Breathe helium, voice pitch rises; breathe SF6, voice pitch lowers.

Variation of sound speed with molecular mass. Kinetic theory shows that the speed of sound in gases varies as 1/ where m is the mass of the molecules. Since the normal mode frequencies of gas filled chambers scale proportionally to the speed of sound it follows that these normal mode frequencies scale as 1/ The sound which is radiated when we speak is a superposition of the normal modes in our mouth cavity after excitation by the vocal cords. Thus if we only have a light mass gas in our lungs the sound of our voice will be higher (because all the normal modes are shifted higher) than if we have a heavy gas in our lungs. In this demo the lecturer first breathes in the light gas Helium and then speaks or sings. The voice is rather high pitched. Subsequently the lecturer breathes in Sulfur Hexafluoride which is heavier than air, and the person_s voice is lowered. Note that you should not inhale multiple breaths of either gas because you will pass out. Also note that the Sulfur Hexafluoride, which is heavier than air, will leave your lungs best if you stand on your head and open your mouth. The Helium will leave easily by simply talking.