Why does inhaling helium cause your voice to sound high pitched?

Answer 1

The sound that we make is caused by the vocal strings in or throats which vibrate the air in specific frequencies. This is very similar to the way that a guitar works.
The sound travels through the air as a sound wave, i.e. the air contracts and expands like a water wave raising and ebbing in the sea. The frequency of this wave depends on the medium that transports it. Usually it is just air, but when you fill your lungs with helium the sound is carried by helium, causing the frequency to go up because helium is lighter then air.

Answer 2

Your voice's fundamental pitch doesn't actually change. Notice that if you sing a song after breathing helium, the notes are correct and the song is totally recognizable. It all just sounds "higher" somehow.

The key to understanding the effect is to realize that our voice is a complex sound consisting of a spectrum of harmonics. The lowest harmonic is the fundamental, and corresponds to the pitch. It's set by the base rate of vibration of the vocal cords. But a whole series of higher, related frequencies are also produced (the harmonics). Harmonics are responsible for the timbre of a sound.

Think of musical instruments. They can all play the same note (fundamental), but sound completely different because of the identities and relative amplitudes of their harmonics. A flute is relatively pure: it produces fewer additional harmonics, at lower relative amplitudes, than say, a clarinet.

So, your vocal cords vibrate and produce a complex spectrum of sound. The vocal tract is a resonant chamber that shapes the output of the cords. That is, if you measured the sound right beside the cords, and again outside your mouth, the spectra would differ. The fundamental frequency (pitch) and the additional harmonics produced by the cords are the same in both cases, but some components are amplified and some attenuated (dampened) by the resonant properties of the vocal tract.

The factors that determine the resonant properties of a container (ie the vocal tract) are shape, size, and the speed of sound inside it. Helium is less dense than air, and sound velocity in a gas varies inversely with density, so the speed of sound in helium is greater than in air. A vocal tract filled with helium has higher resonant frequencies than the same tract filled with air. So what happens with your voice in helium is that it produces the same spectrum of frequencies, but the higher ones are amplified more (or attenuated less) and the lower ones are attenuated more (or amplified less) than in air. The fundamental frequency (pitch) is the same, but the timbre is different.

A little test of this answer would be to find someone who can sing very pure tones (with relatively few harmonics, like a flute). See if their sung notes change very much after breathing helium. I predict they won't, because there'd be no rich spectrum of higher harmonics whose amplifications could increase. The person might find they simply can't sing as loudly (the fundamental would be more attenuated).

Answer 3

The secret to this trick lies in the fact that helium is less dense than air. The less dense the medium, the faster your vocal chords will vibrate, in this case making your voice sound higher pitched.

A lower pitch can be achieved by inhaling a gas that's heavier than air, such as xenon, but a party decorated with xenon filled balloons doesn't sound as fun.

Answer 4

Did you know....that if your sexy (like I am) that inhaling helium will have no effect on the pitch of your voice!!!
Very serious answer!!!