Anatomically, the ear is a usually divided into three parts: the outer ear, the middle ear and the inner ear.

The outer ear consists of the external ear together with a long tunnel, the external ear canal, which leads to the eardrum. The external ear doesn't do a whole lot other than to gather high-pitched sounds and conduct the sound through the ear canal to the middle ear. (An aside: there's a good reason why some people with hearing loss cup their hand next to their ear to try and hear better -- the high-pitched sounds typical of speech can then bounce off their hands and into their ear canals. In effect, they're enlarging the outer ear's sound-gathering area.)

The middle ear begins at the eardrum, which seals the middle ear from the external ear. The middle ear has a boxy shape. The job of amplifying and transmitting sound falls to three tiny bones of hearing (the malleus, incus, and stapes) that are linked together and lie within the middle ear. When sound waves enter the ear canal they set the middle ear into motion. The sound pressure is amplified some 22 times as it passes through the middle ear to the inner ear.

The inner ear is the most critical organ for hearing, converting sound into the electrical energy that is transmitted to the brain, where it is processed. The inner ear's cochlea has 20 to 30,000 microscopic hair cells in it which react to sound and stimulate the auditory nerve going to the brain. Under a microscope, you can actually see living hair cells seemingly dancing to music.

Too much noise can temporarily tire hair cells, cutting down signals to the brain. Usually this corrects itself, but repeated exposure to loud noise can leave you with permanent high tone loss and sometimes chronic ringing called tinnitus a condition that affects tens of millions of Americans.

  Video 28: How the ear works