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Homework Help: Sound problem

  1. Apr 17, 2005 #1
    Here is the trouble problem...

    11) A tornado warning siren on top of a tall pole radiates sound waves uniformly in all directions. At a distance of 15 m the sound intensity of the siren is 0.39 . Neglect any effects from reflection of the sound waves from the ground. a) At what distance from the siren is the intensity 0.19 ? b) What is the total acoustical power output of the siren? c) At what distance is the sound intensity reduced by 15 dB from its level at 15 m?

    a) 21.5 m
    b) 1100 Watts

    Ok... so I figured out parts a and c, but I have no clue for part c- and the same seems to go for most of my class. I think the change might be very small, but I don't know how to get it. Any help would be appreciated. Thanks!
  2. jcsd
  3. Apr 17, 2005 #2


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    What are the dimensions of .39 and .19? Knowing that should lead you to an answer for b) I believe the customary dimensions would give you intensity in watts/meter^2. Intensity is the power per unit area, and the area in your case is the total area of the sphere centered at the source.
  4. Apr 18, 2005 #3
    Sorry... I meant I didn't understand part C... any help?
  5. Apr 18, 2005 #4


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    The decibel scale is based on logarithms. In terms of intensity

    [tex] L = 10log({\frac{I}{I_0}}) [/tex]

    You need to find the intensity ratio that gives you L = -15 and from that determine the distance ratio and then the final distance
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