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Math Help - Sound Intensity Problem

  1. #1
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    Sound Intensity Problem

    At a concert, the loudness of sound, L, in decibels, is given by the equation L=10log(I/I_o), where I is the intensity, in watts per square metre, and I_o, is the minimum intensity of sound audible to the average person, or 1.0 x 10^-12 W/m^2.

    a) The sound intensity at a concert is measured to be 0.9 W/m^2. How loud is the concert?

    My answer (that is correct); 119.54 dB

    b) At the concert, the person beside you whispers with a loudness of 20 dB. What is the whisper's intensity?

    My attempt: (but it is wrong)
    20+119.54=10log(I/10^-12)
    Then I solved for I, to get T=89

    c) On the way home from the concert, your car stereo produces 120 dB of sound. What is its intensity?

    I did not get this answer either, but tried to do it like b...

    Help on parts b and c please
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  2. #2
    Super Member
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    Quote Originally Posted by skeske1234 View Post
    b) At the concert, the person beside you whispers with a loudness of 20 dB. What is the whisper's intensity?

    My attempt: (but it is wrong)
    20+119.54=10log(I/10^-12)
    Then I solved for I, to get T=89
    I don't think you're supposed to add the two decibel levels 20 and 119.54. Just put 20 on the left side:
    20=10log(I/10^-12)

    Now solve for I.

    2=log(I/10^{-12})
    10^2 = 10^{log(I/10^{-12})}
    100 = \frac{I}{10^{-12}}
    I = 10^{-10}

    For part c, the equation would be
    120=10log(I/10^-12). Solve for I.


    01
    Last edited by yeongil; May 20th 2009 at 02:36 PM.
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