How does the ear hear?

What you call your ear is only the start. Hearing happens in three parts, most of them hidden inside the hardest bone in your skull. We are looking at a right ear from the front, so sound comes in from your left.

The outer ear is the pinna, the part you can see, and the ear canal, a tube about 2.5 cm long. The pinna funnels sound in, and its folds help you tell if a sound is above or behind you.

At the end of the canal is the eardrum (tympanic membrane), a skin thinner than paper. Behind it is the air-filled middle ear, with the three smallest bones in your body: the malleus (hammer), incus (anvil) and stapes (stirrup). The stapes is only about 3 mm long. The Eustachian tube joins the middle ear to your throat, so the air pressure on both sides of the eardrum stays equal. That is the “pop” you feel in a plane or a lift.

The stapes pushes on the oval window, a door into the inner ear. There, the snail-shaped cochlea, 2.75 turns of fluid-filled tube, turns vibrations into nerve signals. The round window below lets the fluid bulge back out. Next door, the vestibular system, three semicircular canals plus the utricle and saccule, senses balance. The auditory nerve carries it all to the brain (see BrainClear).

The inner ear is full of fluid. Fluid is much harder to shake than air, so if sound hit it straight from the air, about 99.9% of the energy would just bounce off, like shouting at a swimming pool. The middle ear's job is impedance matching: turning a big, gentle push into a small, strong one.

It does it two ways. First, the eardrum (about 55 mm² that does the work) is about 17 times bigger than the stapes footplate (about 3.2 mm²). The same force on a smaller area means a bigger pressure, like a drawing pin. Second, the malleus and incus form a lever that gives about 1.3 times more force. Together: about 22 times the pressure, roughly +27 dB, which wins back most of that loss.

The ear canal helps too. A tube closed at one end rings at f = c ÷ 4L, the same physics as a flute or organ pipe (see FluteClear and WaveClear on resonance). With L ≈ 2.5 cm and sound at 343 m/s that is about 3.4 kHz, so sounds near 3 kHz arrive about 10 dB stronger. Many speech sounds, like “s” and “t”, live up there.

For very loud sounds, a tiny muscle, the stapedius, tightens the chain: the acoustic reflex. It softens low sounds by 10 to 20 dB, but it takes 25 to 150 milliseconds to kick in, too slow to save you from a firecracker.

Uncoil the cochlea and you get a tube about 3.5 cm long, split along its length by the basilar membrane. When the stapes pushes the oval window, the fluid pushes on this membrane and a ripple, a travelling wave, runs along it from the base towards the apex.

The membrane is not the same all along. At the base it is narrow (about 0.1 mm) and stiff; at the apex it is about five times wider and floppy. Stiff things like to vibrate fast, floppy things slowly. So the wave for each tone grows as it travels, peaks at the one place that matches its frequency, then dies away. High pitches peak near the base, low pitches near the apex. This map of pitch along the membrane is called tonotopy. Georg von Békésy first watched the travelling wave, and won a Nobel Prize for it in 1961.

A formula by Donald Greenwood gives the place for any frequency: f = 165.4 × (10^2.1x − 0.88) Hz, where x is the fraction of the way from the apex. It runs from about 20 Hz at the apex to 20 kHz at the base. It is a bit like a piano keyboard laid along the membrane (see PianoClear): each octave gets a few millimetres.

Real sounds are mixtures. A guitar or flute note has a fundamental plus overtones at 2, 3, 4 times the frequency (see GuitarClear, FluteClear and SynthClear on wave shapes). The cochlea peaks at a place for each one, splitting sound into its parts, like a prism splits light.

Sitting on the basilar membrane is the organ of Corti, and in it are your hair cells: about 3,500 inner hair cells in one row and about 12,000 outer hair cells in three rows, in each ear. They are not hairs at all. Each has a tuft of stiff bristles on top, the stereocilia, lined up like a staircase.

When the membrane moves up and down, it slides against a jelly roof, the tectorial membrane, and the bristles bend. Tiny springs between their tips, the tip links, pull little gates open. Charged potassium ions rush in and the cell sends a chemical signal to the auditory nerve, which fires faster the more the bristles bend. At the quietest sound you can hear, the bristles move only about 0.3 nanometres, the width of a few atoms.

The inner hair cells do the sensing: nearly all the nerve fibres that carry sound to the brain start at them. The outer hair cells are the cochlear amplifier. They stretch and shrink in time with the sound, pumping extra energy into the membrane, which makes quiet sounds up to about 40 dB louder and sharpens the pitch map. The ear even gives out faint sounds of its own because of them, which is how newborn babies' hearing is screened.

Here is the catch: in humans, hair cells don't grow back. Very loud sound, some medicines, infections and age can damage them for good. Birds and fish can regrow theirs, and scientists are studying how. Hearing aids and cochlear implants can help people whose hair cells are damaged. If you notice ringing or muffled hearing that does not go away, see a doctor.

Your ears cope with a huge range. The loudest sound you can stand has about a million times the pressure of the faintest one you can hear. To squash that range into handy numbers we use decibels: dB = 20 × log₁₀(p ÷ p₀), where p₀ = 20 micropascals, about the faintest sound a young ear hears. Every +20 dB is ten times the pressure, and every +10 dB sounds roughly twice as loud. The “bel” is named after Alexander Graham Bell.

Loudness also depends on pitch. The equal-loudness contours (the international standard ISO 226) show that a 100 Hz hum must be much stronger than a 3 kHz tone to sound as loud. Your ears are most sensitive around 3 to 4 kHz, helped by the ear canal's resonance.

Too much loud sound for too long damages hair cells for good: noise-induced hearing loss. The WHO's safe-listening standard allows about 80 dB for 40 hours a week (75 dB for children), and half the time for every 3 dB more. So at 100 dB, like earphones turned right up, the whole week's safe dose is used up in about 24 minutes. The WHO warns that over 1 billion young people are at risk. In India, firecrackers louder than 125 dB at 4 metres are banned. Warning signs are ringing ears (tinnitus) or muffled hearing after a noisy event; if they don't go away, see a doctor.

With age, the highest pitches fade first. This is presbycusis. Many children hear up to about 20 kHz; many adults can't hear tones above 15 or 16 kHz, and it varies a lot. Our quick test is just for fun, not a hearing test: speakers and headphones often can't play the top notes either. For a real check, an audiologist uses calibrated equipment.

Right next to the cochlea sits your vestibular system, a balance sensor that works even with your eyes shut. It has two kinds of parts.

The three semicircular canals are fluid-filled loops at right angles to each other, one for nodding, one for tilting to the side and one for turning like saying “no”. Each loop has a bulge, the ampulla, with a jelly flap, the cupula, across it. When your head starts to turn, the fluid inside lags behind, like tea in a cup that you twist. The lagging fluid bends the cupula and its hair cells tell the brain: “we are turning”.

Keep spinning at a steady speed and the fluid catches up. The cupula springs back in a few seconds, and the feeling of turning fades. Now stop suddenly: the fluid keeps going, bends the cupula the other way, and you feel as if you are spinning backwards. That is why you feel dizzy after a spin on a playground roundabout.

The utricle and saccule sense tilt and straight-line movement. They carry tiny crystals, the otoliths (“ear stones”), on a jelly layer. Tilt your head, or speed up in a lift or a car, and the heavy crystals slide and bend the hair cells below. Motion sickness is thought to come from a mismatch: reading in a moving car, your ears feel every bump and turn while your eyes see a still page. Looking out at the horizon often helps. If dizziness comes on its own or keeps coming back, see a doctor.