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Homework Help: Find the sound power

  1. Nov 25, 2005 #1
    Hello! I am doing some questions about sound waves for my homework assignment and have some problem in one of them.
    Assuming that it radiates sound uniformly, find the sound power from a motor car whose sound level at a distance of 7.5m is 87dB.
    Here are my steps:
    I = sound intensity, I_0 = minimum audible intensity
    By h = 10log(I/I_0)
    87 = 10 log (I / 10x10^-12)
    I = 5.01 x 10^-4 W/m^2
    By I = P / 4 pi r^2
    5.01x 10^-4 = P / 4 pi 7.5^2
    P = 0.354W
    But the answer given by my teacher is 0.18W...Can anyone tell me what's wrong in my calculation.:smile: Thanks!
  2. jcsd
  3. Nov 25, 2005 #2
    I also got P = 0.354W.
  4. Nov 25, 2005 #3


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    Homework Helper

    You were having the Sound Energy travel downward, into the ground.
    Almost all the sound will reflect from the solid Earth -
    spreading over the upper (air) hemisphere, 2 pi r^2 .
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