In my class we just began learning about waves, and so today I figured I’d write about one of my favorite demonstrations. I’m sure you’ve seen this somewhere or another, whether in a classroom or at a party. Someone swallows some helium from a balloon and suddenly they sound like Alvin the Chipmunk.
Have you ever wondered why that is?
Plenty of physics teachers love this demonstration. And why not? It’s eye (or ear) catching, funny, and has a lot of powerful physics behind it. Unfortunately, it’s almost always taught incorrectly, at least from what I’ve seen. Here’s how it’s usually taught, why that’s wrong, and what’s really happening when you swallow a balloon full of helium.
[At this point I should probably include a disclaimer about doing this yourself. Swallowing helium directly from a pressurized tank should never be done by anyone under any circumstances. However, swallowing helium from a balloon is perfectly safe...provided you don't swallow too much or too quickly. The helium displaces the air in your lungs, which means if you do this too quickly or for too long your body will asphyxiate for lack of oxygen. When this happens you will pass out, and can injure yourself by collapsing. It's not fatal (helium is so light that it'll all leave your lungs while you're unconscious and you'll be able to breathe again), but can be dangerous if you hit your head on the way down. I recommend you always have someone watching you while you try this.]
HOW IT’S USUALLY TAUGHT
Most teachers use this demonstration to illustrate the fundamental wave equation, the formula shown below (warning: Maths ahead)
Since helium is less dense than air, the speed of the sound waves produced when you speak is higher with helium in your lungs than with air. Since speed increases, something on the other side of the equation above must also increase. Most teacher will then have their students conclude that when speed increases, frequency increases.
WHY THAT’S WRONG
This is a horrible misconception to be perpetuating in a classroom. The frequency of a wave is like its fingerprint or DNA. Once a wave has been created, nothing can change that frequency. If it did, it would be an entirely different wave. And it’s not the gas in our throat and lungs that is creating the sound of our voice but our vocal chords, which function the same way regardless of what we’ve been breathing. Combining that with the logic and equation above, we see that it’s not the frequency of our voice that changes when we ingest helium but its wavelength.
SO WHY DOES THE PITCH CHANGE?
In order to correctly explain this phenomenon, you need to realize two things:
- The human voice is composed of more than one frequency. When we speak, our vocal chords don’t just vibrate in a single mode but in several, creating harmonics of different frequencies all at once. This is why two people singing the same note sound different from one another.
- When people speak, our throats function in a very similar way to a pipe organ. The source of the sound is our vocal chords, which transfer their vibrations into the air in our lungs as sound. This is equivalent to the strings hidden within an organ. From there, our throat takes over, which serves the same function as the pipes in an organ: Amplification. Both the organ pipes and our throats accomplish this amplification through resonance. When a sound wave with a wavelength matching the length of the tube/throat passes by, it gets amplified.
Now we can begin to make sense of this. First, the frequencies of sound (the pitches) that we produce are exactly the same regardless of what is filling our lungs at the moment. Those frequencies depend only on how we vibrate our vocal chords. Changing the speed (and thus the wavelengths) of those waves does not change the frequency or pitch we hear.
However, it does change which frequencies get amplified via resonance in our throats (because remember that does depend on wavelength). After swallowing a less dense gas like helium, our throats selectively resonate the higher frequencies among the range that our voice always produces. Similarly, if you were to ingest a denser gas [this is far more dangerous than swallowing helium as denser gases will settle in your lungs, producing a much higher risk of suffocation], your throat would selectively resonate the lower frequencies among that range, making you sound more like Darth Vader.