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Wavelength and Frequency Calculations

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Lighter than Air
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Does helium change the pitch of your voice?

Credit: Ilario 1979
Source: http://www.flickr.com/photos/photobluntitaly/513985140/sizes/m/in/photolist-MqiSL-NwpG6-PyEvm-VKqpE-Wifwd-XUu7U-21x9ER-22Hbrs-2x6E3g-2EAHFh-2VfnL5-2WNAWk-2WT4a1-2Yg9Lg-2ZfvK8-32Rqnj-3bnmyP-3Sr4os-3Sr4oG-4aXQBn-4d1HAq-4hcHD9-4nq8uG-4qqZDg-4vDAYe-4wyHCr-4AycHZ-4EeqCw-4Mz7gj-4U6bUg-4Uj7Sr-4V1eSn-565zjo-5g3cru-5nBTwH-5oqzbS-5oXZBi-5oY1pV-5BqGxo-62AV9k-6bi2U1-6f6BWN-6pTcor-6pXkNU-6wrRm1-6wrRmb-6wsQfa-6xQXAX-6J7GBL-6LsFgc-6S2edx/
License: CC BY-NC 3.0

[Figure1]

A common party trick is to breathe in helium from a birthday balloon and yell in delight as your voice achieves chipmunk status.  Helium is six times lighter than air, so when it passes over our vocal folds (aka our vocal cords) the resulting sound waves travel faster than usual and the frequencies of resonance change.  The speed can be found by multiplying the wavelength and frequency together (v = fλ).  What usually happens is that the vocal folds’ muscle tension controls the frequency of the air that passes through them; higher tension creates higher pitch and lower tension creates lower pitch.  However, the helium gas has no effect on pitch, since pitch is determined by the vocal folds and not the air quality. 

Listen to someone talk without helium: http://www.animations.physics.unsw.edu.au/jw/SPEECH_HELIUM/ordinary_speech.wav

http://www.animations.physics.unsw.edu.au/jw/SPEECH_HELIUM/pitch_in_air.wav

Listen to the same person talk with helium:

http://www.animations.physics.unsw.edu.au/jw/SPEECH_HELIUM/helium_speech.wav

http://www.animations.physics.unsw.edu.au/jw/SPEECH_HELIUM/pitch_in_helium.wav

If you listen closely, you’ll notice that the pitch is the same. 

Visit the first link in the resources section if the audio is not working for you.

When listening to someone talk, we usually do not focus on his or her voice’s pitch.  It is the change in timbre (or tone quality) that the helium causes to change.  Hearing different timbres allows us to tell the difference between instruments, like the difference between your voice and a piano.

Creative Applications

Let’s do some math:

Middle C is about 261.626 Hz. (Remember, v = fλ)

  1. In regular air, sound travels at about 340 m/s. What is the wavelength of a middle C sound wave? (Hint: v = fλ)
  2. In helium, sound travels about three times faster than in regular air. How many meters does sound in helium travel in one second? What will be the wavelength?
  3. Sulfur hexafluoride is six times heavier than air and sound moves three times slower through it. What would happen to your voice if you inhaled sulfur hexafluoride? How many meters does sound in sulfur hexafluoride travel in one second? What will be the wavelength?

Resources

http://www.animations.physics.unsw.edu.au/jw/speech.html

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