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Why is sound different in lower temperatures

  1. Jan 31, 2012 #1
    I am the least of a 'physics' person yet I find myself here,
    It is very cold just now in eastern Poland. for a week -20C - 4F
    very sunny and dry

    Why do the nearby church bells (100 meters?) certainly have a different sound/higher tone to my ears at least now in these low dry high pressure temperatures?
    any non-formula suggestions? that a physics idiot could understand?

    thank you and I hope I have placed my query properly.
     
  2. jcsd
  3. Jan 31, 2012 #2
    It has a small amount to do with the fact that the air is "closer" together because it's cold and mostly that in the summer you'll get a bigger difference in the temps at different levels in the air above the ground while in the winter it's more uniform.

    Or at least that's what I remember from back in science class when we went over weather.
     
  4. Feb 1, 2012 #3

    jim hardy

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    hmmm what changes most with temperature?

    my guess would be viscosity of air
     
  5. Feb 1, 2012 #4
    Frequency is independent of the medium through which a sound wave propagates. The reason you hear things differently in different pressures (Pressure is proportional to Temp) is because your eardrums are likely held in a different equilibrium state. Thus, you process the same exact frequency of wave differently.

    Correct me if I'm wrong.
     
  6. Feb 2, 2012 #5
    Perhaps the difference in sound has more to do with the temperature of the materials that the bells are made of than the temperature of the air?

    Here is a very short article on the speed of sound altered by air temperature...
    http://www.ndt-ed.org/EducationResources/HighSchool/Sound/tempandspeed.htm

    Here is a much more detailed article on wiki...
    http://en.wikipedia.org/wiki/Speed_of_sound
    The section under "Speed of Sound in Solids" would seem to indicate that the temperature of the bells may play a role though I can not decipher to what extent.

    Here is an excerpt from a book describing the effect of weather on the sound of church bells.
    http://books.google.com/books?id=5Y...A#v=onepage&q=sound bells temperature&f=false
    Again it focuses more on the air and does not appear to say anything about the temperature of the metal, so perhaps I am wrong about the temperature of the metal playing much of a role.


    Edit: Ok, apparently bells are rather sophisticated acoustic devices...
    https://www.msu.edu/~carillon/batmbook/chapter5.htm
     
    Last edited: Feb 2, 2012
  7. Feb 2, 2012 #6
    It's most probably the inversion.

    We had this air to ground shooting range with was pretty noisy obviously. Normally it would not be a problem for the nearest village a few kilometers away, but sometimes the windows would vibrate and it looked like the explosions were very close.

    So the Dutch Aerospace laboratory (NLR) did extensive research and found out that it was the inversion layer, when the earth cools, cooling the lowermost part of the atmosphere, that acted as a sort of reflector for the sound, keeping the energy at ground level.
     
  8. Feb 2, 2012 #7
    But this supports your original suspicion. Temperature changes the pitch of the bell. All it says is that the partials stay in the same ratio to the fundamental with temperature changes. The "tuning" he's talking about is the design plan they have to follow to make sure the partials are in tune with the fundamental when they make the bell. This, unlike other instruments, is not a given with a bell. Anyway, if you hear the same bell every day, you'll probably notice if the pitch changes a quarter tone, without, perhaps, realizing what's different.

    The second paragraph suggests that another difference would arise from the colder temperature of the surrounding surfaces: nothing would be absorbing the sound as well. Inversion, as per Andre, would also make a large difference, preventing the sound from losing intensity by deflecting upward.
     
  9. Feb 2, 2012 #8
    Yes, I realized that. I made sure to add the second portion to allow that there are apparently a number of variables that can make the sound of a bell seem different. I can not see any way of realizing to what extent which variables are influencing the acoustics more so it seems most honest to point out that there are several.

    I was also rather stupefied by the apparent complexity of such a "simple" instrument. It would seem like any number of factors, certainly including the expanding and contracting of the metal, could make it sound different.
     
  10. Feb 2, 2012 #9

    jim hardy

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    i was thinking maybe something analogous to diffraction patterns for light waves.

    the sound arriving at one's ear has got there by several paths
    line of sight,
    bounce off ground
    refracted through vertical temperature gradient
    reflected off other structures

    those path lengths when expressed as wavelengths will change with temperature by a substantial fraction of any specific wavelength, causing interference , so when sensed by the ear their sum will have a different spectral content at temperature A than at temperature B.
    i suppose the effect would be nore noticeable in lower frequencies.

    It would be interesting to walk toward the bell and see if its perceived tone varied .



    my uneducated guess.
     
  11. Feb 2, 2012 #10
    Ah, I see.
    I agree. It turns out to be much more complicated than a person would anticipate.
     
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