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: Finding sound level intensity

  1. Mar 13, 2008 #1
    1. The problem statement, all variables and given/known data
    A person standing a certain distance from an airplane with four equally noisy jet engines is feeling a sound level intensity bordering on pain, 114 dB. What sound level intensity would this person experience if the captain shut down all but one engine?



    I have no idea how to do this problem. I thought this was relative intensity so i plugged it into the formula and found intensity. Now i'm not sure what to do. Thanks!
     
  2. jcsd
  3. Mar 13, 2008 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It is a relative intensity problem. Which formula did you use? You were supposed to enter that into Part 2 of the posting template.

    Show us what you did. That was supposed to go into Part 3 of the posting template.
     
  4. Mar 13, 2008 #3
    Yeah, i was in a rush when i posted that because my computer's being repaired and i'm at the library and time was running out. Here's my work

    I used relative intensity=10log (I/Io)

    114=10log(I/1e-12) and got 0.251 w/m2 for I. I do not know what to do after this.
     
  5. Mar 13, 2008 #4
    OK, you have the value for I, divide by four and calculate the new sound level intensity.
     
  6. Mar 13, 2008 #5
    Is it 108 Hz?
     
  7. Mar 13, 2008 #6
    i just submitted and it was correct. THanks for your help :)
     
  8. Mar 13, 2008 #7
    No problem.

    The thing to realise is, you had the value for the power / m^2 at whatever arbitrary distance from the plane, then 3/4 of the engines were shut down, so you automatically know the new power / m^2 at this arbitrary distance.
     
  9. Jul 1, 2009 #8
    I have the same problem. Except instead of 114 dB it's 121 dB. I do not understand where the 1e-12 came from. Can someone tell me please?

    Also, here is what I have done so far:

    121=10log(I/Io)
    121/10=log(I/Io)
    10^12.1=I/Io

    Now I do not know what to do... I am stuck there since I have no clue to what Io is and we're trying to solve for I- two variables one equation...

    Please Help.
     
  10. Jul 1, 2009 #9

    LowlyPion

    User Avatar
    Homework Helper

    Io = 10-12 w/m2

    This is the lowest threshold of human hearing.

    So the ratio of the sound intensity to that is 121/10 or

    I = 121dB can also be written as 1012.1*10-12 w/m2
     
  11. Jul 1, 2009 #10
    I did that and I got an answer of .315 dB
    I need the answer to be in dB to submit. Can you tell me what I am doing wrong here?
     
  12. Jul 1, 2009 #11

    LowlyPion

    User Avatar
    Homework Helper

    Perhaps you should show your work. And maybe even what the problem is, because, if it is the same as the OP here where 3 jet engines cut out, you won't be getting .315 dB, if you calculated it correctly.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook