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Halliday 2 Cap 18 - P18 087

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    December 2018
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87. The source being isotropic 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 (a) With I1 = 9.60 × 10−4 W/m2 , r1 = 6.10 m, and r2 = 30.0 m, we find I2 = 0.960(6.10/30.0)2 = 3.97 × 10−5 W/m2 . (b) Using Eq. 18-27 with I1 = 9.60 × 10−4 W/m2 , ω = 2π(2000 Hz), v = 343 m/s and ρ = 1.21 kg/m3 , we obtain  2I sm = = 1.71 × 10−7 m . ρvω 2 (c) Eq. 18-15 gives the pressure amplitude: ∆pm = ρvωsm = 0.893 Pa .