ramos (dar2447) – oldhomework 35 – Turner – (58120)
1
This
printout
should
have
15
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
001
(part 1 of 3) 10.0 points
An electric guitar’s amplifier is at a distance
of 5.3 m.
Find the intensity of its sound waves when
its power output is 0.31 W.
Correct answer: 0
.
000878213 W
/
m
2
.
Explanation:
Let :
r
= 5
.
3 m
and
P
= 0
.
31 W
.
I
=
P
4
π r
2
=
0
.
31 W
4
π
(5
.
3 m)
2
=
0
.
000878213 W
/
m
2
.
002
(part 2 of 3) 10.0 points
Find the intensity of the sound waves when
the amplifier’s power output is 0.61 W.
Correct answer: 0
.
0017281 W
/
m
2
.
Explanation:
Let :
P
= 0
.
61 W
.
I
=
0
.
61 W
4
π
(5
.
3 m)
2
=
0
.
0017281 W
/
m
2
.
003
(part 3 of 3) 10.0 points
Find the intensity of the sound waves when
the amplifier’s power output is 2.8 W.
Correct answer: 0
.
00793225 W
/
m
2
.
Explanation:
Let :
P
= 2
.
8 W
.
I
=
2
.
8 W
4
π
(5
.
3 m)
2
=
0
.
0017281 W
/
m
2
.
004
(part 1 of 2) 10.0 points
The
area
of
a
typical
eardrum
is
about
4
.
7
×
10
−
5
m
2
.
Find the sound power (the energy per sec
ond) incident on an eardrum at the threshold
of hearing
(
1
×
10
−
12
W
/
m
2
)
.
Correct answer: 4
.
7
×
10
−
17
W.
Explanation:
Let :
A
= 4
.
7
×
10
−
5
m
2
and
I
0
= 1
×
10
−
12
W
/
m
2
.
At the threshold of hearing,
I
=
P
A
P
=
I A
=
(
1
×
10
−
12
W
/
m
2
) (
4
.
7
×
10
−
5
m
2
)
=
4
.
7
×
10
−
17
W
.
005
(part 2 of 2) 10.0 points
Find the sound power incident on an eardrum
at the threshold of pain
(
1 W
/
m
2
)
.
Correct answer: 4
.
7
×
10
−
5
W.
Explanation:
Let :
I
= 1 W
/
m
2
.
At the threshold of pain,
P
=
(
1 W
/
m
2
) (
4
.
7
×
10
−
5
m
2
)
=
4
.
7
×
10
−
5
W
.
006
10.0 points
Two sound sources 3 m apart have the same
frequency of 870 Hz. The sources emit energy
in the form of spherical waves.
An observer
initially hears a minimum sound, since the
sound sources are out of phase, at a distance
22 m along the perpendicular bisector of the
line joining the two sources.
What distance
x
must the observer move
along a line parallel to the line joining the
two sources before hearing another sound of
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ramos (dar2447) – oldhomework 35 – Turner – (58120)
2
minimum intensity?
The speed of sound is
354 m
/
s.
Correct answer: 3
.
0186 m.
Explanation:
Let :
d
= 3 m
,
y
= 22 m
,
f
= 870 Hz
,
and
v
= 354 m
/
s
.
At the beginning, the path difference between
the two sources for the observer is 0 but the
phase difference is
π
, since he heard the sound
of minimum volume, which means that an
original phase difference exists between the
two sources.
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 Spring '08
 Turner
 Physics, Work, Wavelength, Standing wave, 0.000878213 W, 0.61 W

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