Take a deep breath of helium, the same gas that’s used for inflating party balloons, and you’ll talk like a cartoon character. It’s an amusing effect but one with a very complicated cause. To understand why this inert gas changes the pitch of a voice we have to delve into three topics: what sound is, how we hear, and how our voice is formed.
Sound is actually changes in air pressure that move out from the source like ripples on a pond. The air squashes up and then spreads out again as the pressure wave moves past a fixed point, but just like the water in the pond, the air doesn’t actually move away from the source; it’s just the shape of the wave that moves. These pressure waves move the tiny bones in our ear, and nerves transmit that motion to the brain, which is how we hear.
The speed at which sound moves through any medium, such as air, water, or helium, is determined by the elasticity of the material. Elasticity, or springiness, is a function of atomic structure. Water is less springy than air, so sound moves faster through water than it does through air. Helium is also much less elastic than air, so sound waves pass through it much more quickly. In fact while the speed of sound in air is around 343 meters per second, (about 760mph,) at sea level, it’s over 1,000 meters per second in helium.
This means that the pressure waves move through helium much faster than through air. When they move faster we say that they have a higher frequency and higher frequency sounds are higher pitched. (Think of guitar strings: when the guitarist’s fingers shorten the free length of the string it vibrates faster and the note we hear is higher.)
Now how does this come in to play when we inhale helium? Well let’s review how we actually make sound in our larynx.
As we breathe out our lungs push air over our vocal chords. Actually “chords” is a bit of a misnomer because it suggests something like the strings of a harp. In fact our vocal chords are soft membranes that vibrate as air flows over them. These vibrations are then amplified by a process called “resonance.”
Resonance is the mechanism that lets us hear an acoustic guitar. The vibrating string makes very little noise by itself, but it causes the air in the guitar body to vibrate in sympathy. The frequency, or pitch, of this vibration is dictated by a combination of the forcing frequency – the speed the string vibrates at – and the shape of the body.
If the vibrations are moving through a stiff medium, like helium, they will move faster than if they were moving through air, and this means that a listener will hear a higher pitch. If you don’t like the idea of inhaling helium, try using it to fill the body of a guitar instead and hear how it sounds then.
The human voice works the same way. As we speak we create various shapes that cause the vibrations of our vocal chords to resonate. As children we learn how to make the sounds that comprise speech, and even when our voicebox is full of helium we make the same shapes. But as those vibrations move faster their frequency is higher, so while we speak the same way with a mouth full of helium, the sound is moving faster and creating that funny cartoon character voice.
If you’re tempted to try this home you should heed these safety tips: helium itself is harmless, but because it displaces air from the lungs, it is possible to suffocate if you breathe too much. Also, helium comes out of a gas cylinder with enough pressure to damage the lungs, so be very careful about placing a tube between your lips.
