You clap your hands, and a friend right across the playground turns and hears it. But your clap happened over there, next to your hands — and your friend's ears are all the way over here. So how did the clap get across? Nobody carried it. Nobody threw it. And yet it arrived.
The answer is that a sound travels. A sound never just sits where it is made. The moment it is born it sets off, spreading out in every direction at once — forwards, backwards, up, down, sideways — like an invisible ripple racing away from your hands. This page follows that ripple all the way from where it starts to the inside of an ear.
Every sound begins with something that shakes — your clapping hands, a plucked guitar string, a buzzing bee's wings. That shaking is the secret. When the thing wobbles, it bumps into the tiny bits of air pressed up right next to it and gives them a little shove.
Now here is the clever part. Those bits of air don't fly all the way to your friend. Instead each bit shoves the bit next to it, which shoves the bit next to that, on and on and on — passing the shake along from neighbour to neighbour. It is exactly like a ripple spreading across a pond: the water doesn't travel across the pond, but the ripple does. In the same way, the air stays roughly where it is, but the shake journeys all the way to your friend's ear, where it wobbles their eardrum — and they hear your clap.
It is easy to think sound only travels through air, because that is how we usually hear it. But the shake can be passed along by anything made of stuff — and some stuff is even better at it than air.
Sound travels beautifully through water: whales boom out long, low songs and other whales hear them from oceans away. It travels through solid things too — walls, doors, the ground under your feet, a long metal rail. Because the tiny bits in water and solids are packed much closer together than in air, they pass the shake along faster and more clearly. Press your ear to a table and gently tap the far end: the knock arrives loud and sharp through the wood, better than through the air above it.
Take two paper cups, poke a small hole in the bottom of each, and join them with a long piece of string knotted at both ends. Pull the string tight, put one cup to your ear and have a friend whisper into the other from right across the room. You will hear them clearly — even in a whisper too quiet to carry through the air that far! Their voice shakes the bottom of their cup, the shake races along the taut string as a ripple, and it shakes your cup into your ear. Let the string go slack, though, and the whispering stops dead: a floppy string can't pass the shake along.
A sound needs something to travel through — some stuff whose bits can pass the shake from neighbour to neighbour. Take that stuff away, and the ripple has nothing to ripple through. It simply stops.
Out in space there is almost no air at all — nothing but empty nothing. So a shake made out there has nobody to shove, no neighbour to pass to. Even the biggest, brightest explosion in space would be utterly silent. Two astronauts floating side by side cannot hear each other shout; they have to talk by radio instead. Empty space is the quietest place there is.
In films, spaceships scream past with a mighty whoooosh and laser guns go pew pew. It looks thrilling — but it is completely made up! Space is empty, so there is nothing out there for a wobble to push against, and every one of those sounds would really be silent. The film-makers add the noises afterwards because a totally quiet space battle feels strange to us. Now you know the truth: in real space, the loudest explosion makes no sound at all.
On the left is something making a sound; on the right is an ear. The sound spreads out from the source as rings, just like ripples on a pond. First, slide the ear further away and watch the sound arrive fainter — that is why a shout sounds tiny from right across a field. Then change what is in between: through Air, Water or a Wall the shake gets passed along and the ear hears it — but switch to empty Space and the rings vanish, because there is nothing there to carry the shake. The ear hears… nothing.
You saw it in the box above: the further the ear, the fainter the sound. Why? Because the ripple spreads out. Close to the source, all the shake is squeezed into a tiny ring, so it is strong and loud. As the ring races outward it grows bigger and bigger, and the same amount of shake has to stretch all the way around it — so it gets thinner, and weaker, and quieter.
That is why you can only whisper to someone standing right beside you, but you have to shout to be heard across the park — and why, from far enough away, even the loudest sound fades to nothing at all. The ripple never quite stops; it just spreads so thin that your ears can no longer feel it.
Sound is quick. It crosses a room before you can blink and races down a street in about the time it takes to say "one". But there is one thing that leaves sound completely in the dust: light. Light is the fastest thing in the whole universe, and next to it sound is a slowcoach. This gap between speedy light and plodding sound shows up in one of the most exciting places of all — a thunderstorm.
In a thunderstorm, the lightning and the thunder happen at the very same instant — a giant spark rips through the sky and makes a colossal crack. Yet you always see the flash first and hear the bang a few seconds later. Why? The light gets to your eyes almost instantly, but the sound has to crawl all the way to your ears far more slowly, arriving late.
You can turn this into a trick. When you see the flash, start counting slowly: "one… two… three…" until the thunder arrives. Roughly every three seconds you count means the storm is about a kilometre away. If the flash and the crash come almost together, the storm is right on top of you! Count again a few minutes later — if the number is getting smaller, the storm is heading your way.