Why Does Helium Make Voices Sound Squeaky?S

Your birthday party isn't complete without the helium-induced squeak, but what does physics say? Discover how people think it works, how it really works, and many, many ways to get hurt while demonstrating it. Don't worry, no chipmunks are involved.

The common misconception is that, because helium is a lighter element than the main components of air, your vocal chords will vibrate faster if immersed in it. This makes a certain amount of sense. People move faster through air than they do through heavier stuff like water. Less resistance should enable vocal chords to move faster as well.

As it turns out, though, vocal chords vibrate at pretty much the same frequency no matter what gas rushes over them – though chlorine would eventually pose a problem. That factor isn't what changes the sound of your voice.

When an audio speaker – or anything else – makes a sound, it vibrates at a certain frequency. That vibration compresses the air around the speaker. The compression moves through the air, hits a person's ear, and is interpreted as sound. The frequency of the compressions will determine the quality of the sound. A low frequency means a low note. When a high number of waves of compression hit the ear every second, the note will be high.

There are two major things that determine the sound of a person's voice. The first is the size and shape of the vocal chords themselves. The other is the shape of the throat that the air passes through. Vocal chords are not the precision instruments many people imagine them to be. They create not a single note, but a jumble of sound. Of that sound, certain notes are emphasized.

Specifically emphasized are the resonant notes. These are the notes which ‘fit' inside the throat.

Why Does Helium Make Voices Sound Squeaky?S

Look at the figure to the left. Most people have idly played with a rope like this, whether that rope is a jump-rope, a shoe-lace, a gold necklace that their mother told them not to touch, or a particularly unfortunate snake. A number of different frequencies fit the rope. In the same way, a number of different frequencies fit a person's throat.
Frequencies that don't fit are choked off in much the same way as the waves in the picture above would be if someone were to hold the rope still just beyond point ‘A'.

How does helium change that? Sound travels faster in lighter gas. Because helium hangs out at the top of the periodic table, while nitrogen, oxygen and carbon dioxide, major components of the air we breathe, are heavier, sound will travel faster in a throat filled with helium than it will in a throat filled with air.

The faster speed of sound will allow more frequencies to resonate within a throat. Look again at the figure above. If the rope were made of ordinary twine, it would be relatively easy for two people with decent coordination to get all of those frequencies going. If the rope were made of inch-thick iron links, it would be a different story. Getting metal to move fast is hard, so a strong group would have a tough time getting even the top wave to work. They would, however, have the advantage later on when the inevitable fight broke out. But that's a different story.

When you breathe in helium, your regular voice is joined by a whole host of new, higher resonant frequencies. You'll still be able to sing or speak the same note as you do with regular air, but because the higher frequencies are included, your voice will sound high, odd, and hilariously funny to drunk people.

Speaking of drunk people inhaling things, this is the part of the article in which the dangers of this practice are explained in an earnest attempt to prevent injury and keep from getting sued. Helium inhalation is, as said before, hilarious to the intoxicated. It is also a painless, effective and relatively natural-feeling way to keep oxygen from getting to a person's brain. And yes, people do die from it. One shallow breath is all it takes to say something funny. Take one puff, and pass the balloon along. Any more than that — and it is very possible to die sounding like a gerbil.

Another common mistake is to try to inhale helium straight from the compressed air tank it often comes in. To understand what's wrong with that idea, it's probably best to picture an overfilled balloon popping. Now picture it filled with blood and connected to nerve-endings. Those are lungs. They are designed to take in air at a certain speed only, and everyone should respect that.

What's more, people, in their intrepid search to find more ignominious ways to kill themselves, have figured out that while helium makes a voice go up, heavier-than-air gases have the opposite effect. Heavier gases will limit the number of frequencies that resonate in a person's throat, and make their voices sound deeper. Great stuff, right?

Here's the problem. Even if a person could find a gas that won't kill them to inhale, lungs are very picky about what kind of physical conditions they can work under. They can push out air. They can't push out heavier things. One site, which under no circumstances will io9 link to, recommends hanging upside-down after inhaling these gases, in order to clear the lungs. That technique is included in this article as a way of letting readers know what to do if they have had a heavy gas forced into their lungs by a supervillain, killer robot, lab accident, or vicious-but-nerdy street gang. Do not, repeat, do not go to the park, inhale a bunch of xenon and try to climb onto the monkey bars. It won't end well. It didn't even begin well.

Besides, with any luck, the physics behind this will impress and amuse people much more than the trick itself.

[Via UNSW Physics and CCMR]