How You Hear (Inside the Ear)
Hearing is a relay race. A sound in the air is a pressure wave; your ear catches it, turns it into mechanical vibration through three tiny lever-bones, then into a fluid wave inside the spiral cochlea, and finally into electrical nerve signals your brain can read. Watch a tone travel from the outer ear, rock the ossicles, and ripple down the cochlea — then look at the uncoiled cochlea graph below the anatomy: the traveling-wave peak lands at a different place for every pitch (high near the base, low near the apex). Drag Frequency (20 Hz–20 kHz) to move the peak, and Loudness (dB) to swell the wave and drive the auditory-nerve firing rate in spikes per second.
What's happening
The Science in Plain Language
1. The outer ear catches the sound. The visible flap (the pinna) and the ear canal funnel air-pressure waves inward and focus them onto a thin, tightly stretched membrane — the eardrum (tympanic membrane). When the wave arrives, the eardrum flutters in and out exactly in time with the sound, so a high tone shakes it faster than a low one.
2. Three tiny bones amplify the vibration — as a lever. The eardrum is glued to a chain of the three smallest bones in your body: the hammer (malleus), the anvil (incus), and the stirrup (stapes). They pivot about a shared axis like a see-saw, so the chain works as a mechanical lever. Combined with the fact that the large eardrum concentrates its force onto the stirrup's tiny footplate, this lever-and-funnel trick multiplies the pressure roughly 20–22×. Without it, most of the sound energy would simply bounce off the fluid inside the inner ear instead of entering it — this pressure matching is called impedance matching.
3. The stirrup pushes fluid inside the cochlea. The stirrup's footplate rocks against the oval window, a membrane sealing the cochlea — a snail-shaped, fluid-filled spiral about 35 mm long if you uncoiled it. Each inward push sends a ripple traveling through the fluid, and a matching outward bulge at the round window lets the incompressible fluid move.
4. Hair cells turn motion into electricity. Riding on the flexible basilar membrane are about 16,000 hair cells, each topped with a bundle of microscopic bristles called stereocilia. When the traveling wave rocks the membrane, the bundle bends, tip-links pull open tiny ion channels, potassium and calcium rush in, and the cell releases transmitter onto the auditory nerve. Bending is the whole trick: hair cells are mechanical-to-electrical translators, and they respond in well under a millisecond.
5. The traveling wave and the place code (tonotopy). Georg von Békésy won the 1961 Nobel Prize for showing that the stirrup does not shake the whole membrane evenly. Instead it launches a traveling wave that starts at the base, grows as it moves inward, peaks at one specific place, then dies away sharply. Because the membrane is stiff and narrow at the base and floppy and wide at the apex, where the wave peaks depends on frequency: high pitches peak near the base, low pitches travel all the way to the apex. The map is astonishingly orderly — the Greenwood function assigns roughly 20 kHz to the very base and 20 Hz to the apex, with 1 kHz landing a little past the middle. Drag the Frequency slider and watch the peak in the uncoiled graph slide to exactly the right spot — that is tonotopy, and it is why which hair cells fire tells your brain what pitch you heard.
6. Loudness is a rate code. Pitch is coded by place; loudness is coded mostly by rate. A louder sound makes a bigger traveling wave, bends the stereocilia further, and drives each auditory-nerve fiber to fire more action potentials per second — from a resting “spontaneous” trickle of a few spikes per second up toward a saturating ceiling around 200–300 spikes/s, plus recruitment of more fibers. Push the Loudness slider and watch the firing-rate readout climb and saturate the way a real fiber does.
7. The auditory nerve carries it to the brain. Each firing hair cell hands its signal to fibers of the auditory (cochlear) nerve, which preserve the tonotopic order all the way up through the brainstem to the auditory cortex, where the pattern is decoded into voices, music, and warnings.
Why loud noise causes hearing loss. Hair cells are fragile and, in humans, they do not grow back. Sustained sound above about 85 dB (and brief blasts far louder) physically overdrives and kills them, starting with the high-frequency cells crowded at the base — which is why noise damage usually shows up first as trouble hearing high pitches and consonants. Age, certain drugs, and infections damage them too. Because the loss is permanent, protecting hair cells (lower volume, ear protection, distance from the source) is the only real prevention.