So just what is 'normal hearing', your ear consists of three sections, that play all equally important parts. The external, middle and inner ear. Sounds travel through the air, which the external ear catch a bit like a satellite dish.
The sound travels into the ear canal which bounce against the ear drum. Causing it to vibrate, this in turn transfers the vibration to the three smallest bones in the body located in the middle ear.
( the three bones are called hammer, anvil & stirrup).

This vibration is then transferred to the Cochlea of the inner ear, when the three small bones move they cause waves of fluid in the spiral of the cochlea. These waves of fluid stimulate over 16000 very small and delicate hearing hair cells. The hair cells generate an electrical current to the auditory nerve when stimulated.
The electrical current travels through the auditory nerve system to the brain, and clever old brain recognizes the currents as sounds.

The fact that you have two separate ears, means that your brain can work out which side the sound is coming from.  So depending on the levels of sounds entering into each ear, you automatically know where around you, the sound has originated. This is something hearing people take for granted.

(see 'deaf aids' page too.)