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Homework Help: Amplitude and intensity of rock concert

  1. Apr 28, 2008 #1
    1. The problem statement, all variables and given/known data

    At a rock concert, the engineer decides that the music isn't loud enough. He turns up the amplifiers so that the amplitude of the sound, where you're sitting, increases by 87.0%. By what percentage does the intensity increase? Do not enter unit.

    2. Relevant equations

    I used the fact that intensity is proportional to amplitude squared.

    (A2/A1)^2 = I2/I1

    3. The attempt at a solution

    85% increase in A, so:




    3.4225x100= 342%

    342%-100% = 242%

    Answer=242 <-- which is incorrect.

    where did I make my mistake?
  2. jcsd
  3. Apr 28, 2008 #2
    Hey there,

    For a start is it 87% or 85%?

    Last edited: Apr 28, 2008
  4. Apr 16, 2010 #3
    the problem is that you didn't compute the percent by using a percent difference. All of your work is right but you need to use (I1-I2)/I1 = percent difference, use this instead of a straight fraction like you calculated. if you use this equation you will get the desired answer.
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