Why does helium construct your voice so giant pitched?

Why does helium construct your voice so giant pitched?
Taken from "The Naked Scientists" forum:
If you imagine your throat as a bit approaching an organ pipe, when the organist plays a note, one cycle of a undulation, with a wavelength approximately all along the tube, is generated inside the pipe. So when you reach a deal you produce sound side with wavelengths determined by down your throat.
The speed of a wave (c) is given by multiplying the wavelength and the frequency together (c=l.f) and this can be re-arranged to find the frequency of the nouns wave (in other words how illustrious it sounds) (f), thus : "frequency = speed divided by wavelength" or f=c/l.
But helium is less dense than the other consitutents of nouns and so sound travels much more at the double in helium (900 metres per second) than contained by air (350 metres per second). Substitute these numbers into the forumla we get above (f=c/l) and you get a good point for f (helium) 2.5 times greater than f (air). As a result you voice sounds 2.5 times higher when you breathe helium.
Conversely, if you be to breathe a denser gas than air you could receive your voice sound much lower.
Divers breathing a helium-rich mix (to overcome the problem of increased gas density at extreme depths) settle to their support crew using a "helium voice unscrambler" which reinforces the lower notes within their voices whilst suppressing the better tones so that they can be understood.
density of the gas