It is how the ear packs sound info into the rapid electrical pulses sent up the nerve to your brain.
Sound bends a hair cell, which fires identical all-or-nothing spikes; their timing codes pitch and their rate codes loudness, up to a ~1,000/sec ceiling.
What it is
How the ear turns sound into electrical nerve spikes whose timing and rate tell the brain pitch and loudness.
Key facts
~30,000 fibres in the human auditory (cochlear) nerve carry every sound to the brain.
One spike = ~1 millisecond event; a fibre's hard ceiling is ~1,000 spikes/sec because of its ~1 ms refractory (recovery) gap.
Phase-locking: fibres fire in step with a sound's wave up to ~4,000-5,000 Hz, then timing info fails and only place (which fibre) codes pitch.
Rate coding: louder sound = more spikes per second + more fibres recruited (population coding), NOT a bigger spike.
All-or-nothing: every nerve spike is the same size (~100 mV swing); loudness is in the RATE, never the amplitude.
Dynamic range of ONE fibre is only ~20-50 dB, but high/low-threshold fibre types overlap to cover the ear's full ~120 dB range.
Tonotopy: the cochlea is laid out by pitch, base = high freq (up to ~20,000 Hz), apex = low freq (down to ~20 Hz).
Volley principle: groups of fibres take turns firing so the population tracks frequencies above any single fibre's 1,000 Hz limit.
Adaptation: spike rate spikes at a sound's onset then drops within ~10-20 ms even if the sound holds steady.
Healthy hearing range 20 Hz-20,000 Hz; speech intelligibility lives mostly in 500-4,000 Hz, the band where timing code is strongest.
How it works
Sound wave bends inner hair cell stereocilia, opening ion channels.
Hair cell releases neurotransmitter, triggering the nerve fibre.
Fibre fires an all-or-nothing spike, then needs ~1 ms to reset.
Pitch = which fibres fire (place) + spike timing locked to the wave (phase-lock).
Loudness = how fast the fibres fire + how many fibres join in.
Brainstem and cortex decode the spike pattern into what you hear.
Real examples
A 1 kHz tone: fibres fire roughly once per wave, locked to its timing.
A 10 kHz cymbal: too fast to phase-lock, so the brain reads pitch from WHICH fibres fire.
Quiet whisper: a few low-threshold fibres tick over slowly.
Loud kick drum: many fibres blast near their ~1,000 spikes/sec ceiling.
Dense, hot mix: fibres max out and saturate, so detail blurs into mush.
How it helps in live sound
Keep mix peaks below clipping; saturated nerve fibres = mushy detail for the audience, not just for the desk.
Carve space in 500-4,000 Hz (where timing code is sharpest) so vocals stay intelligible in a dense mix.
Give kick and bass their own slots; two loud low sources hammer the same fibres and turn to mud.
Use gentle compression to control crests, not brick-wall limiting; preserve transients the ear codes at onset.
Watch SPL: sustained >100 dB(A) pushes fibres into adaptation, so the crowd 'stops hearing' detail mid-set.
High-end above ~5 kHz reads by place not timing, so excess HF feels harsh before it feels detailed; ease off.
Everyday analogy
It is Morse code down a wire: the dots and dashes are nerve spikes, and their timing and speed (not their size) spell out the pitch and loudness.
Watch out
Myth: a louder sound makes a bigger nerve spike. Wrong: spikes are all-or-nothing and identical in size; loudness is coded by firing RATE and number of fibres recruited.
Fun fact
Above ~5 kHz your auditory nerve literally cannot fire fast enough to track the waveform, so the top octaves of music reach your brain by 'which wire' (place) alone, not by timing.
Key takeaways
Nerve spikes are all-or-nothing; loudness lives in rate, not spike size.
One fibre caps at ~1,000 spikes/sec due to a ~1 ms refractory gap.
Phase-locking codes pitch by timing up to ~4-5 kHz, then place takes over.
~30,000 fibres, each only ~20-50 dB range, overlap to cover ~120 dB.
Overload the code (too loud/too dense) and the ear blurs to mush.