Sound
Sound
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What causes sound?
Sound Waves
Speed of Sound
Frequency and Pitch
The Doppler Effect
What Causes Sound?
• Every sound is produced by an object
that vibrates.
• For example, your friends’ voices are
produced by the vibrations of their
vocal cords, and music from a carousel
and voices from a loudspeaker are
produced by vibrating speakers.
Sound Waves
Sound waves are
compressional waves.
• A compressional wave is
made up of two types of
regions called
compressions and
rarefactions.
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• You’ll see that when a
radio speaker vibrates
outward, the nearby
molecules in the air are
pushed together to
form compressions.
Sound Waves (2)
• As the figure shows,
when the speaker moves
inward, the nearby
molecules in the air have
room to spread out, and
a rarefaction forms.
• As long as the speaker
continues to vibrate
back and forth,
compressions and
rarefactions are
formed.
Sound Waves (3)
• Compressions and rarefactions move
away from the speaker as molecules in
the air collide with their neighbors.
• A series of compressions and
rarefactions forms that travels from
the speaker to your ear.
• This sound wave is what you hear.
Moving through Materials
• Most sounds you hear travel through air to reach your
ears.
• If you’ve ever been swimming underwater and heard
garbled voices, you know that sound also travels
through water.
• Sound waves can travel through any type of
mattersolid, liquid, or gas.
• Remember the matter that a wave travels through is
called a medium.
• Sound waves cannot travel through empty space.
The Speed of Sound in Different
Materials
• The speed of a sound
wave through a medium
depends on the
substance the medium is
made of and whether it
is solid, liquid, or gas.
• In general, sound
travels the slowest
through gases, faster
through liquids, and
even faster through
solids.
The Speed of Sound in Different
Materials (2)
• Sound travels faster in
liquids and solids than in
gases because the individual
molecules in a liquid or solid
are closer together than the
molecules in a gas.
• However, the speed of sound
doesn’t depend on the
loudness of the sound.
• Loud sounds travel through a
medium at the same speed as
soft sounds.
Pitch
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If you were to sing a
scale, your voice would
start low and become
higher with each note.
Pitch is how high or low
a sound seems to be.
The pitch of a sound is
related to the
frequency of the sound
waves.
Frequency and Pitch
Frequency is a measure of how many
wavelengths pass a particular point each
second.
 For a compressional wave, such as sound,
the frequency is the number of
compressions or the number of
rarefactions that pass by each second.
 Frequency is measured in hertz (Hz).
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Frequency and Pitch (2)
When a sound wave with high frequency
hits your ear, many compressions hit
your eardrum each second.
 Your brain interprets these fast
vibrations caused by high-frequency
waves as a sound with a high pitch.
 As the frequency of a sound wave
decreases, the pitch becomes lower.
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Frequency and Pitch (3)
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This figure shows
different notes and
their frequencies.
A healthy human ear can
hear sound waves with
frequencies from about
20 Hz to 20,000 Hz.
The human ear is most
sensitive to sounds in
the range of 440 Hz to
about 7,000 Hz.
The Doppler Effect
The change in pitch or wave frequency
due to a moving wave source is called
the Doppler effect.
 The Doppler effect occurs when the
source of a sound wave is moving
relative to a listener.
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The Doppler Effect (2)
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As a race car moves, it
sends out sound waves in
the form of compressions
and rarefactions.
The race car creates a
compression, labeled A.
Compression A moves
through the air toward
the flagger standing at
the finish line.
The Doppler Effect (3)
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By the time compression
B leaves the race car,
the car has moved
forward.
Because the car has
moved since the time it
created compression A,
compressions A and B
are closer together than
they would be if the car
had stayed still.
The Doppler Effect (4)
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As a result, the flagger hears a higher pitch.
The Doppler effect happens any time the source of a
sound is changing position compared with the observer.
It occurs no matter whether it is the sound source or
the observer that is moving.
The faster the change in position, the greater the
change in frequency and pitch.
Using the Doppler Effect
The Doppler effect also occurs for
other waves besides sound waves.
 For example, the frequency of
electromagnetic waves, such as radar
waves, changes if an observer and wave
source are moving relative to each
other.
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Using the Doppler Effect (2)
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Radar guns use the
Doppler effect to
measure the speed
of cars.
Weather radar also
uses the Doppler
shift to show the
movement of winds
in storms, such as a
tornado.
The Doppler Effect (5)
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The animation to the left
shows a stationary sound
source.
Sound waves are produced
at a constant frequency
and the wave fronts move
away from the source at a
constant speed.
The distance between
wave fronts is the
wavelength.
All observers will hear the
same frequency.
The Doppler Effect (6)
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In the animation at the left the same
sound source is radiating sound waves
at a constant frequency in the same
medium.
However, now the sound source is
moving to the right.
The wave fronts are produced with the
same frequency as before. However,
since the source is moving the center
of each new wave front is now slightly
displaced to the right.
As a result, the wave fronts begin to
bunch up on the right side (in front of)
and spread further apart on the left
side (behind) the source.
An observer in front of the source will
hear a higher frequency and an
observer behind the source will hear a
lower frequency.
Mach 1 – Breaking the Sound
Barrier
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Now the source is moving at the speed of
sound in the medium (at sea level
approximately 750 mph).
The wave fronts in front of the source are
now all bunched up at the same point.
As a result, an observer in front of the
source will detect nothing until the source
arrives.
The pressure front will be quite intense (a
shock wave), due to all the wave fronts
adding together, and will not be perceived
as a pitch but as a “thump” of sound as the
pressure wall passes by.
Mach 1 – Breaking the Sound
Barrier (2)
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The figure to the
left shows a bullet
travelling at Mach
1.01.
You can see the
shock wave just
ahead of the bullet.
Mach 1.4 - Supersonic
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The sound source has now broken through
the sound speed barrier, and is travelling at
1.4 times the speed of sound.
Since the source is moving faster than the
sound waves it creates, it actually leads the
advancing wave front. The sound source will
pass by a stationary observer before the
observer actually hears the sound it creates.
As you watch the animation, notice the clear
formation of the Mach cone. It is this
intense pressure front on the Mach cone that
causes the shock wave known as a sonic boom
as a supersonic aircraft passes overhead.
Mach 1.4 – Supersonic (2)
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The figure at the
left shows a bullet
travelling at Mach
2.45.
The Mach cone and
show wave fronts
are very noticeable.
Mach 1.4 – Supersonic (3)
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The picture shows
the shock wave
front created by a
T-38 Talon, a twinengine, high-altitude,
Super Sonic jet
trainer.
Mach 1.4 – Supersonic (5)
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This picture shows
the sonic boom
created by the
THRUST SSC team
car as it broke the
land speed record
(and also broke the
sound barrier on
land).