The ear changes to-and-fro vibrations to a sequence of nerve impulses which are transmitted to the brain. As a result, you can hear a range of sounds, from loud to quiet and from a high-pitched squeak to a low-pitched growl. And all at the same time.
There are three parts to the ear: the outer ear, the middle ear and the inner ear. Each has a job.
The outer ear collects and channels the vibrations to the middle ear. The middle ear transfers the to-and-fro motion of the air into matching vibrations of the bones in the middle ear. These vibrations become to-and-fro motions in the liquid of the inner ear. This vibration is changed into nerve impulses, which are then transmitted to the brain as vibrating electrical signals.
There is much more to hearing, but that is a good start.
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