A sound arrives at you as a wave — a pattern of squashed and stretched air, wobbling back and forth, racing outward from wherever it was made. But a wobble in the air is not yet a sound you hear. Something has to catch that wave, feel its wobble, and turn it into a message your brain can understand. That something is your ear.

This page follows one sound wave on the last leg of its journey: from the moment it reaches the side of your head to the moment you actually hear it. On the way it is funnelled, drummed, passed along by three of the tiniest bones in your body, sloshed through a coil of fluid, and finally turned into a spark of nerve signal. It is one of the most beautiful little machines in nature — and you have two of them.

Three rooms: outer, middle and inner

The ear is really three parts in a row, and the sound passes through them one after another, like rooms in a house:

• The outer ear — the flap on the side of your head (its proper name is the pinna) and the tunnel behind it, the ear canal. The pinna is a funnel: it catches the sound wave out of the air and channels it down the canal to a thin skin stretched across the end.
• The middle ear — that thin skin is the eardrum, and behind it sit three minute bones called the ossicles. The wave sets the eardrum vibrating, and the bones carry that vibration across the middle ear, making it stronger as they go.
• The inner ear — a coiled, fluid-filled tube called the cochlea. Here the vibration finally becomes a shivering of the fluid, which bends thousands of tiny hair cells and fires off nerve signals to the brain. Only then do you hear.

So the full relay runs: pinna → ear canal → eardrum → ossicles → cochlea → nerve → brain. Each part hands the sound to the next. Let's watch it happen.

See the relay, part by part

Below is a simple map of the ear, laid out left to right the way the sound travels. Slide the control to walk through the five stops in order — the highlighted part is the one handling the sound right now, and the caption tells you what it does. Notice there is no gap: each part touches the next, so the wobble is never dropped.

The eardrum drums, the bones amplify

The eardrum is a taut little membrane, tighter than the skin of a real drum and no bigger than your fingernail. When the pressure wave reaches it, it pushes the eardrum in; as the wave stretches back, the eardrum springs out. In and out, in and out — the eardrum copies the wobble of the sound wave exactly. A high, fast wave makes it flutter fast; a big, loud wave makes it swing far.

Glued to the back of the eardrum are the three ossicles — the hammer, the anvil and the stirrup, the three smallest bones you own (the stirrup is about the size of a grain of rice). They work as a chain of tiny levers. The eardrum shakes the first bone, which shakes the second, which shakes the third, and by the time the wobble reaches the end it is pressing on a much smaller spot — the oval window — so the push is concentrated and amplified. This matters because on the far side of the oval window is fluid, and it takes a firm push to get liquid moving.

The cochlea: from wobble to nerve signal

Past the oval window lies the cochlea, a tube coiled like a snail's shell and filled with fluid. When the stirrup taps the oval window, it sets that fluid rippling . Lining the inside are thousands of microscopic hair cells, and as the fluid ripples past, it bends them. Each bend fires off a tiny electrical nerve signal, and those signals stream up the auditory nerve to your brain, which reads them as sound.

Cleverly, different spots along the coil respond to different pitches: the hair cells near the entrance are tickled by high notes, the ones deep in the coil by low notes. So the cochlea doesn't just detect that a sound arrived — it sorts out which pitches were in it, all at once. It is the point where a physical wobble finally becomes information.

What you can — and can't — hear

Your ears are marvellous, but they are not limitless. They can only detect wobbles within a certain band of pitches. A young, healthy human ear hears sounds whose frequency lies roughly between:

Hertz (Hz) just counts how many wobbles the wave makes each second — so higher Hz means a faster wobble, and a higher pitch. Below the bottom of the band, a sound is called infrasound — too low and slow for us to hear, though elephants and whales use it to call across huge distances. Above the top, it is ultrasound — too high and fast for us, but easy for bats, dolphins and dogs. A sound can be perfectly loud and still be silent to you simply because its pitch sits outside your band.

Try the whole range yourself

Here is the ladder of pitches, from very low on the left to very high on the right. Drag the slider and watch two things at once: the little wave in the middle wobbles faster as the frequency climbs — that's the pitch rising — and the label tells you whether your ear could actually pick it up. Slide down past 20 Hz and the sound drops into infrasound; slide up past 20 kHz and it soars into ultrasound. Only the shaded band in between is yours to hear.

Why the high notes fade first

The top of your hearing range does not stay put — it slowly sinks over a lifetime. A small child can often hear right up to 20 kHz, but by the time you are an adult the ceiling has usually dropped to more like 15 kHz, and it keeps falling with age. That is why grandparents sometimes miss the thin, high sounds a child hears easily.

The reason is those delicate hair cells deep in the cochlea. Unlike a cut on your skin, they do not grow back once they are damaged — and the ones tuned to the highest pitches are the most fragile. Age wears them down, and so does loud noise: a very loud sound slams the fluid so hard it can flatten and kill hair cells outright. Lose them, and you lose the pitches they listened for, for good. This is why blasting music straight into your ears, or standing next to the speakers at a concert, can quietly steal the top of your hearing — and why ear protection is worth wearing.