It is a stretchy strip inside the cochlea that responds to different pitches at different spots along its length.
Uncoiled basilar membrane: the travelling wave peaks at a different spot for every pitch, high at the stiff base, low at the floppy apex.
What it is
A stretchy strip inside the cochlea that splits sound into pitches, each frequency vibrating its own spot.
Key facts
Length ~32-35 mm, coiled inside the snail-shaped cochlea (~2.5 turns); a built-in frequency analyser.
Tonotopy: each pitch lights its own location. BASE end is stiff + narrow (~0.1 mm) = HIGH freqs up to 20,000 Hz; APEX end is floppy + wide (~0.5 mm) = LOW freqs down to ~20 Hz.
It runs BACKWARDS from a guitar string: stiff narrow handles HIGH, not low. Stiffness drops ~100x base to apex.
Mapping is LOGARITHMIC: each octave (2x frequency) takes roughly equal distance along the strip, just like an EQ scale.
Human range 20 Hz to 20 kHz; most sensitive 2,000-5,000 Hz, peaking 3-4 kHz (ear-canal resonance + speech consonants).
Travelling wave (von Bekesy, Nobel 1961) rolls base toward apex and peaks where the frequency belongs.
Masking: a loud low note floods nearby apex spots so a quieter note next to it vanishes.
How it helps in live sound
HPF-first: high-pass every non-bass channel at 80-120 Hz so the apex isn't flooded and mids stay clear.
Hunt feedback by ear: ringing usually sits 2-5 kHz where the membrane is most sensitive; notch with a tight Q.
Treat 3-4 kHz harshness gently; a few dB of cut relaxes a fatiguing, piercing PA.
Protect your own analyser: wear -15 to -25 dB musician earplugs; base-end hair cells die first and never return.
Use a 31-band graphic or RTA as an external basilar membrane to SEE the spectrum you are EQing.
Mind masking: carve competing instruments into different frequency lanes so each lights its own spot.
Everyday analogy
It is a piano laid out as one long ramp: tap a bass note and only the floppy far end shivers, hit a cymbal and only the stiff near end buzzes, so position equals pitch.
Watch out
Myth: stiff narrow = low pitch like a thick bass string. Reality: it is BACKWARDS, the stiff narrow base handles HIGH frequencies and the floppy wide apex handles LOW.
Fun fact
Outer hair cells physically dance, contracting up to 20,000 times a second; that active boost leaks back out as faint tones your ear actually EMITS (otoacoustic emissions), used to test newborn hearing.
Key takeaways
The basilar membrane is a biological spectrum analyser: each pitch lights its own location (tonotopy).
Stiff narrow BASE = HIGH (20 kHz); floppy wide APEX = LOW (20 Hz). It runs backwards from intuition.
Mapping is logarithmic, ~one octave per equal step, just like an EQ's frequency scale.
Most sensitive at 2-5 kHz, peaking 3-4 kHz, so that is where harshness and feedback bite.
Hair cells that fire the signals never regrow; protect them or lose high frequencies first.