It is the orderly map where each pitch has its own fixed place, kept neat all the way from your ear up to your brain.
A pitch peaks at its own fixed spot on the basilar membrane, and that low-to-high order is copied all the way to the brain.
What it is
An orderly frequency map where each pitch keeps its own fixed spot, from the cochlea all the way up to the brain.
Key facts
Cochlea is a coiled fluid-filled tube ~35 mm long, about 2.5 turns.
Human hearing range: 20 Hz to 20,000 Hz (20 kHz) in young, healthy ears.
High frequencies map to the BASE (near oval window); low frequencies map to the APEX (tip). The basilar membrane is stiff/narrow at base, wide/floppy at apex.
Each spot has a fixed 'characteristic frequency': 2 kHz always fires the same place.
~3,500 inner hair cells transduce; ~12,000 outer hair cells amplify (+40 to 60 dB cochlear amplifier).
Tonotopic order is preserved end-to-end: auditory nerve to cochlear nucleus to inferior colliculus to A1 cortex.
Critical bands: cochlea analyses sound in ~24 overlapping filters (Bark scale), each roughly 1/3 octave wide.
Speed of sound in air = 343 m/s at 20 degrees C; octave = frequency x2; cochlea spacing is logarithmic, not linear.
+6 dB = double sound pressure; +10 dB = ~double perceived loudness; +3 dB = double power; -3 dB = half power.
How it works
Sound vibrates the eardrum, then the 3 ossicles.
The stapes pushes the oval window, sending a pressure wave into cochlear fluid.
A travelling wave runs along the basilar membrane and peaks at the spot tuned to its frequency.
High pitches peak early (base), low pitches peak late (apex) = the place code.
Hair cells at that spot bend and fire the matching nerve fibre.
The brain reads WHICH fibres fire, keeping the low-to-high map all the way to A1 cortex.
Real examples
A 2 kHz snare crack and a 60 Hz kick land on totally different cochlear spots, so you hear both clearly.
Boosting 3 kHz on a vocal EQ lights up the same fixed cochlear region every time = predictable presence.
Years of loud gigs kill base (high-Hz) hair cells first = classic noise-induced cymbal/hi-hat hearing loss.
A cochlear implant lays electrodes along the cochlea low-to-high, faking the natural tonotopic map.
You pick a guitar's note out of a full band because its pitch owns its own place on the map.
How it helps in live sound
Trust your EQ: a cut at 4 kHz hits the same ear region for you and the crowd, so feedback ringing is findable.
Ring out monitors by sweeping a 1/3-octave EQ; the howl sits in one critical band you can pinpoint and notch.
Highs fatigue first: protect 2-5 kHz hearing with -15 to -25 dB earplugs at loud shows.
Carve by frequency, not volume: park kick (~60-100 Hz), bass (~80-250 Hz), vocal (~2-4 kHz) in separate cochlear lanes.
Narrow Q for surgical notches, wide Q for tonal shaping, matching how critical bands overlap.
Reference around 85 dB SPL: tonotopic sensitivity (Fletcher-Munson) is flattest there for honest mixes.
Everyday analogy
Like a piano keyboard rolled into a snail shell: low notes at the deep end, high notes at the mouth, and the order never gets shuffled.
Watch out
Myth: the cochlea is linear like a ruler. Reality: it is logarithmic. 100-200 Hz is the same perceived 'jump' as 1000-2000 Hz, so think in octaves, not Hz steps.
Fun fact
Your cochlea tunes its frequency map BEFORE birth: a foetus starts hearing low bass (~500 Hz) by about 25 weeks, with the apex-to-base layout already locked in.
Key takeaways
Every pitch owns a fixed physical place from ear to brain = tonotopy.
Base = high Hz, apex = low Hz, mapped roughly logarithmically.
Place coding rules the highs; timing/phase-locking rules the lows.
This map is WHY frequency tools like EQ make intuitive sense to your ears.
High-frequency hair cells (base) die first from loud-noise damage.