Why does helium make your voice high pitched when you inhale it? I’ve seen this on TV a lot and have tried it a couple times myself just to see if it really works, but never thought about why it does that until recently. Anyone have any ideas?
It’s less dense than regular air, so your vocal cords vibrate faster, therefore higher-pitched sound.
Hell, Gunslinger, I’m less dense than air, and I have a rounded and sonorous voice, rich with vibrato and undertones. 
Cartooniverse
BratMan: A common misconception, actually. Acoustically speaking, you voice doesn’t change pitch. The resonances of your vocal tract occur at higher frequencies. The pitch of your voice actually remains pretty much the same.
Gunslinger: You are correct about helium having a lower density than air, and it is the lower density of helium that alters the resonant frequencies. But while your vocal folds do vibrate slightly faster in helium, the change in vibration is slight and doesn’t have a significant effect on your voice.
I recently wrote an excruciatingly detailed Mailbag article on this very topic. Keeped your eyes peeled.
I think I’m afraid to find out.
(Mr. Cynical, get back in your box!)
Stuyguy:
What the fuck does that mean?
(Get your head out of your ass)
Oh, I get it. You guys think you’re going to bring some sort of feud over here from another forum.
You’re not. Knock it off.
A little off-topic but related…
I checked the article in the link above and noticed someone mentioning that inhaling Xenon makes your voice lower (although the wisdom of inhaling Xenon seems to be in question).
FYI:
You don’t need to inhale Xenon for this effect. Go to your dentist and see if he/she will give you laughing gas (N2O). Your voice will go down by a few octaves before you pass out.