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Help with homework problem

  1. Apr 29, 2006 #1
    Hello. I was hoping someone could give me a direction to go with this homework problem. I'm having trouble trying to figure it out. Here's the question:

    Two people relaxing outdoors listen to a bird sing. One person, only 1.00m from the bird, hears the sound with an intesity of 2.80x10^-6 W/m^2. (a) What is the power output of the bird's song? (b) What intensity is heard by a second person, which is 4.25m from the bird?

    The answer I get doesn't correlate with the correct answer given. This is what I did for the first part. I can't solve the second part w/o the correct power from the first.

    P=2.80x10^-6(1m) = 2.80x10^-6

    Can anyone give me a clue of another direction to go? The answer for part (a) is 3.52x10^-5.

    Thank you! :confused:
  2. jcsd
  3. Apr 29, 2006 #2

    Doc Al

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    Staff: Mentor

    Consider the bird as a point source of sound emanating in all directions. At a distance of 1 m, how much area does the sound cover?
  4. Apr 29, 2006 #3
    Thank you so much for your help. So basically I need to use pi(r^2) and r would =.5m.

    I have two more questions but will post them seperatly as they relate to thermal processes and heat as it relates to energy.

    Thanks again for your help.
  5. Apr 29, 2006 #4


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    Homework Helper

    Not at all ... the BIRD is in the CENTER of a sphere of sound (it is emitting),
    the listener is at the EDGE of the sphere (at one wave front).
    The Power from the bird into sound waves , from 1/340 second ago ,
    has spread to cover the entire surface of the sphere.

    Did you draw a diagram of this situation?
  6. Apr 29, 2006 #5


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    Homework Helper

    No. What is the formula for surface area of a sphere ?
  7. Apr 29, 2006 #6
    Got it! Thank you. I was thinking of it as an area of a circle instead of a sphere and after you said that it made me realize I needed to use a different formula. I=Pav/4(pi)(r^2).

    Thanks to you both :). Appreciate it.
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