Variation in noise levels is wide. In rural areas, ambient noise can be as low as 30 dB; even in residential areas in or near cities, this low level is seldom achieved. In urban areas, the noise level can be 70 dB or higher for eighteen hours of each day. Near freeways, 90 to 100 dB levels are not unusual. Many industries have high noise levels. Heavy industries such as iron and steel production and fabricating and mining display high levels; so do refineries and chemical plants, though in the latter few people are exposed to the highest levels of noise. Automobile assembly plants, saw-mills and planing mills, furniture factories, textile mills, plastic factories, and the like often employ many people in buildings with high noise levels throughout. Hearing impairment of such employees is probable unless corrective measures are taken.
The construction industry often exposes its employees to hazardous noise levels and at the same time adds greatly to community noise. Community noise may not be high enough to damage hearing (within buildings) and yet have an unfavorable effect on general health.
Transportation contributes largely to community noise. The public may suffer more than the employees—the crew and passengers of a jetliner do not receive the high noise level found along the takeoff and approach paths. The drivers of passenger cars often are less bothered by their own noise than are their fellow drivers, and they are less annoyed than residents nearby for psychological reasons.
Noise levels high enough to be harmful in their immediate area are produced by many tools, toys, and other devices. The dentist's drill, the powder-powered stud-setting tool used in building, home workshop tools, and even hi-fi stereo headphones can damage the hearing of their users. They are often overlooked because their noise is localized.
Some typical noise sources are listed in Table 6.2.1 and are classified by origin.
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