Chapter 16 Waves and Sound - Millersville Universitysites.millersville.edu/tgilani/pdf/131...
Transcript of Chapter 16 Waves and Sound - Millersville Universitysites.millersville.edu/tgilani/pdf/131...
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Traveling waves
A healthy human ear can detect sound waves of frequency in the range of 20 Hz to 20 kHz.
API =
24 rPIπ
=
Main Points from last lecture
Chapter 16 Waves and Sound
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16.7 Sound Intensity
Conceptual Example: Reflected Sound and Sound Intensity Suppose the person singing in the shower produces a sound power P. Sound reflects from the surrounding shower stall. At a distance r in front of the person, does the equation for the intensity of sound emitted uniformly in all directions underestimate, overestimate, or give the correct sound intensity?
24 rPIπ
=
Underestimates
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16.8 Decibels
The decibel (dB) is a measurement unit used when comparing two sound intensities.
( )
=
oIIlogdB 10β
212 mW1000.1 −×=oI
Note that log(1)=0, so when the intensity of the sound is equal to the threshold of hearing, the intensity level is zero.
Because of the way in which the human hearing mechanism responds to intensity, it is appropriate to use a logarithmic scale called the intensity level:
Human threshold of hearing
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16.8 Decibels
( )
=
oIIlogdB 10β 212 mW1000.1 −×=oI
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16.8 Decibels
Example: Comparing Sound Intensities Audio system 1 produces a sound intensity level of 90.0 dB, and system 2 produces an intensity level of 93.0 dB. Determine the ratio of intensities.
( )
=
oIIlogdB 10β
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16.9 The Doppler Effect
The Doppler effect is the change in frequency or pitch of the sound detected by an observer because the sound source and the observer have different velocities with respect to the medium of sound propagation.
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16.9 The Doppler Effect
MOVING SOURCE
Tvs−=′ λλ
sssso fvfv
vTv
vvf−
=−
=′
=λλ
−
=vv
ffs
so 11
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16.9 The Doppler Effect
−
=vv
ffs
so 11source moving
toward a stationary observer
source moving away from a stationary observer
+
=vv
ffs
so 11
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16.9 The Doppler Effect
Example: The Sound of a Passing Train A high-speed train is traveling at a speed of 44.7 m/s when the engineer sounds the 415-Hz warning horn. The speed of sound is 343 m/s. What are the frequency and wavelength of the sound, as perceived by a person standing at the crossing, when the train is (a) approaching and (b) leaving the crossing?
−
=vv
ffs
so 11
+
=vv
ffs
so 11
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16.9 The Doppler Effect
MOVING OBSERVER
+=
+=+=
vvf
fvfvff
os
s
os
oso
1
1λλ
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16.9 The Doppler Effect
+=
vvff o
so 1
−=
vvff o
so 1
Observer moving towards stationary source
Observer moving away from stationary source
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16.9 The Doppler Effect
±=
vvvv
ffs
o
so
1
1
GENERAL CASE
Numerator: plus sign applies when observer moves towards the source
Denominator: minus sign applies when source moves towards the observer
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16.10 Applications of Sound in Medicine
By scanning ultrasonic waves across the body and detecting the echoes from various locations, it is possible to obtain an image.
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16.10 Applications of Sound in Medicine
Ultrasonic sound waves cause the tip of the probe to vibrate at 23 kHz and shatter sections of the tumor that it touches.
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16.10 Applications of Sound in Medicine
When the sound is reflected from the red blood cells, its frequency is changed in a kind of Doppler effect because the cells are moving.
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Reading For Next Class Ch. 17: 17.1 – 17.3
Assignment#10 is due
by Sunday, Dec 4th