The ear is composed of three main parts. Shape of the outer ear gives us information about the direction sound is coming from and funnels sound into the middle ear. The middle ear overcomes the impedance mismatch between air and liquid and transmits sound to the cochlea via the ossicles. The cochlea gives us our sense of balance and turns mechanical vibrations into nerve impulses. There are two theories of hearing, the place theory and the temporal theory. Neither can account for the richness of our perception of sound such as the missing fundamental, critical bands or other interesting aural illusions. Both theories may operate but in different frequency ranges. Age related hearing loss, presbycusis, generally lowers the loudness of high frequencies as we grow older. Loud sounds, especially if repeated, can also cause either conductive or sensorineural hearing loss.