Asked by: Charlie Gregson, by email
Sound travels faster through a lighter gas than a heavier one because the individual molecules have less mass and so can move more quickly in response to the pressure changes of the sound wave. The speed of sound in helium is almost three times faster than in air and this changes the resonant frequency of your throat so that high frequencies sound louder than low ones.
If you inhale a gas that is denser than air, such as sulphur hexafluoride, the sound travels at just 39 per cent of its speed in air and your voice sounds deeper.
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