I'm sure most of us have sucked back a quick inhale from a helium balloon and had a good laugh. Sounding like Donald Duck or the Chipmunks is enough to make anyone crack up. What is it about helium that makes us sound so funny?
No one will disagree that helium changes your voice, but not everyone believes it is helium that causes your voice to become high-pitched. What it does is change the timbre of your voice. This has nothing to do with falling trees. Timbre is what allows you to tell the difference between the same note played at the same volume on two different instruments. It also helps you hear the different parts of speech, like vowel sounds.
You need to know how sound works to understand what happens. When you talk, air travels from your lungs up through the larynx and out of your mouth. The larynx has a pair of vocal folds which form a V-shaped slit. When the air is leaving, it hits the underside of the vocal folds and makes them vibrate. Are you still with me? This vibrating excites air molecules in your vocal tract. By moving your tongue, lips, roof of your mouth, etc. you can create recognizable sounds. So that's how sound works. Let's get back to helium.
Helium is much lighter than air. Because of this, the speed of sound is much higher in helium. When you inhale helium you increase the speed of the sound of your voice. The shape of your vocal tract doesn't change. If you increase the speed of a sound you also increase the wavelength. However, on the way to the ear the sound travels thru air again and the wavelength goes back to normal. It's not the change in frequency that gives you the squeaky voice but a change in pitch. The sound has the same frequencies but helium causes a shift in your timbre. This means the higher frequencies are getting a higher enhancement than normal so the pitch changes. The end result? You sound like a chipmunk.
Although inhaling helium is fun it's not exactly healthy. Every time you inhale pure helium, you are not inhaling oxygen. Whatever you do, never inhale helium from a high pressure tank! Besides the serious risk of lung damage, you could end up with bubbles of helium in the arteries that lead to your brain. This can lead to stroke-like symptoms and, of course, death. Still laughing? Didn't think so.
