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The Sound
CHAPTER 12
Edit By: Fahad A.
ملخص حفظ
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[Unless stated otherwise, assume
C
º
20
=
T
and
s
m
343
sound
=
v
in air.]
12–1
Characteristics of Sound
1.
(I) A hiker determines the length of a lake by listening for the echo of her shout
reflected by a cliff at the far end of the lake. She hears the echo 2.0 s after shouting.
Estimate the length of the lake.
SOL.
3.
(I) (a) Calculate the wavelengths in air at 20°C for sounds in the maximum range of
human hearing, 20 Hz to 20,000 Hz. (b) What is the wavelength of a 10-MHz
ultrasonic wave?
SOL.
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7.
(II) Calculate the percent error made over one mile of distance by the “5-second
rule” for estimating the distance from a lightning strike if the temperature is
(a) 30°C, and (b) 10°C.
SOL
12–2
Intensity of Sound; Decibels
8.
(I) What is the intensity of a sound at the pain level of 120 dB? Compare it to that of
a whisper at 20 dB.
SOL
.
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9.
(I) What is the sound level of a sound whose intensity is
?
m
W
10
0
.
2
2
6
-
´
SOL
.
12. (II) A cassette player is said to have a signal-to-noise ratio of 58 dB, whereas for a
CD player it is 95 dB. What is the ratio of intensities of the signal and the background
noise for each device?
SOL
14. (II) A 50-dB sound wave strikes an eardrum whose area is
.
m
10
0
.
5
2
5
-
´
(a) How much energy is absorbed by the eardrum per second?
(b) At this rate,
how long
would
it take your eardrum to receive a total energy of 1.0 J?
SOL
17. (II) Human beings can typically detect a difference in sound level of 2.0 dB. What is
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the ratio of the amplitudes of two sounds whose levels differ by this amount? [Hint:
See Section 11–9.]
SOL
.
18. (II) If the amplitude of a sound wave is tripled, (a) by what factor will the intensity
increase? (b) By how many dB will the sound level increase?
SOL
.
19. (II) Two sound waves have equal displacement amplitudes, but one has twice times
the frequency of the other. What is the ratio of their intensities?
SOL
.
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12–4 Sources of Sound: Strings and Air Columns
26. (I) (a) What resonant frequency would you expect from blowing across the top of an
empty soda bottle that is 18 cm deep, if you assumed it was a closed tube? (b) How
would that change if it was one-third full of soda?
SOL
.
12–7 Doppler Effect
49. (( )مهمI) The predominant frequency of a certain fire engine’s siren is 1550 Hz when
at rest. What frequency do you detect if you move with a speed of 30.0 m/s (a)
toward the fire engine, and (b) away from it?
SOL
.
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54. (II) A bat flies toward a wall at a speed of 5 m/s As it flies, the bat emits an ultrasonic
sound wave with frequency 30.0 kHz. What frequency does the bat hear in the
reflected wave?
SOL
.
THE SOUND
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56. (II) A Doppler flow meter uses ultrasound waves to measure blood-flow speeds.
Suppose the device emits sound at 3.5 MHz, and the speed of sound in human tissue
is taken to be 1540 m/s What is the expected beat frequency if blood is flowing in
large leg arteries at 2 cm/s directly away from the sound source?
SOL
.
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57. (III) The Doppler effect using ultrasonic waves of frequency 2.25 x 10
6
Hz is used to
monitor the heartbeat of a fetus. A (maximum) beat frequency of 500 Hz is observed.
Assuming that the speed of sound in tissue is 1.54 x 10
3
m/s calculate the maximum
velocity of the surface of the beating heart.
SOL
.
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General Problems
66. Approximately how many octaves are there in the human audible range?
SOL
.
68. A single mosquito 5.0 m from a person makes a sound close to the threshold of human
hearing (0 dB). What will be the sound level of 1000 such mosquitoes?
SOL
.
71. A stereo amplifier is rated at 150 W output at 1000 Hz. The power output drops by
10 dB at 15 kHz. What is the power output in watts at 15 kHz?
SOL
.
72. Workers around jet aircraft typically wear protective devices over their ears.
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Assume that the sound level of a jet airplane engine, at a distance of 30 m, is 140 dB, and
that the average human ear has an effective radius of 2.0 cm. What would be the power
intercepted by an unprotected ear at a distance of 30 m from a jet airplane engine?
SOL
.