Basilar membrane:

The image above shows the cochlea unrolled to make the basilar membrane easier to see. the basilar membrane is the entire blue/grey structure.

the basilar membrane is found in the cochlea; it forms the base of the organ of Corti, which contains sensory receptors for hearing. Movement of the basilar membrane in response to sound waves causes the depolarization of hair cells in the organ of Corti. The hair cells transduce auditory signals into electrical impulses.

Read more: Know your brain - Cochlea

Watch this 2-Minute Neuroscience video to learn more about the cochlea.