The Ears and Auditory Sensation
The apparatus for the ears is mechanical up to the point where sounds are transduced into neural impulses. The sound waves that stimulate our ears are produced when, for example, something in the world around you clicks, rings, vibrates, or scrapes against or hits something else. Each produces cyclical displacements of molecules in the air — that is, contractions and expansions of the air. Although you can't see these waves, you can get a clear idea of how sound is produced if you remove the front cover of a speaker system, turn up the volume, and watch what the bass and midrange speakers do.
One instance of expansion and contraction is a cycle, and sound waves are measured in cycles per second called hertz (Hz) — in honor of a prominent physicist. People vary considerably in the levels and the range of sound waves they can detect, but the typical range is 20 to 20,000 Hz. This is also the range that home electronics manufacturers try to cover as faithfully as possible, although modern systems often go below and above this range just to make sure that everything on a CD is reproduced.
Repeated exposure to extremely loud sounds can cause permanent damage to the structures of the ear and diminish hearing in certain ranges. Think about this the next time you're tempted to stand in front of the speakers at a rock concert.
How the Ear Works
Sound waves enter through the pinna — the outer part of the ear that you can see — and travel through the auditory canal to the tympanic membrane, or eardrum, which then vibrates as a result. The vibrations are amplified by several bony structures called the ossicles and transmitted to the cochlea (which, because of its coiled shape, derives from the Greek word for “snail”). While the cochlea is an incredibly complex structure, it is only about the size of a pea. Within the fluid-filled cochlea the vestibular canal carries the vibrations in the form of ripples to the basilar membrane, where thousands of tiny hair cells move in response to the ripples and transform the transduction into neural impulses. From there, the thousands of receptor neurons of the auditory nerve relay the signals to the thalamus and eventually to the auditory cortex of the temporal lobes. Partial or complete hearing loss can occur anywhere along the way, such as through the following:
Obstruction of the auditory canal
Inflammation of the auditory canal because of infection
Puncturing of the eardrum
Fusion or other damage to the ossicles
Less-efficient hearing associated with aging
Damage to the hair cells of the basilar membrane by viral infections or exposure to very loud sounds
Damage to the auditory nerve
Most of these conditions are correctible either through medical intervention or use of a hearing aid. The last two are not. Damage anywhere from the basilar membrane on is irreversible.