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Homework Help: Sound Intensity of a baby

  1. Mar 1, 2010 #1
    1. The problem statement, all variables and given/known data

    A baby's mouth is 30 cm from her father's ear and 150 cm from her mother's ear. Compute the difference, in dB, between the sound intensity levels heard by the father and by the mother.

    2. Relevant equations

    Intensity = Power/ Area; power = 1/2 * mass per unit length*angular frequency^2*Area^2*v
    Intensity = 1/2*Bulk modulus*Area^2*k*angular frequency
    B = 10log (I/Io)
    3. The attempt at a solution

    i understand how to get the velocity (344m/s), but what about the mass per unit length, angular frequency and area

    I dont know how to do this problem with such little info, unless i'm using the wrong formulas

    ANY help would be much appreciated
  2. jcsd
  3. Mar 1, 2010 #2
    You're subtracting logs, what rule is there for subtracting logs?
  4. Mar 1, 2010 #3
    u divide them, i understand that, however but where does the distance come into play? in the area?
  5. Mar 1, 2010 #4
    Oh, use the first formula. You're dividing so the power should cancel, right?
  6. Mar 1, 2010 #5
    yeah, the powers cancel out, but how does 30 cm and 150 cm relate to area

    so far i have

    answer =10 log (A * 1/A)
  7. Mar 1, 2010 #6
    Well the sound is propagating like a spherical shell, A=4πr^2.
  8. Mar 1, 2010 #7

    answer = 10 log (r^2/R^2) = 10 log ( 30^2 / 150^2) = -13.9794 dB

    am i supposed to put 150^2/30^2 in the log, or is it fine the way it is?

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