Sound

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Sound
Sound is a longitudinal wave that travels through
a medium.
The speed of sound depends on the properties of
the medium. It travels faster in a solid than in
a liquid, and it travels faster in a liquid than
a gas.
Air 330 m/s Water 1500 m/s Aluminum 6400
m/s Wood 4000 m/s
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Poll
You tap the edge of the pool. Friend A is under
water 10 m from where you tapped. Friend B is out
of the water 10 m from where you tapped. Who will
hear the sound first?

1. Friend A
2. Friend B
3. Neither, because they will hear the sound at the
   same time.

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Sinusoidal sound wave
A continuous, single-frequency tone is described
by a sinusoidal wave. It can be produced by a
speaker that oscillates in simple harmonic motion.
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Superposition
Most periodic sound waves are a superposition of
sine waves. Superposition means to add sine waves.
Examples voice recognition, BOSE noise-
canceling headphones
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Beats
Beats occur when two sine waves of different
frequencies are added together. The amplitude of
the sum of the waves varies between zero and a
maximum value with a frequency called the beat
frequency.
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y1
t
1.0 s
y2
t
1.0 s
Two subsonic (below frequencies audible to
humans) signals are shown here over a one second
time interval. A creature that can hear at these
frequencies hears a beat frequency of _______
Hz.
A. 1 B. 2 C. 12 D. 24
E. 25
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Poll
A person is tuning his guitar. The low E string
is tuned to 156 Hz. When playing the same note on
another string, he hears a beat frequency of 2
Hz. What is the frequency of the other string?

1. 158 Hz
2. 154 Hz
3. It could be either 154 Hz or 158 Hz.
4. None of the above.

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Poll
Suppose that the guitarist in the previous
question decides to increase the tension on the
string he is tuning (in order to get it in tune).
He hears the beat frequency increase as he
increases the tension. What should he do?

1. Continue to increase the tension until the
   strings are in tune.
2. Turn the key the other way and instead decrease
   the tension until the strings are in tune.
3. Give up and hire someone else to tune the guitar.

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Sound level (loudness)
Sound level is related to intensity. Intensity is
proportional to amplitude squared. The units of
sound level are decibels.
where
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Example
To increase the sound level by 20 dB, by what
factor must you increase the sound intensity?

1. 2
2. 20
3. 10
4. 100
5. 200

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Typical sound levels
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Example
What is the intensity of sound with a measured
sound level of 100 dB?
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Poll
The sound level of a motorcycle is about 100 dB.
The sound level of normal conversation is about
70 dB. How much more intense is the sound from a
motorcycle than the sound from a persons voice
during conversation?

1. 30 times more intense
2. 1.4 times more intense
3. 300 times more intense
4. 3 times more intense
5. 1000 times more intense

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Doppler Effect
The frequency of sound heard by the detector is
different than the frequency of sound emitted by
the source, depending on the relative motion of
the source and detector.
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Doppler shift
The upper sign in each case corresponds to the
detector and source moving away from each
other. The lower sign in each case corresponds to
the detector and source moving toward each other.
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The speaker below is mounted on wheels. When it
is being pulled in the direction shown at
constant speed, the frequency that the ear hears
will be be ____________ when the speaker is
stationary.
A. lower than B. the same as C.
higher than
v
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NYPD
AMBULANCE
An ambulance and a police rescue vehicle are
traveling in the same direction toward the scene
of an accident. The ambulance has its siren on.
When the ambulance is going faster than the
rescue vehicle, the siren frequency heard by the
police officers in the rescue unit will be
_________________ when both vehicles are
stationary
A. higher than B. the same as C.
lower than
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NYPD
AMBULANCE
An ambulance and a police rescue vehicle are
traveling in the same direction toward the scene
of an accident. The ambulance has its siren on.
When the rescue vehicle is gaining on the
ambulance, the siren frequency heard by the
police officers in the rescue unit will be
_________________ when both vehicles are
stationary
A. higher than B. the same as C.
lower than
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Example
Sound of frequency 1000 Hz is coming toward you
with a velocity of 20 m/s. You are at rest. The
speed of sound in air is about 330 m/s. What
frequency will you hear?
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Doppler Shift for Light
Light from a star moving away from us is shifted
to lower frequency (longer wavelength) which is
toward the red end of the visible spectrum -- red
shifted. Light from a star moving toward us is
shifted to higher frequency (shorter wavelength)
which is toward the blue end of the visible
spectrum -- blue shifted.
We use Doppler shifts to measure (radial) speeds
of stars.