Q. Why does helium make my voice go squeaky?

A.
The pitch of your voice is largely determined by the shape and size of your throat. As you breath through your voice box, the air sets up standing waves in your throat. The throat is roughly an open-ended cylinder, so, like a flute or a pipe organ, the air will vibrate with standing waves of specific wavelengths – called resonances. The resonances will have antinodes (points of maximum vibration) just beyond the open ends of cylinder.

The first harmonic then will be a standing wave with antinodes at either end of the throat and a node (place of no vibration) in the center of the larynx. Other resonances have more nodes and antinodes along the throat. The wavelength of this wave is approximately the length of the throat and the frequency of this sound is then the speed of sound divided by this wavelength.

The speed of sound in air is approximately 350m/s, but the speed of sound in helium is 900m/s, so the frequency of the resonances in your throat are ~2.5 times higher than with regular air.

The sound that comes out your mouth is driven by the resonances in your throat (modified by the shape of your mouth and tongue), so even though the sound is travelling through the air at only 350m/s, the frequency of the sound remains high.

Although helium is not intrinsically poisonous, when you breath in helium, you are depriving your self of a lungful of oxygen, so there are risks involved in this experiment. I recommend that you always exercise care when you are breathing helium, and ensure that there is someone around ot summon help if you have trouble breathing.