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Unformatted text preview: 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 =1012 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 ) In this problem, we first have to find the intensity I = P/(4 R 2 ) = (200 W)/(4 (100 m) 2 ) = 1.59 x 103 W/m. Then we find 10 dB log (1.59x103 /1x1012 ) = 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 1012 W/m 2 B. 2.04 1012 W/m 2 C. 2.98 1012 W/m 2 D. 3.16 1012 W/m 2 E. 4.25 1012 W/m 2 We use the same formula as in problem 2, but solve it for I: 10 dB log(I/I ) = 5 dB. Then log(I/I ) = 5/10 = 1/2 leading to I = I * 10 1/2 = 1x1012 W/m 2 *3.16 = 3.16x10*3....
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This note was uploaded on 12/23/2011 for the course PHYSICS 2130 taught by Professor Zhixianzhou, during the Fall '09 term at Wayne State University.
 Fall '09
 ZHIXIANZHOU,
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