What part of the ear determines pitch a sound?

The part of the ear that determines the pitch of a sound is the basilar membrane. The basilar membrane is located within the cochlea, which is a spiral-shaped organ in the inner ear. The cochlea is filled with fluid, and when sound waves enter the cochlea, they cause the fluid to vibrate. These vibrations are then transferred to the basilar membrane, which is made up of thousands of tiny hair cells. The hair cells on the basilar membrane move in response to the vibrations of the fluid, and the different frequencies of sound waves cause different patterns of movement on the basilar membrane. The brain uses these patterns of movement to determine the pitch of a sound.