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Halliday 2 Cap 18 - P18 088

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    December 2018
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88. The source being a “point source” means Asphere = 4πr2 is used in the intensity definition I = P/A, which further implies  2 I2 P/4πr22 r1 = = . 2 I1 P/4πr1 r2 From the discussion in §18-5, we know that the intensity ratio between “barely audible” and the “painful √ threshold” is 10−12 = I2 /I1 . Thus, with r2 = 10000 m, we find r1 = r2 10−12 = 0.01 m.