Asked by: Richard Cosgrove, Hove
Although it’s often said that sound travels faster through denser materials, this is not true. The speed of sound increases with the stiffness of a medium and decreases with its density (it’s actually the square root of the stiffness divided by the density). The reason sound travels faster through water than through air is because water is so much less compressible (stiffer) than air that it more than compensates for the increase in density.
Helium and air are both gases with very similar compressibility so the much lower density of helium causes sound waves to propagate about 2.7 times faster. Your voice doesn’t actually change pitch with a lungful of helium: your vocal chords still vibrate at the same frequency. Rather, what changes is the natural frequency of your throat, so it resonates more strongly with the higher harmonics than the lower ones. The low frequency component of your voice is still there, but it is much quieter than the higher frequency component. The relative strength of these harmonics is called the ‘timbre’ and it is this that changes when you breathe helium. This is why you sound more like Donald Duck than Mickey Mouse.