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Speed of sound in turbulent air?

  1. Jan 8, 2015 #1
    If I turn on a standing fan in my room how does the speed of sound change in the fast moving air compared to the still air around it? Would it be relatively faster, slower, or does it depend on the direction in which the fan is blowing?
     
  2. jcsd
  3. Jan 8, 2015 #2

    Doug Huffman

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    Sound is subject to shear. "It may be seen that refraction effects occur only because there is a wind gradient and it is not due to the result of sound being convected[sic] along by the wind." (Singal, S. (2005). Noise Pollution and Control Strategy. Alpha Science International, Ltd. p. 7. ISBN 1-84265-237-0.)
     
  4. Jan 8, 2015 #3
    Under conditions of equilibrium, it can be shown that the speed of sound in still air is some function of mean axial molecular velocity along the sound axis connecting the emitter with the sensor. This function is complex and non-linear. When the fan is blowing, conditions of equilibrium do not exist, and the function becomes even more complex.

    In general, sound will travel faster in the wind direction and slower in all other directions. Since the relationship is non-linear, a ten mps wind speed will not add 10 mps to the speed of sound in the windward direction. Moreover, the drop in the speed of sound in all other directions will be a function of the angle between the wind speed axis and the sound speed axis.

    In addition, there will be changes in the intensity of the sound with wind direction.
     
  5. Jan 9, 2015 #4
    Could I used turbulent air to deflect a sound wave then? As in my image.

    scaled_image.jpg
     
  6. Jan 9, 2015 #5

    phinds

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    It will depend on the direction in which you are measuring the speed of sound. Have you not ever experienced having a wind gust reduce or even briefly eliminate the sound from a source? A fan is likely to be a bit limited in its capability to do that but I think it would be measurable with sensitive enough instruments. On the other hand, you ask about the "still air around it". If the air is indeed still, why do you think, turbulence elsewhere would matter to the speed of sound in still air.
     
  7. Jan 9, 2015 #6
    My thinking started with wind gusts. I was more thinking if one is bed and the neighbor is noisy could a fan be used to deflect the sound waves around one' s head.

    I didn't mean the turbulent air would change the speed of sound in the still air, I meant relative to the still air. I don't expect to get a value for the speed of sound in still air and the speed of sound in air moving at 3m/s, in the direction of the sound wave, but this would be interesting.

    I was thinking along these lines,

    but for sound waves.
     
  8. Jan 10, 2015 #7
    If your goal is simply to get to sleep in the presence of extraneous noise, then there are numerous small devices that replicate soothing sounds (surf, raindrops, meadow noises, etc.) that might help. I have found the simple motor noise of a fan to be useful. Your diagram shows a rather simplistic view of sound waves. This view rapidly deteriorates with increases in distance, time, and scale. If you have a fan in any reasonably-sized bedroom, then you don't have any still air in that room.

    As one who has dealt with the free atmosphere for more years than I care to remember, I would even go so far as to say that still air does not exist except under rigid laboratory conditions. What we call still air is simply air whose net movement is less than some arbitrary limit. And it doesn't take sophisticated scientific instruments to detect this movement. A simple strand of gossamer will do the trick.
     
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