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Sound
Chapter 12
Pg.

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12.1Sound Waves

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What do you think?
• What is sound?
• What do all of the sounds that you hear have in
common?
• How do they differ?
• Can sounds travel through solids? Liquids?
Gases?
• Is one type of material better for transmitting sound
waves?
• When race cars or emergency vehicles pass
you, the sound changes. In what way, and
why?

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What is Sound?
Sound is a longitudinal wave.
Allsound waves are produced by
vibrating objects.
Tuningforks, guitar strings, vocal cords,
speakers
The vibrating object pushes the air
molecules together, forming a
compression.
Itthen spreads them apart, forming a
rarefaction.

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Graphing Sound Waves
The diagram shows compressions (dark) and
rarefactions (white). If you measured the pressure or
density of the air and plotted these against position,
how would the graph appear?

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Characteristics of Sound
Frequency is the number of waves per second.
You have heard of ultrasound. What is it?
Frequencies audible to humans are between 20 Hz
and 20 000 Hz.
MiddleC on a piano is 262 Hz.
The emergency broadcast signal is 1,000 Hz.
Infrasound frequencies are lower than 20 Hz.
Ultrasound frequencies are greater than 20,000
Hz.

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Pitch
Pitch is the human perception of how high or
low a sound appears to be.
Pitch is primarily determined by frequency.
Pitch also depends slightly on other factors.
Higher frequencies appear to have a higher
pitch when played loudly, even though the
frequency does not change.

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Speed of Sound
Sound waves travel through solids, liquids and
gases.
In which would the speed generally be greatest?
Why?
Solids. Because the molecules are more
closely packed, the particles respond more
rapidly to compressions.
How might the temperature of air affect the
speed of sound waves? Why?
Higher temperature increases the speed of the
waves because the particles are moving faster
and colliding more often.

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Speed of Sound

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Spherical Waves
Sound propagates in three dimensions.
The diagram shows:
Crests or wave fronts (blue circles)
Wavelength (λ)
Rays (red arrows)
Rays indicate the direction of
propagation.
How would these wave fronts appear
different if they were much farther from
the source?

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Spherical Waves
Wave fronts and rays become more nearly
parallel at great distances.
Plane waves are simply very small segments of a
spherical wave a long distance from the source.

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Doppler Effect
 An observed change in
frequency when there is
relative motion between the
source waves and the
observer.
 In our example, when the
ambulance is moving there is
relative motion between the
ambulance and the two
stationary observers.

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Doppler Effect
Why are the waves closer together on the left?
Waves are closer because the vehicle moves to the
left along with the previous wave.
• For observer A
the wavelength is
less, so the
frequency heard
by observer A is
greater than the
source
frequency.
• Continued on the
next slide….

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Doppler Effect
• Now for observer B
the wave fronts reach
observer B less often.
So the wavelength is
greater and the
frequency heard by B
is less than the
source frequency.

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Doppler Effect
• How would the wave pattern change if the
vehicle moved at a faster speed? How would it
sound different?
– At a higher speed, waves would be even closer
together and the pitch difference would be even
greater.

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Now what do you think?
What is sound?
What do all of the sounds that you hear have in
common?
How do they differ?
Cansounds travel through solids? Liquids?
Gases?
Isone type of material better for transmitting sound
waves?
When race cars or emergency vehicles pass
you, the sound changes. In what way, and
why?

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12.2 Sound Intensity and Resonance

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What do you think?
• Members of rock bands generally protect their ears
from the loud sounds to prevent damage to their
hearing.
• How do we determine the loudness of a sound?
• What quantity is loudness measuring?
• What units are used?
• Name three ways you can reduce the loudness of the
music heard by a person in the audience.

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Sound Intensity
Vibrating objects do work on the air as
they push against the molecules.
Intensity is the rate of energy flow
through an area.
Power (P) is “rate of energy flow” - ΔE/t
Since the waves spread out spherically, you must
calculate the area of a sphere.
A = 4π2r
So, what is the equation for intensity?

