Chapter 12 Sound
Student: ___________________________________________________________________________
2. A sound wave radiates from a source in all directions. If the power of the sound source is 200 watts, then the
intensity level of the sound wave 100 m from the source is, in dB:
A. 98.5.
B. 92.0.
C. 91.5.
D. 89.6.
E. 88.3.
Sound decibels (dB) are defined with respect to the “threshold of hearing” which corresponds to an intensity of I
0
=10
12
W/m
2
. Moreover, each 10
fold increase in intensity increases the dB by 10. As a formula, this gives: 10 dB log
10
(I/I
0
)
In this problem, we first have to find the intensity I = P/(4
R
2
) = (200 W)/(4
(100 m)
2
) = 1.59 x 10
3
W/m. Then we find
10 dB log (1.59x10
3
/1x10
12
) = 92 dB.
25. The sound of a bird singing is given as 5 dB. What is the intensity of the sound of a bird singing?
A. 1.95
10
12
W/m
2
B. 2.04
10
12
W/m
2
C. 2.98
10
12
W/m
2
D. 3.16
10
12
W/m
2
E. 4.25
10
12
W/m
2
We use the same formula as in problem 2, but solve it for I: 10 dB log(I/I
0
) = 5 dB. Then log(I/I
0
) = 5/10 = 1/2 leading to I = I
0
* 10
1/2
= 1x10
12
W/m
2
*3.16 = 3.16x10
12
W/m
2
.
28. A 3.0 m organ pipe is open at both ends and contains air. The velocity of sound in air is 331 m/s. What is
the wavelength of the lowest frequency mode?
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 Fall '09
 ZHIXIANZHOU,
 Power, Hertz, Velocity, Hz

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