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Helium and a tuning fork

  1. Apr 24, 2008 #1
    Would a tuning fork in an atmosphere of Helium sound different than in air?

    I know the speed of sound is much faster and the fork (100Hz) would have a different wavelength but how would that affect sound?
  2. jcsd
  3. Apr 24, 2008 #2


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    The frequency you hear determines the tone.
  4. Apr 24, 2008 #3
    So it would still sound the same because it would still resonant at 1000Hz--it would just have a different wavelength?
  5. Apr 24, 2008 #4

    Andy Resnick

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    That's a good question- The density of helium is different than air, so I'd expect the velocity to be different, so the relationship between frequency and wavelength is different.

    Our voices sound different with helium (and nitrous oxide), so I would expect the sound of a tuning fork to change as well. Has anyone tried this?
  6. Apr 24, 2008 #5
    our voices (and woodwinds brass organpipes) are produced by vibrations of the air. If the mass of the air changes, the speed of sound will change, and so the frequency will change.
    Changes in temperature can also make these instruments go out of tune.

    the sound in a tuning fork is produced by vibrations in the metal. the surrounding gas doesn't change the frequency
  7. Apr 24, 2008 #6


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    Our voices are produced by vibration of our vocal chords; an oscilating solid creating waves in a gas. A tuning fork produces sound in much the same way.

    Luois, I would say at first thought that a tuning fork would keep its pitch if it were inside a room full of helium, but if it were in a localised pocket of helium, those outside the pocket would here a higher pitch. As the soundwaves travelling at high speed through the helium hit nitrogen-rich atmosphere, they would bunch up, hitting the eardrum at higher frequency.
    Last edited: Apr 24, 2008
  8. Apr 24, 2008 #7
    Im not sure but I have the feeling the frequency should stay the same. Yes, something should happen at the boundary between two mediums with different propagation velocity, but I think this is not a change of frequency but the sound should be deflected into a slightly different direction (dont know if deflected is the correct term, like a ray of light entering a prism and that stuff, you know ?)

    EDIT: Sorry, I meant "refracted", not deflected. sin(a1) / sin (a2) = n2 / n1, that stuff.
    Last edited: Apr 24, 2008
  9. Sep 30, 2008 #8
    I have this same question in my lab. With a tuning fork, the frequency is constant. It still oscillates at the same frequency. With f constant, then v would be proportional to the wavelength. The speed of sound in helium is around three times that in air, so I would guess that the pitch would stay the same but the wavelength would increase.
  10. Sep 30, 2008 #9


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    Helium is lighter than air so your voice goes up.

    But have you heard someone on Sulphur Hexafluoride, which is five times heavier than air??

    Adam Savage is awesome!
  11. Sep 30, 2008 #10
    Is the conjecture in my post before yours correct?

    The major factor in determining the speed of sound is not density but the elastic modulus of the medium.
  12. Sep 30, 2008 #11


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    I do not know, so I chose not to address it.
  13. Sep 30, 2008 #12


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    The helium trick never worked for me - I've seen it happen with other people, but never knew if they were pretending - so I cheated and looked at this site which seems to agree with what you say: http://www.phys.unsw.edu.au/jw/speechmodel.html

    Their explanation is that helium does not change the pitch of a voice, but only the tone colour. For example, different vowels like are "Ah" and "Oo" can be sung at the same pitch, but constitute different tone colours. The pitch is determined by what frequencies are present, eg. A below middle C would be 220, 440, 880, 1100, 1320 Hz tones etc. The tone colour is the spectral envelope of a note, and helium enhances the relative prominence of the upper frequencies.
  14. Sep 30, 2008 #13
    The frequency of the fork would not change in a helium environment. The frequency of the sound is dependent on the structure of the fork, not the medium surrounding it. In a hard vacuum, the pitch would still be 1000Hz. Since the density of helium is less than that of air the speed of sound will increase along with its wavelength.

    The reason why the pitch of your voice changes is because your are pushing air past your vocal cords and a change in density affects the mechanics of producing sound. For example, try talking without breathing. A tuning fork and your vocal cords don't operate in the same manner. You can also use a tuning fork to tune anything to the correct frequency whether your 500ft below sea level or on Mt. Everest, the frequency will remain constant. Hence, why it is called a tuning fork.
  15. Oct 2, 2008 #14
    Since the OP's question seems to be answered, I use the same thread for a related question.

    Why is tuning fork U/Y shaped?
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