The sound is funneled through the outside of the ear, which is called the pinna, into the ear canal. These two parts are called the outer ear. The sound then vibrates the ear drum, which in turn sets the ossicles (a set of three tiny bones in the middle ear) in motion. This motion of the ossicles creates waves in the fluid of the snail-shaped cochlea. The cochlea is located deep inside, in an area called the inner ear. The cochlea is the place where the sound energy is converted into electrical impulses by thousands of tiny hair cells. The auditory nerve passes this information to the brain, where the details of the sound such as its characteristics, pitch, loudness, and direction, are then understood, so that the boy recognizes the sound of the engine as a car approaching from behind him. This is a rapid process that happens in less than a second. The speed of the hearing process allows the boy to quickly react.
Our hearing system, especially the hair cells in the inner ear, is very sensitive and can easily be damaged by loud sounds. Once these hair cells are damaged, the damage is permanent.

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