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Homework Help: Sound Intensity of a Whisper

  1. Apr 21, 2009 #1
    1. The problem statement, all variables and given/known data

    What is the sound intensity of a whisper at a distance of 3.0 m, in W/m^2 ?

    What is the corresponding sound intensity level in dB?

    2. Relevant equations


    3. The attempt at a solution

    I tried looking up what the power of a whisper was and I can't find any information. This would not be a difficult problem if the information was given, but I don't know how to go about it without a value for P.

    Any help on this subject would be greatly appreciated.
  2. jcsd
  3. Apr 21, 2009 #2
    I tried using 10^-10 as the power emitted (got that from a website) and this is what I did:

    I= 10^-10/ 4pi*3^2

    I= 8.8*10^-13

    That was wrong, so I'm really at a loss here.
  4. Apr 21, 2009 #3


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    Homework Helper

    I think you mostly have the idea.

    I would think that a whisper is frequently listed as 20 decibels or 1*10-10 as you have indicated.

    Dividing by roughly 100 ~ 9*4*3 yields then just about 10-12 w/m2which is the reference level for detectable sound.
    So that in decibels that looks like 0.
    Maybe your answer just has too much precision? After all, it's not all that precise to begin with.
  5. Apr 21, 2009 #4
    Why would you be asked for a precise answer if they don't give you a value for P? Is this a book problem or something your teacher gave you?
  6. Apr 21, 2009 #5
    It's a mastering physics problem.
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