The basilar membrane inside the cochlea vibrates as a traveling wave. High frequencies peak near the base (stapes end), low frequencies near the apex — a biological Fourier analyzer.
Basilar Membrane — Displacement Envelope
Current frequency envelope
Place of maximum response
Base (stapes, ~20 kHz)Apex (~20 Hz)
Traveling Wave Animation
Instantaneous displacement of the basilar membrane. The wave travels from base to apex, building up and then rapidly decaying at the characteristic frequency place.
Characteristic place x_BF = L·log(f_max/f)/log(f_max/f_min)
Human cochlea: L≈35mm, f_max≈20kHz, f_min≈20Hz.