Wow, that was a lot of heavy slogging. But we’ve come out on the other side with a lot of great stuff. We’ve seen helium born in three different ways. We’ve seen how it helps give rise to virtually everything we know. And we’ve seen how the strange behavior of electrons determines helium’s chemical properties. Now let’s take it a little easier.

We’ve all seen it, heard it, and probably done it. The title of this entry comes from the movie version of “Parenthood.” The grandma sucks in on a helium balloon and her voice goes all squeaky. Now I’m not going to give you all a disclaimer about how breathing pure helium is dangerous and you shouldn’t do it blah blah blah. If you want that, go here.

But why does it work? Why does your voice go all crazy in the presence of helium? Surprisingly, the answer isn’t as simple as you might think. First of all, your voice doesn’t just get high-pitched. Instead, something called the timbre of your voice changes. This is the quality of sound that makes a clarinet playing middle C sound different from a trumpet playing middle C. It’s the same note in both cases, but these two instruments bounce around air quite differently. And that’s the key.

Your voice is produced by vibrations in your throat. These vibrations don’t change based on the kind of gas surrounding them, any more than a guitar string would vibrate differently in a helium atmosphere. The solid vibration is the same, resulting in the same pitch. But the speed of sound is very different in different gases, and that affects something called resonance.

Both those points bear some explaining. First, why does sound move at different speeds in different gases? This is because of a beautiful connection. Sound is movement! If you hear someone’s voice from across the room, it’s because the air between you and her has actually moved. Now, be careful here. No air from her throat is actually impacting your eardrums. Instead, air molecules in her throat impact air molecules in her mouth impact air molecules just past her lips impact . . . . and on and on until the air molecules in your own ear move enough to impact your eardrum. This is the great power of science, that things that seem so different, sound, and light, and baseballs, and electric current, can have these beautiful, unexpected connections. Sound is movement!

Now, think about moving helium atoms vs. moving air (oxygen or nitrogen) molecules. Helium is a lot lighter, making it easier to move. Easier to move translates to faster movement. The speed of sound in lighter gases is faster than the speed of sound in heavier gases.

Next, the timbre of your voice depends to a great extent on the shape of your mouth and throat. This is because certain shapes set up sympathetic vibrations, or resonances. You’ve felt resonance if you’ve ever felt a powerful drum line making your chest shake. The vibrations of the drum heads resonate with your body. In a similar way, your throat and mouth resonate with the vibrations of your vocal cords, making your voice sound the way it does. But this assumes that the gas in your throat and mouth is air. When it is another gas like helium, the pattern of resonance changes, generally emphasizing the higher-pitched resonances. So while the overall pitch doesn’t change, the higher-pitched resonances are more important, so your voice sounds like her:

Sound is movement! Cool!