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I Why sound produced by rapids varies in pitch based on height

  1. Nov 7, 2016 #1
    The other day I was at a river listening to some rapids.
    The sound produced by the rapids appeared to vary in pitch depending on how high up I was (squatting down to the ground and standing up produced about a fifth in the variation of the pitch); and I can't think why it should be?
    Many Thanks
     
  2. jcsd
  3. Nov 7, 2016 #2
    Odd! Which direction? Also, being rapids I presume there was some surrounding walls or bluffs, correct? Could you describe the topology? I'm not saying that is the cause, but it is something to consider. Location dependent sound suggests reflecting surfaces to me, and the hardness, shape, and texture of a reflecting surface can certainly affect the quality of the sound.
     
  4. Nov 7, 2016 #3

    FactChecker

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    A couple of possibilities come to mind and the sound may be influenced by a combination of these and other factors:
    1) The vibrations of the rapids may be transmitted through the ground and the closer you get to the ground the more those frequencies are heard. So if the earth responds to different frequencies than the air, it would sound different.
    2) High frequencies are much more directional and "line-of-sight" than low frequencies. So the high frequencies may be blocked by terrain between you and (at least part of) the rapids.
     
  5. Nov 9, 2016 #4
    doppler effect maybe? when you are standing you are at a greater distance than when you are crouching?
     
  6. Nov 9, 2016 #5

    NascentOxygen

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    I believe you'll find that the attenuation by air of sound is frequency dependent. If you place your ear really close to trickling water or a TV speaker you can discern higher frequencies than when you are a metre away; all the frequencies are louder because you are close to the source of the sound, but the upper frequency content appears even louder still when you get closer.

    This graph illustrates how the higher tones suffer higher attenuation:

    1300px-Atmospheric_sound_absorption_coefficient_2.svg.png
    https://en.m.wikibooks.org/wiki/Engineering_Acoustics/Outdoor_Sound_Propagation
     
  7. Nov 9, 2016 #6

    FactChecker

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    CORRECTION: @NascentOxygen has questioned this and I think I agree with him.
    There could also be a temperature effect. The air near the ground may have a different temperature and the sound frequency might change.
     
    Last edited: Nov 9, 2016
  8. Nov 9, 2016 #7

    NascentOxygen

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    That doesn't seem right. How could temperature bring a frequency change after the sound is generated?
     
  9. Nov 9, 2016 #8

    FactChecker

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    I think you are right.
     
  10. Nov 9, 2016 #9

    David Lewis

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    When you pour liquid into a drinking glass, the resonant pitch starts out low, and increases as the glass fills up.
     
  11. Nov 9, 2016 #10
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