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Sound Intensity
SI unit: W/m2
This is an inverse square relationship.
Doubling r reduces intensity by ¼.
What happens if r is halved?
Intensity increases by a factor of 4.

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Example Problem
What is the intensity of the sound waves
produced by a trumpet at a distance of 3.2 m
when the power output of the trumpet is
0.20W?
P= 0.20W r = 3.2m
00
.2 W
I t ni y=
ne st
π3 m
4 ( .2 )2
Intensity= 1.55 X 10-3 W/m2

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Intensity and Decibels
An intensity scale based on human
perception of loudness is often used.
The base unit of this scale is the bel. More
commonly, the decibel (dB) is used.
0.1 bel = 1 dB,1 bel = 10 dB, 5 bels = 50 dB, etc.
The lowest intensity humans hear is assigned a
value of zero.
The scale is logarithmic, so each increase of
1 bel is 10 times louder.
An increase in intensity of 3 bels is 1 000 times
louder.

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Audible Sounds
The softest sound humans can hear is
called the threshold of hearing.
Intensity = 1 × 10-12 W/m2 or zero dB
The loudest sound humans can tolerate is
called the threshold of pain.
Intensity = 1.0 W/m2 or 120 dB
Human hearing depends on both the
frequency and the intensity.

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Forced Vibrations
Sympathetic vibrations occur when a
vibrating object forces another to vibrate
as well.
A piano string vibrates the sound board.
A guitar string vibrates the bridge.
This makes the sound louder and the
vibrations die out faster.
Energy is transferred from the string to the
sound board or bridge.

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Resonance
A phenomenon that occurs when the
frequency of a force applied to a system
matches the natural frequency of vibration
of the system, resulting in a large
amplitude of vibration.

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Resonance
Thered rubber band links the 4
pendulums.
Ifa blue pendulum is set in motion,
only the other blue pendulum will
have large-amplitude vibrations.
Theothers will just move a small
amount.
Since the vibrating frequencies of
the blue pendulums match, they are
resonant.

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Resonance
Bridges have collapsed as a result of
structural resonance.
Tacoma Narrows in the wind
A freeway overpass during an earthquake
http://www.youtube.com/watch?v=3mclp9QmCGs

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Now what do you think?
Members of rock bands generally protect their
ears from the loud sounds to prevent damage to
their hearing.
How do we determine the loudness of a sound?
What quantity is loudness measuring?
What units are used?
Name three ways you can reduce the loudness of the
music heard by a person in the audience.

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11.4 Standing Waves & 12.3 Harmonics

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What do you think?
• A violin, a trumpet, and a clarinet all play
the same note, a concert A. However, they
all sound different.
• What is the same about the sound?
• Are the frequencies produced the same?
• Are the wave patterns the same?
• Why do the instruments sound different?

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Standing Waves
Standing waves are produced when two
identical waves travel in opposite
directions and interfere.
Interference alternates between constructive
and destructive.
Nodes are points where interference is
always destructive.
 No motion happens here
Antinodes are points between the nodes
with maximum displacement.

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Standing Waves
A string with both ends fixed
produces standing waves.
Only certain frequencies are possible.
A single loop = 12
wavelength
The one-loop wave (b) has a
wavelength of 2L.
The two-loop wave (c) has a
wavelength of L.
What is the wavelength of the
three-loop wave (d)?
2/3L

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Standing Waves on a String
There is a node at each end because the
string is fixed at the ends.
The diagram shows three possible
standing wave patterns.
Standing waves are produced by
interference as waves travel in opposite
directions after plucking or bowing the
string.
The lowest frequency (one loop) is called
the fundamental frequency (f1).

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Standing Waves on a String
Tothe left is a snapshot of a single loop standing
wave on a string of length, L.
What is the wavelength for this wave?
Answer: λ = 2L
What is the frequency?
Answer: f1

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Harmonics
n is the number of loops or harmonic number.
v is the speed of the wave on the string.
Depends on tension and density of the string
L is the length of the vibrating portion of the
string.
How could you change the frequency (pitch) of a
string?