Download chapter-14-2-with

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
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.
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.
Superposition
Most periodic sound waves are a superposition of sine
waves. Superposition means to add sine waves.
Examples: voice recognition, BOSE noise- canceling
headphones
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.
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
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.
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.
Sound level (loudness)
Sound level is related to intensity. Intensity is
proportional to amplitude squared. The units of
sound level are decibels.
where
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
Typical sound levels
Weakest sound heard
0dB
Whisper Quiet Library
30dB
Normal conversation (3-5')
60-70dB
City Traffic (inside car)
85dB
Subway train at 200'
95dB
Level at which sustained exposure may result
in hearing loss
90 - 95dB
Power mower at 3'
107dB
Motorcycle
100dB
Power saw at 3'
110dB
Rock Concert
115dB
Pain begins
125dB
permanent damage - Loudest recommended
exposure WITH hearing protection
140dB
Jet engine at 100'
140dB
Death of hearing tissue
180dB
Loudest sound possible
194dB
Example
What is the intensity of sound with a measured
sound level of 100 dB?
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 person’s 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
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.
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.
NYPD
NYPD
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
NYPD
NYPD
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
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?
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.