Cochlea.
The part of the ear where sound wave compressions and rarefactions cause the eardrum to vibrate is the middle ear. The 8th nerve in the inner ear actually converts the mechanical energy to electrical energy for transmitting to the brain. A membrane called the tympanic membrane separates the middle ear from the outer ear. Whenever a sound reaches the ear, it creates a sound wave that creates vibration in the eardrum. The pressure when high pushes the membrane inwards while low pressure sound waves helps the eardrum to come outwards.
These sound waves are then transduced when it reaches the cochlea where hair-like structures interprets the sensory information and is relayed to the brain.