Békésy showed that we distinguish pitch because different spots on the basilar membrane in the cochlea vibrate for different sounds. By dissecting the cochlea, immersing it in saline, and filming the membrane with special optics, he observed a traveling wave: low notes peak deep inside, high notes peak near the entrance. This mechanism forms the basis of frequency resolution. It guides the frequency response design of hearing aids and helps diagnose noise-induced hearing loss. His work also revealed the remarkable efficiency of the auditory system: sound is physically sorted before nerve signals ever reach the brain.

Using nearly fresh mammalian cochleae, Békésy combined stroboscopic photography with phase-contrast microscopy to analyze basilar-membrane motion. He demonstrated that when sound pressure reaches the perilymph via the ossicles and the oval window, fluid inertia plus the membrane’s graded stiffness and mass produce a traveling wave that peaks at the site whose characteristic frequency matches the stimulus. He quantified how the stiffness gradient and fluid impedance create a spatial frequency filter whose layout is roughly logarithmic. This work established the prototype of the auditory filter-bank theory, later inspiring gammatone models and the channel spacing of cochlear implants. He also discussed how cochlear mechanics influence neural phase locking and masking phenomena, bridging psychoacoustics and biophysics.

Békésy uncoiled the spiral cochlea, removed Reissner’s membrane, and visualized basilar-membrane responses under open perilymph conditions. The observed phase-delay gradient and group-velocity dispersion were reproduced by a 1-D fluid-structure coupled model, showing that the vibration maximum occurs where local characteristic impedance is minimized. His data were later used to derive Zwicker bandwidths and critical ratios, confirming that passive mechanics alone can yield sharp tuning slopes of about 80 dB/decade. The results provided quantitative references for how presbyacusis-related stiffening shifts resonance patterns and serve as benchmarks for modern micromechanical OCT measurements. Although the concept of outer-hair-cell active amplification emerged in the 1970s, Békésy’s passive wave theory remains an indispensable prerequisite.