Helium makes the pitch of the voice rise, Sulphur Hexafluoride makes it go down. Ignoring the question 'Why would anyone want to?', is it possible to make a mixture of Oxygen (and possibly Nitrogen), Helium and Sulphur Hexafluoride so that when you breathe it in, your voice sounds exactly the same as if you had not? Has anyone done it?

Peter Finan, Haworth, West Yorkshire