The ‘professor’ inhales helium from a hose. With an extremely high Mickey Mouse voice, he explains that the gas easily escapes from his lungs because it is a very light gas. Then he calls his colleague who has inhaled a really heavy gas known as Sulfur Hexafluoride (SF6). His voice is extremely deep! He has to do a handstand in order to remove the gas from his lungs.
How does it work?
Our voices are mainly formed in the mouth and throat. When someone inhales a light gas instead of normal air, the speed of the sound in their throat increases. This means the sound waves propagate faster and the pitch (the frequency of the sound) increases too. While helium is seven times lighter than air, the SF6 gas is five times heavier. The speed of sound in SF6 is therefore significantly lower than in air. When speaking after inhaling SF6, the voice gets lower.
The vibrations of the vocal chords are actually only marginally affected by the gas.
