From air pressure waves to music and speech: the eardrum and ossicles, the cochlea's piano-keyboard of hair cells, sound localisation with microsecond precision, hearing loss and cochlear implants.
Senses & Perception
Intermediate
Hearing
The Editors
October 2, 2026
3 min read
In plain English
Sound waves make your eardrum vibrate. Tiny bones pass the vibration to the cochlea, a fluid-filled spiral in your inner ear, where hair cells turn different pitches into nerve signals. Your brain then works out what the sound is and where it came from. Loud noise can permanently damage hair cells.
Intermediate level: Builds on the basics. Introduces the real vocabulary and the mechanisms behind it.
Concepts to know first
Sound is a wave of pressure changes travelling through air. Human ears can detect frequencies from about 20 to 20,000 hertz (vibrations per second) in young people; the upper limit falls with age.
The outer and middle ear
The outer ear funnels sound to the eardrum, which vibrates. Three tiny bones — the malleus, incus and stapes (hammer, anvil and stirrup, the smallest bones in the body) — transmit and amplify the vibration to the oval window of the inner ear. This matching is needed because sound in air would mostly bounce off the fluid-filled inner ear.
The cochlea
The cochlea is a snail-shaped, fluid-filled tube. Running along it is the basilar membrane, which is stiff and narrow at the base and wider and floppier at the apex. High frequencies vibrate it most near the base, low frequencies near the apex — Georg von Békésy won the 1961 Nobel Prize for showing this travelling wave. The cochlea therefore works like a piano keyboard laid out by pitch, a tonotopic map preserved all the way to the auditory cortex.
Along the membrane sit hair cells topped with bundles of tiny stereocilia. When vibration bends them, mechanically-gated channels open and the cell signals to the auditory nerve. Each cochlea has about 3,500 inner hair cells, which send most information to the brain, and about 12,000 outer hair cells, which actively amplify vibrations — so actively that healthy ears emit faint sounds of their own (otoacoustic emissions), used to screen newborns' hearing.
Where is that sound?
The brainstem compares the two ears. A sound on your left reaches your left ear slightly earlier — differences as small as about 10 microseconds can be detected — and slightly louder, because your head casts a sound shadow. The shape of the outer ear helps judge elevation.
Hearing loss
Hair cells in mammals don't regenerate, so loud noise and ageing (presbycusis) cause permanent loss, often of high frequencies first. Hearing loss is now recognised as one of the largest modifiable risk factors for dementia. Cochlear implants bypass damaged hair cells by electrically stimulating the auditory nerve with an electrode array threaded into the cochlea; hundreds of thousands of people use them, and children implanted early often develop spoken language well.
Everyday examples & things to try
Analogy
A keyboard rolled into a snail shell
Uncoil the cochlea and you'd have something like a piano keyboard: high notes at one end, low notes at the other. Each hair cell is a key that responds best to its own note.
Real world
Why concerts leave your ears ringing
After a loud concert, hair cells are stressed and may temporarily stop working properly, causing muffled hearing and ringing (tinnitus). Repeated exposure can kill them for good — and they don't grow back. Earplugs at loud events genuinely protect your future hearing.
Key takeaways
- Eardrum and ossicles transmit sound to the fluid-filled cochlea.
- The basilar membrane maps pitch along its length; hair cells transduce vibration.
- Mammalian hair cells don't regenerate; cochlear implants can bypass them.
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