What is "secondary tympanic membrane"

The secondary tympanic membrane (or round window membrane) covers the round window, sealing off one of two openings into the inner ear. It separates the scala tympani of the cochlea from the middle ear. It vibrates with opposite phase to vibrations entering the cochlea through the oval window as the fluid in the cochlea is displaced when pressed by the stapes at the oval window. This ensures that hair cells of the basilar membrane will be stimulated and that audition will occur.