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(Sound) waves crashing on the shore

June 06, 2003

The real surprise is not that we lose our hearing due to loud noise but that we hear at all.

We are bombarded with sounds. Sound waves move through the atmosphere like ripples and swells across an ocean. And just like water waves, sound waves can strike our ears with painful and damaging force or cause gradual erosion over time.

Three sections of the ear work together to gather and focus sound waves. The outer ear is like a giant funnel which collects sound waves. The eardrum is the boundary between the outer ear and the middle ear.

In the middle ear, three tiny bones, known as the ossicles (nicknamed the hammer, anvil and stirrup), mechanically amplify sound vibrations and transmit them to the inner ear.

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The inner ear features a fluid-filled, coiled tube, like a snail shell, called the cochlea. The inside of the cochlea is lined with sensitive cells, each of which has a bundle of hairs extended into the fluid. These hairs are affected by sound waves in the fluid - like seaweed floating in waves at the beach.

The cells convert the physical motion of the hairs into electrical impulses, which are transmitted via nerve cells to the brain. The brain interprets these signals as sound.

According to the National Institute of Deafness and Other Communication Disorders (its Web site is www.nidcd.nih.gov), loud noises can damage the hair cells.

Both loud impulse sounds, such as a gun shot or auto horn close to the ear, and medium-loud continuous sounds, such as the noise of a chain saw or tractor engine, can damage the tiny, sensitive hairs in our inner ear.

The hairs are able to repair minor damage to themselves within 16 to 48 hours, according to researchers. But the effect of severe sound waves can break or rip out the hairs, leaving ears permanently changed.

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