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Jet airplane (sound waves) problem

  1. Dec 12, 2006 #1
    The intensity of the sound wave from a jet airplane is 9.00 W/m2 at a distance of 4.8 m. At what rate does the jet airplane radiate energy in the form of sound waves? Assume that the sound wave radiates from the airplane equally in all directions.

    I am having trouble starting this problem.
    Would I first start using I/r^2.
    where I would equal I = 9.00 W/m2
    I'm not sure how I could find r
     
  2. jcsd
  3. Dec 12, 2006 #2

    OlderDan

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    The units of intensity are watts per square meter because intensity is energy per time distributed over an area. If the intensity is the same at all points that are 4.8m from the center of the source, how much area is the energy distributed over?
     
  4. Dec 12, 2006 #3

    would it just be 4.8 m * 9.00 W/m2 = 43.2 W/m
    and then from there I'm not sure what to do
     
  5. Dec 12, 2006 #4
    Think more about how sound propagates -- what shape?
     
  6. Dec 12, 2006 #5
    a circle?
    so area = pi r ^ 2

    i am still not getting this
     
  7. Dec 12, 2006 #6

    OlderDan

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    The units of "rate of energy" are the units of energy/time. In this problem the units you would want to use for energy and time are Joules and seconds. 1 Joule/sec = 1W. You need to multiply the given 9W/m² by an area in units of m² to get the number of Watts being radiated. The only question is, what is that area? Look around you. Imagine you can see through any obstacles that might be in your way. Where are all the points in space that are 4.8m from your eye? If you connect all those points together, what shape do they form? What is the area of that shape?
     
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