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I How can a medium carry multiple sounds at once?

  1. Mar 23, 2016 #1
    When one hears a police siren, music, and a car alarm all at once, how can these multiple types of vibrations be accommodated by the medium? (Let's take the simple case of the diatomic gases in air.) Is it possible for individual molecules to vibrate and propagate multiple sound waves at once, or do different groups of molecules somehow end up carrying a certain sound, while other groups carry the other sound? Do they "trade roles" quickly? Is oxygen more apt to carry certain types of sounds than nitrogen? In the case of one sound, do all/most molecules vibrate to propagate the sound, or only a small amount?

    Thank you for your wisdom!
     
  2. jcsd
  3. Mar 23, 2016 #2

    andrewkirk

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    The pressure waves add to give one overall pressure wave, and that's all that our eardrum and inner ear detects. What separates it out into separate sounds is our remarkable brains, which I suspect perform a type of Fourier analysis on the waveform to split the single wave into separate waves of different frequencies. It's a pretty stunning feat when you think about it, but then brains are pretty amazing things.
     
  4. Mar 23, 2016 #3
    It is very stunning indeed. Fourier analysis of any kind seems like a miracle. A follow-up question then: Is there a maximum complexity or amplitude of sound that can be carried by normal air? (I assume there is a limit to what our ears can detect, correct?)
     
  5. Mar 23, 2016 #4
    Sure, there is a limit to sounds that human ears can detect. other species have their own range as a result of what worked for natural selection in their case.
    Hearing 'everything' is of no advantage, it's just white noise.
     
  6. Mar 24, 2016 #5

    A.T.

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    The frequency splitting is done by the ear, not by the brain:
    https://en.wikipedia.org/wiki/Basilar_membrane#Frequency_dispersion
     
  7. Mar 24, 2016 #6

    andrewkirk

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    Thanks. Great link. What an amazing mechanism!
     
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