Mechanical tuning on the basilar membrane
When the stapes vibrates against the oval window, it causes the fluid to move down the vestibular canal. This vibration eventually reaces the helicotrema and then moves around the bend to cause fluid motion in the tectorial canal. Together these movements cause a sort of shearing action that produces pressure waves across the chelear duct. These pressure waves, in turn, cause mechanical waves to travel down the basilar membrane from the stiffer, narrower base to the looser, broader apex.