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Uncertainty of room mode frequencies

  1. Aug 7, 2011 #1
    Hi - I'm new here - I'm just writing a paper based on an acoustical analysis of a rock/pop performance venue in a college I work at, and I'm up to my uncertainties section.

    I've read up on uncertainties, and feel confident in working out uncertainties for simple additions, multiplications, divisions, squares and roots - but my equation is more complex than that.

    Here's the main equation: -
    [PLAIN]http://dl.dropbox.com/u/11341635/Room%20modes.png [Broken]

    nx, ny and nz are integer values from one to infinity.

    lx = 13.640 m ± 0.005 m
    ly = 8.109 m ± 0.005 m
    lx = 6.241 m ± 0.005 m

    c is the speed of sound and is calculated using the equation: -
    [PLAIN]http://dl.dropbox.com/u/11341635/Speed%20of%20sound.png [Broken]
    (the 101.1% is due to humidity at 30 degrees C and 30 % humidity - the maximum that could have been reached during the experiment).

    T was 23 degree C ± 2

    I'm not sure how to deal with the integer values - I'll need to put values to each at some point - I was assuming each would be either 1, or the maximum reached during the experiment (around 15), but this would mean that the end uncertainty would be a variable number depending on what these integers were.

    Assuming each integer value were 1, how do you deal with 1/x in uncertainties? I can't find any reference to this in all my guidance documents.

    Then finally there's the square root to deal with. I know that to deal with a square root you usually divide the relative uncertainty by 2, but what if it's a square root of a bunch of uncertainties - like in this case.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Aug 7, 2011 #2
  4. Aug 7, 2011 #3
    The key is to take the log of both sides and differentiate then.
  5. Aug 7, 2011 #4
    Thanks - I'm reading that document now.

    What about a simpler one - I have two sets of data for reverberation time measurements - one at 500 Hz and one at 1,000 Hz. The estimated mean average for the 500 Hz set is 0.908 s with a relative uncertainty of 0.00861 s and for the 1,000 Hz set these are 0.793 s and 0.0194 s.

    I need the mid-frequency reverberation time, which is a mean average based on only the 500 Hz and 1,000 Hz measurements. So in this case it is (0.908+0.793)/2 = 0.850 s. How do I work out the uncertainty for this value?
  6. Aug 7, 2011 #5
    I think I'm getting it. For the last example I arrived at the following equation: -

    [PLAIN]http://dl.dropbox.com/u/11341635/Uncertainty%20of%20MidFrequency%20Reverberation%20Time.png [Broken]

    Think I've got in about as deep as I need for now.

    Many thanks.
    Last edited by a moderator: May 5, 2017
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