Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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)
    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


    User Avatar
    Science Advisor
    Homework Helper

    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


    User Avatar
    Science Advisor
    Homework Helper

    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.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook