The human ear is the envy of even the most sophisticated acoustic engineer. Without a moment's thought or the slightest pause, you can hear the difference between a violin and a clarinet, you can tell if a sound is coming from your left or your right, and if it's distant or near. And you can discriminate between words as similar as hear and near, sound and pound.

Nearly everyone experiences trouble hearing from time to time. Common causes include a buildup of earwax or fluid in the ear, ear infections, or the change in air pressure when taking off in an airplane. A mild degree of permanent hearing loss is an inevitable part of the aging process. Unfortunately, major hearing loss that makes communication difficult also becomes more common with increasing age, particularly after age 65.