Cochlea

It is the small snail-shaped, fluid-filled part deep in your ear that turns sound vibrations into signals your brain can read.

Sound vibrates the eardrum, the stapes pumps cochlear fluid, a wave runs the coil, and hair cells fire nerve signals to the brain.

What it is

The snail-shaped, fluid-filled inner-ear organ that converts sound vibrations into electrical nerve signals for the brain.

Key facts

Coiled ~2.75 turns; uncoiled length ~33 to 35 mm; whole organ ~size of a pea (~9 mm), set in the temporal bone.
~16,000 hair cells per ear: ~3,500 Inner Hair Cells (the real sensors) + ~12,000 Outer Hair Cells (amplifiers, up to ~40 to 60 dB gain).
Healthy hearing range: 20 Hz to 20,000 Hz (20 kHz).
Tonotopic map: BASE of coil = HIGH freq (20 kHz), APEX (tip) = LOW freq (20 Hz); the basilar membrane is stiff at base, floppy at apex.
Three fluid chambers; endolymph is high-potassium (K+) at the endocochlear potential ~+80 mV = the battery powering hearing.
Most fragile region ~3 to 6 kHz: the audiogram noise-notch (usually 4 kHz) dips here first.
Hair cells DO NOT regrow in humans: damage is permanent and cumulative for life.
Exposure limit (Safe Work Australia): 85 dB(A) over 8 hours; every +3 dB HALVES safe time (88 dB = 4 h, 91 dB = 2 h, 94 dB = 1 h, 100 dB ~15 min).
Peak limit: 140 dB(C) peak causes instant damage, no safe duration.
Speed of sound ~343 m/s at 20 deg C (used for PA delay/time-alignment, not inside the cochlea).

How it works

Sound waves hit the eardrum and vibrate it.
Three ossicles (malleus, incus, stapes) amplify and push the stapes onto the oval window.
Stapes pumps the cochlear fluid (perilymph), sending a travelling wave down the coil.
The basilar membrane ripples; the wave peaks at the spot matching the sound's pitch.
Hair cell stereocilia bend, open ion channels, and let K+ rush in.
Hair cells fire electrical spikes down the auditory nerve to the brain = you hear.

Real examples

A 1 kHz tone (vocal/snare body) peaks roughly mid-coil and bends those specific hair cells.
A sub-bass 40 Hz kick travels all the way to the apex (tip) of the spiral.
Cymbals/hiss at 12 kHz excite hair cells right at the base, the first to die from loud gigs.
After a loud unprotected gig, ringing (tinnitus) = those base hair cells are bruised or dying.
An audiogram 'noise notch' at 4 kHz literally maps the dead patch on the basilar membrane.

How it helps in live sound

Wear earplugs at FOH: musician filters (Etymotic ER20, ~20 dB flat) keep the mix tonally correct.
Treat 3 to 6 kHz as the danger band: dont park yourself in front of horns/cymbals for hours.
Use the 85 dB / 8 h rule with +3 dB halving: at 100 dB(A) safe time is only ~15 min.
Carry a dB meter (or phone app) on C-weighting; watch for 140 dB(C) peaks from kicks/snare cracks.
Give your cochlea recovery time: quiet breaks between sets stop temporary threshold shift becoming permanent.
Roll off ear-fatiguing 2 to 5 kHz harshness on the PA: protects the crowds hair cells too.

Everyday analogy

Like a curled-up garden hose full of water with thousands of tiny tuning-fork hairs inside, each hair wobbling only for its own note and shouting that note to your brain.

Watch out

Myth: 'my hearing came back the next morning so no harm done.' Truth: that's a temporary threshold shift; repeated shifts kill hair cells permanently and they never regrow.

Fun fact

Your outer hair cells are tiny active amplifiers that physically dance, contracting and stretching up to ~20,000 times per second, and they actually emit faint sounds back out your ear (otoacoustic emissions) that doctors can record.

Key takeaways

The cochlea is where sound literally becomes hearing: vibration to nerve signal.
It is a frequency analyser: base = treble, apex = bass (tonotopic map).
~16,000 hair cells per ear, and they never regrow once killed.
85 dB(A) for 8 h is the limit; every +3 dB halves your safe time.
3 to 6 kHz dies first; that is why gig hearing loss starts in the treble.
Earplugs are non-negotiable PPE for anyone working loud PAs.