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How does air pressure affect pitch?

  1. Dec 15, 2012 #1
    1. The problem statement, all variables and given/known data
    I thought that only the length that the sound travels affects the pitch. However, in low pressures/breathing helium the distance traveled by the gas is the same so how is pitch affected?


    2. Relevant equations
    none
    3. The attempt at a solution
    this is what i found from another post "That's true, but the atmospheric pressure has as much to do with propagation of the sound as the pitch. The frequency is primarily determined by the length of the vocal chords; the longer they are, the lower the frequency. Thinner air, though, absorbs the energy of the sound before it can go very far.
    That's the reason that people sound so 'duckish' after inhaling helium. The change of ambient gas density effectively (but not physically) shortens the chords. That is because the initial vibrations are attenuated according to the density of the medium that they are propogating through."

    I don't understand how thin air can absorb more energy than normal air and how does decreasing the energy decreases the pitch shouldn't it only decrease the loudness since there is less energy?

    Thanks for the help!
     
  2. jcsd
  3. Dec 15, 2012 #2

    BruceW

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    Well, in thin air, the sound will travel less far. And the energy must go somewhere, so what must we conclude?

    I'm not sure what you mean here. Do you mean decreasing the energy of vibrations which caused the sound? Think about the simple case of an oscillating string. How is the energy of a particular harmonic related to the frequency?
     
  4. Dec 28, 2012 #3
    Hi why would sound travel less far in thin air? Since when we take a breathe and speak, the air that vibrates starts from the lungs to mouth has traveled the same distance. So how should the pitch change?
     
  5. Dec 28, 2012 #4

    haruspex

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    As I understand it, attenuation of sound in air is chiefly a function of humidity. For a given humidity, density has not much to do with it. The attenuation that results varies according to frequency, so it will change the pitch. See e.g. http://www.kayelaby.npl.co.uk/general_physics/2_4/2_4_1.html
    Helium does not, strictly speaking, change the pitch of the voice. It alters the resonant frequencies within the mouth and throat, so changes the power distribution amongst the harmonics. Google "helium pitch".
     
  6. Dec 28, 2012 #5

    CWatters

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    What haruspex said.

    If your vocal chords vibrate at 500Hz then 500Hz is what reaches your ears regardless of the density or speed of sound of the air/helium.
     
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