I just read Alphagene’s report Why does helium make your voice squeaky? I believe I have a nit to pick.
The explanation is that:
So your vocal cords vibrate at their normal speed, but because sound moves more quickly through helium, individual elements of speech have their frequency shifted.
This makes it sound like I could use a graphic equalizer to get the same result, distorting the pattern by boosting high frequencies and cutting low frequencies. I don’t believe that’s what is going on.
I believe that the vocal cords are vibrating at their normal speed, and I agree that the speed of sound is higher in helium and that it results in a doppler shift. The shift occurs when the soundwaves that are moving quickly through helium reach normal air and slow down. The soundwaves “pile up” at the boundary, packing together to make shorter wavelengths and higher frequencies.
The effect is that all the frequencies across the spectrum are shifted by an equal amount. The pitch does change, after leaving the vocal cords.
I don’t think that the filtering effect of the throat, mouth, tongue and lips would be changed significantly. The mouth is full of helium, so the soundwaves have not reached air and changed their pitch yet. I could be wrong, though. Maybe the speed of sound through the mouth would have an impact on the filtering effect.
Well that’s a lot of text for a nitpick. To sum up, I think the effect is mainly due to a uniform frequency shift, not to a change in timbre.