Once sound reaches the inner ear, it enters a bony tube coiled like a snail shell — the cochlea . Uncoiled, that tube runs roughly 33 to 35 mm long, and it varies noticeably from person to person. Running along its length is a strip called the basilar membrane . Everything that follows happens on that strip.it is not uniform . At the end nearest the incoming sound (the base) it is narrow and stiff; the further in you go toward the apex , the wider and more flexible it becomes.traveling wave runs along the membrane from base toward apex, growing in amplitude and slowing as it goes, until it reaches a point of maximum displacement. Where it peaks depends on frequency: high frequencies peak at the base, low frequencies at the apex.place . A membrane whose width and flexibility vary systematically vibrates maximally at different positions for different stimulus frequencies, which turns the whole strip into a map running from high pitch to low — this is tonotopy .when this happens: the splitting is mechanical, and it happens before hair cells convert vibration into neural signals. The cochlea is not only an amplifier and a transducer — it is also a mechanical frequency analyzer that decomposes a complex waveform into simpler elements before anything is handed to the nerve.in part to the geometry of the basilar membrane — it lays down the broad map; the rest is on the next page.