Why does helium make your voice squeaky?
Somebody at a party breathes in from a balloon, says four words, and everybody falls over laughing. The obvious description is that helium put their voice up high. Record it and measure the note, though, and the note is almost exactly where it was before. They are singing precisely as low as they always did, and they still sound like a cartoon duck. So what actually changed?
Your voice is made in two stages by two different parts of you, and helium only interferes with the second one.
The buzz and the tube are separate machines
Put two fingers on the front of your throat and hum. That vibration is your vocal folds: two small flaps of muscle and tissue that slam together and apart, chopping your outgoing breath into puffs.
How many puffs per second decides your pitch, meaning how high or low the note sounds. A grown man runs at roughly a hundred and twenty puffs a second, a child much faster, and you change it by stretching the folds tighter or letting them slacken.
The buzz those folds make, on its own, sounds nothing like a person. It is a harsh flat rasp. Everything that makes a voice recognizable happens afterward, in the space above.
That space is a bent tube about 17 cm(7 in) long, running from your vocal folds up through your throat and out at your lips. Every sound you have ever made had to travel through it.
A tube does not pass every note equally
Blow across the top of an empty bottle and it answers with one particular note. Fill it halfway with water and it answers with a higher one. You did the same thing with your lips both times, so the note is coming from the space, not from you.
Air inside a container will vibrate happily at certain frequencies and hardly at all at others. That preference is called resonance, and the size of the space decides which frequencies win. Short spaces prefer high notes.
Your throat and mouth are a container of exactly this kind, and they boost a few frequencies strongly while leaving the rest alone. Those boosted bands have a name of their own: formants.
Formants are how you tell one vowel from another. Say “ee” and then “oo” while carefully holding the same note. The pitch does not move — you can verify that against a piano — yet the sound transforms completely, because moving your tongue and lips reshaped the tube.
That difference in character, with the pitch left alone, has its own name: timbre. Two instruments playing an identical note sound like different instruments entirely, and timbre is the whole of that difference.
So a voice is a buzz passed through a filter, and the two are entirely independent. Change the buzz and the note changes. Change the tube and the character changes.
Sound moves three times faster through helium
Now the part that does the damage. How fast sound travels through a gas depends on how heavy that gas is, and helium is extraordinarily light — a helium atom weighs about a seventh of what an average air molecule weighs.
Light molecules move faster at the same temperature, so they pass a vibration along much more quickly. In warm air sound travels at about 350 m/s(780 mph). In warm helium it manages roughly 1,000 m/s(2,200 mph).
Speed of sound through helium against ordinary air, both measured in meters per second at body temperature.
A resonance depends on how long a vibration takes to travel the tube and back. Fill the tube with helium and every trip is nearly three times quicker, so every formant jumps to nearly three times its usual frequency.
Your throat never got smaller. The vibration simply started crossing it three times faster.
Your ears have spent your entire life using formants to estimate how big a speaker is, because a genuinely small person has a short tube and high formants. High formants arriving from an adult-sized person is an impossible combination, and your brain settles for cartoon.
What the gas does not do
There is a real hazard here, and it deserves saying plainly rather than being tacked on at the end. Helium is not poisonous, but it is not oxygen either, and a lungful of it displaces the air you needed. People have passed out doing this trick, and a few have died breathing directly from a pressurized cylinder.
Which is why the experiment further down uses bottles instead. You can watch the whole mechanism without putting anything unusual in your lungs.
The machine that spoke without a voice box
The tube was understood before anybody could measure it, and the proof was a box of bellows. In 1791, when your great-great-great-great-grandparents were alive, Wolfgang von Kempelen published a description of a machine that talked.
He had a reputation to overcome. Years earlier the same man had toured Europe with a chess-playing automaton that beat everybody it met, and which turned out to have a chess master hidden inside the cabinet.
The speaking machine was not a trick. It had a bellows for lungs, a vibrating reed where the vocal folds go, and a soft leather funnel for a mouth. Squeezing the funnel into different shapes produced different vowels, from one unchanging reed.
That is the whole argument, built in wood. The vowels were never in the buzz. They were in the shape of the space the buzz traveled through.
Measuring it properly took another two centuries. Hermann von Helmholtz worked out in 1863 that vowels are combinations of resonances, using glass spheres held to his ear to pick out one frequency at a time. In 1960 Gunnar Fant in Sweden set the whole thing out as the theory every voice app now runs on: a source, and a filter, and nothing else.
Build a set of tubes and tune them
Shorter air column, higher note, every time. Helium achieves the identical result by a completely different route. It leaves the tube exactly as long as it always was, and accelerates the vibration traveling along it, which no resonance can distinguish from a shorter tube.
The humming test is the one that surprises people. You hold a single note, your friend hears something completely different, and not one thing about the buzz changed. That is your filter, working the way it works all day.
You have been doing this the whole time
Nothing about a helium voice is exotic. You reshape your own tube several times a second, every time you speak, and the person listening reconstructs your words almost entirely from where the boosted bands land.
The gas simply moves all of them at once, by an amount no mouth shape could ever produce, and leaves your pitch alone underneath. The result is a voice that belongs to a body the size of a mouse, arriving from somebody the size of a person.
Which is also why the joke works on a whisper. Whisper a sentence and your vocal folds are not buzzing at all — there is no pitch there to raise. Do it with helium in your mouth and it still sounds ridiculous.
