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Homework Help: Physics Help sound level intensity

  1. Jun 25, 2005 #1
    A person standing a certain distance from an airplane with three equally noisy jet engines is feeling a sound level intensity bordering on pain, 120dB. What sound level would this person experience if the captain shut down one engine?

    this is what I did:
    and ended up with 125 dB as the intensity

    What am I doing wrong since I am reducing the factor by 3 and its still not working?
  2. jcsd
  3. Jun 25, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I see two problems:

    (1) The sound level should be lower with only two engines running: log.33 is negative.

    (2) You have the wrong ratio of intensities: What's the ratio of intensities for 3 engines versus 2 engines?
  4. Jun 25, 2005 #3
    let I0 be ref sound intensity level
    sound level (db) w/3 engines = 120 db = 10 log10( 3(I/I0) )
    = 10 log10(3) + 10 log10( I/I0 )
    = 10 log10(2*(3/2)) + 10 log10( I/I0 )
    = 10 log10(3/2) + 10 log10(2) + 10 log10( I/I0 )
    = 10 log10(3/2) + 10 log10( 2(I/I0) )
    = 10 log10(3/2) + (sound level (db) w/2 engines)
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