Quote Originally Posted by jayem View Post
Helium is lighter than air (you get the opposite effect with denser substances). Because of that sound travels faster through it, and it's to do with that.

I think it goes...

The same pitched sound in helium has a longer wavelength than the wavelength in air. A sound with the same wavelength has a higher frequency (or pitch).
This is because of speed=wavelength*frequency.

When the waves are being formed they are formed at a given wavelength, which (as above) corresponds to a higher frequency.
At the helium/air boundaries, the frequencies stay the same (because the point on the boundary has to be in both camps) and the wavelength changes (and it remains the higher frequency).
Interesting. Thank you for the info.