How does the cochlea separate pitches?

One sound can contain several frequencies, yet the ear begins separating them through motion at different places in the cochlea. The stapes, a small middle-ear bone, moves at the oval window and drives fluid motion. That motion sets a traveling wave along the basilar membrane, beginning a mechanical separation before later neural processing.

The membrane’s mechanical properties change along its length. Near the base it is relatively narrow and stiff; toward the apex it is wider and more flexible. These differences make different regions respond most strongly to different frequencies. This is a connected mechanical system, not a row of isolated detectors each hearing only one note.

A wave travels from the base toward the apex. For a particular frequency, its motion grows toward a characteristic region, reaches a maximum, and then falls rapidly. High frequencies peak near the stiff base; lower frequencies travel farther before peaking. A mixture can therefore produce overlapping patterns associated with several frequency components. This frequency-to-place organization is called tonotopy.

Membrane movement bends sensory hair bundles and changes hair-cell voltage. Inner hair cells provide most of the sensory input to auditory nerve fibers. Outer hair cells feed mechanical energy back into the system, increasing sensitivity and sharpening tuning, particularly for faint sounds. The living cochlea therefore performs better than a passive mechanical model alone.

Uncoiling the cochlea makes its spatial ordering easier to see, but the three peak positions and wave envelope are qualitative teaching geometry. They are not a calibrated frequency map. Neural timing and further brain processing also contribute to pitch perception, so a motion peak marks an important first separation rather than the whole experience of hearing.