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Homework Help: Intensity of Sound

  1. Nov 22, 2006 #1
    What is the intensity of sound at the pain level of 120 dB? Compare this to that of a whisper at 20 dB?

    b=10 log (I/ I0)
    120 dB=10 log (I/1)
    10^12=(I/ 1)
    I= 10^12 W/m^2


    b=10 log (I/I0)
    20 dB=10 log (I/1)
    10^2=(I/1)
    I=10^2 W/m^2


    The intensity of 120 dB would be 10^12 W/m^2 compared to the whisper with the intensity at 10^2 W/m^2 which equals 10^10 more intense.

    Sound good?
     
  2. jcsd
  3. Nov 22, 2006 #2

    OlderDan

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    How did I0 become 1? Where did you get that from?
     
  4. Nov 22, 2006 #3
    This is the pain threshold
     
  5. Nov 22, 2006 #4
    This should look better:

    SL=10 log(Sl/10)
    I=Io10^(sl/10) = (1.0*10-12 W/m^2) = 1 W/m^2


    SL=10 log(Sl/10)
    I=Io10^(sl/10) = (1.0*10-12 W/m^2) = 1.0*10^-10 W/m^2
     
  6. Nov 22, 2006 #5

    OlderDan

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    It sounds to me like you are trying to use the answer to find the answer. The things I highlighted in red in your quote are not equalities. I think your first equation was OK, but I_o is not the pain threshold; it is the threshoold of hearing, the 1.0*10-12 W/m^2. If you had used that instead of 1 in your original equation, you would have found the correct value for I. The value you got initially would knock your head off.

    http://hyperphysics.phy-astr.gsu.edu/hbase/sound/intens.html
     
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