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Homework Help: Sound wave intensity

  1. Dec 4, 2007 #1
    1. The problem statement, all variables and given/known data
    Michelle is enjoying a picnic across the valley from a cliff. She is playing music on her radio (assume it to be an isotropic source) and notices an echo from the cliff. She claps her hands and the echo takes 1.2 s to return.
    (a) Given that the speed of sound in air is 343 m/s on that day, how far away is the cliff?
    -i got the correct answer for this one, and it was 206m

    (b) If the intensity of the music 1.0 m from the radio is 1.0 10-5 W/m2, what is the intensity of the music arriving at the cliff?

    i cant find the answer for b. can someone help me out?
  2. jcsd
  3. Dec 4, 2007 #2
    Intensity is proportional to 1/r^2.

    That is, if the source is twice as far away, the intensity is 1/4; three times as far away, 1/9 etc....
    So if the source is 206 times as far away.......?
  4. Dec 4, 2007 #3
    i put in 2.35 x 10^-5 and it was still wrong
  5. Dec 4, 2007 #4
    how did you get 2.35 x 10^-5 ? - show your working.

    this is not what I got.
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