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Homework Help: Sound intensity

  1. Jun 24, 2007 #1
    1. The problem statement, all variables and given/known data
    A person is a certain distance away from a plane with three equally noisy engines at 120 dB. What is the sound intensity if two of the engines are shut off?

    2. Relevant equations
    B (in dB) = log I/Io

    3. The attempt at a solution
    using this equation:
    120 dB = 10 log I/Io
    10^12 = 10 log (I/10^-12 W/m2)
    I = (10^12)(10^-12)
    I = 1 W/m2

    So, intensity for three engines is 1 W/m2. If two engines turned off, then I'm pretty sure you don't divide the answer by three to get 0.33 W/m2 per engine because the answer is 116. I can't figure out what I'm doing wrong.
  2. jcsd
  3. Jun 24, 2007 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    On the second line in your solution, where did 1012 come from?
  4. Jun 24, 2007 #3
    Sorry, this should read:

    120 dB = 10 log (I/10^-12 W/m2)
    10^12 = I \(10^-12)
    I = (10^12)(10^-12)
    I = 1 W/m2
  5. Jun 25, 2007 #4
    any ideas?
  6. Jun 25, 2007 #5

    Doc Al

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    Staff: Mentor

    I think you need to better understand the meaning of decibel. This might help: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/db.html" [Broken]
    Last edited by a moderator: May 2, 2017
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