary
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As a result, the frequency heard is less than the source
frequency and a lower pitch is perceived
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This frequency shift is known as the
Doppler Effect
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The Doppler effect occurs whenever there is
relative motion between the source of the
waves and an observer