Outer hair cells have a special function within the cochlea. They are shaped cylindrically, like a can, and have stereocilia at the top of the cell, and a nucleus at the bottom. When the stereocilia are bent in response to a sound wave, an electromotile response occurs. This means the cell changes in length. So, with every sound wave, the cell shortens and then elongates. This pushes against the tectoral membrane, selectively amplifying the vibration of the basilar membrane. This allows us to hear very quiet sounds.
When you are exposed to loud music or noise, it is your hair cells which are damaged. Hearing loss occurs because loud sounds are really just large pressure waves (like when you stand next to a subwoofer and can "feel" the bass). These large pressure waves bend the stereocilia too far, sometimes to the point where they are damaged. This kills the hair cell. Since cochlear hair cells can not grow back, this manifests as a permanent hearing loss.
This is just a simple explanation of very complex processes which occur during the process of hearing. For more information, we recommend two websites: National Institute on Deafness and Other Communication Disorders (National Institutes of Health) and the Department of Neurophysiology at the University of Wisconsin - Madison.
Project supported by research grants from the Hearing Health Foundation, and the National Institutes of Health DC00354, DC02775 (to William E. Brownell) from NIDCD. Illustrations by Carl Clingman.
Learn more about: How the Ear Works | Outer Hair Cells: Images