Science Puzzle
Why Does Helium Make Your Voice Squeaky?
Breathe in a little helium from a party balloon and you sound like a cartoon character, all squeaky and high. Most people assume the helium makes their voice box vibrate faster.
What does the helium really change?
The Answer
Your voice box keeps buzzing at much the same rate. What changes is how the tube of your throat and mouth rings, because sound travels nearly three times faster in helium than in air.
Your voice has two parts. The vocal cords in your voice box vibrate, making a buzzing sound full of many notes at once: a basic note plus higher ones called harmonics. That buzz then passes through your throat and mouth, which act like a tube that rings. Just as blowing across a bottle makes it sound one particular note, your throat and mouth resonate at certain frequencies, boosting the harmonics near those frequencies. Moving your tongue and lips changes the resonances, which is how you make different vowel sounds.
The frequencies a tube rings at depend on how fast sound travels inside it. In air, sound moves at about 340 metres per second; in helium, at about 1,000. Fill your throat with helium and its resonances jump up, so the high harmonics are boosted instead of the low ones. The basic note from your voice box hardly moves, but the tone of your voice becomes thin and squeaky.
A very heavy gas such as sulphur hexafluoride does the opposite and makes voices sound deep and booming. One warning: never breathe gas from a pressurised cylinder, and don't keep inhaling helium. Your lungs need oxygen, and helium contains none.
The principle: Resonance depends on the speed of sound. A tube resonates at frequencies set by how fast sound travels through the gas inside it, so filling the throat with helium raises its resonances and changes the tone of the voice rather than its basic pitch.