hw13 - strong(ams4684 HW13 ditmire(58216 This print-out...

strong (ams4684) – HW13 – ditmire – (58216) 1
This print-out should have 21 questions.
Multiple-choice questions may continue on
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before answering.
001 10.0 points
If the frequency of sound is doubled, how will
its speed change? How will its wavelength
change?
1. Both speed and wavelength will double.
2. Its speed will halve, and its wavelength
will double.
3. Its speed will double, and its wavelength
will not change.
4. Its speed will not change, and its wave-
length will halve. correct
5. There will be no change in its speed and
wavelength.
Explanation:
v = f λ = (2 f )
1
2
λ
The speed of sound is constant in a given
medium. If the frequency of sound is doubled,
its speed will not change, but its wavelength
must be “compressed” to half. The speed of
sound depends only on the medium through
which it travels, not on its frequency, wave-
length, or intensity (until the intensity is so
great that a shock wave results.)
002 10.0 points
A stereo speaker represented by P in the fig-
ure emits sound waves with a power output of
102.3 W.
P
x
13 . 2 m
What is the intensity of the sound waves at
point x , 13.2 m away?
Correct answer: 0 . 0467216 W / m
2
.
Explanation:
Let : P = 102 . 3 W and
r = 13 . 2 m .
I =
P
4 π r 2
=
102 . 3 W
4 π (13 . 2 m) 2
= 0 . 0467216 W / m
2
.
003 10.0 points
What is the time lapse between seeing a
lightning strike and hearing the thunder if the
lightning flash is 31 km away? The speed of
lightwaves in air is 3 × 10
8
m / s and the speed
of sound waves in air is 333 m / s.
Correct answer: 93 . 093 s.
Explanation:
Let : c = 3 × 10
8
m / s ,
v = 333 m / s , and
d = 31 km .
The time for the light to travel 31 km is
t
1
=
d
c
and the time for the sound to travel 31 km is
t
2
=
d
v
2
,

strong (ams4684) – HW13 – ditmire – (58216) 2
so the time lapse is
Δ t = t
2
- t
1
= d
1
v
2
-
1
c
= (31 km)
1
333 m / s
-
1
3 × 10 8 m / s
= 93 . 093 s .
004 10.0 points
At a maximum level of loudness, the power
output of a 75-piece orchestra radiated as
sound is 72.0 W.
What is the intensity of these sound waves
to a listener who is sitting 28.8 m from the
orchestra?
Correct answer: 0 . 00690777 W / m
2
.
Explanation:
Let : P = 72 . 0 W and
r = 28 . 8 m .
I =
P
4 π r
2
=
72 W
4 π (28 . 8 m)
2
= 0 . 00690777 W / m
2
.
005 10.0 points
The speed of sound in water is 1432 m / s. A
sonar signal is sent from a ship at a point just
below the water surface and 2 . 25 s later the
reflected signal is detected.
How deep is the ocean beneath the ship?
Correct answer: 1611 m.
Explanation:
Let : v = 1432 m / s and
t = 2 . 25 s .
The sound has to travel to the ocean floor
and back, so
2 d = v t
d =
v t
2
=
(1432 m / s) (2 . 25 s)
2
= 1611 m .
006 10.0 points
How many times louder is the intensity of
sound at a rock concert in comparison with
that of a whisper, if the two intensity levels
are 130 dB and 1 dB, respectively?