Noise enters our ears as powerful waves of mechanical energy
Scien¬tists measure sound intensity in decibels (db) , with each doubling of en¬ergy adding ten decibels. Ordinary conversation measures about 60 db; a child's scream hits around 90 db. On this logarithmic scale, the scream is potentially 1000 times more powerful.
Each day, over five million Americans are exposed on the job to at least 90 db , the maximum safe level for an eight-hour period , according to the federal Occupational Safety and Health Administration. "This standard isn ' t ideal , because noise affects individuals differently , " says William Clark of Washington University's Central Institute for the Deaf in St. Louis. "In theory, the standard should protect the lifetime hearing of 90 percent of workers. However, it assumes that a worker's ears will have 16 hours of quiet each day during which to recover an unlikely assumption for most peo¬ple."
Sound causes thousands of tiny hairs in the inner ear to vibrate. These vibrations trigger nerve impulses to the brain, which are perceived as sound. Prolonged exposure to 85 db or more , or far shorter exposure to very intense levels such as the 140-db shock waves from a shotgun blast can ir-revocably damage some of these delicate inner-ear hairs. Ronald Reagan suffered permanent injury during his acting days when a 38-caliber pistol loaded with blanks was fired near his right ear. As a result , he now wears a hearing aid. Audiologists predict that by the year 2010, as many people could be wearing hearing aids as now wear contact lenses.
Many people believe that weaker hearing is an inevitable part of aging. But studies show that those who live in low-noise environments tend to have very little hearing loss in old age.
In noisy industrial nations, however, even young people suffer dam¬aged hearing. David Lipscomb , a former professor of audiology at the Uni¬versity of Tennessee at Knoxville, tested over a thousand incoming fresh¬men and discovered that six of every ten had hearing loss typical of the el¬derly. Rock music is one cause. The noise in a rock-concert hall can easily
exceed 120 db, roughly the level of an air-raid siren. High-tech gadgets (i$rtt) such as powerful portable stereos also threaten to put our hearing into a downward spiral.
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