Some noise sources are so intense, so widespread, or so unavoidable that they must be characterized as specific cases.
Pile driving and building demolition involve violent impacts and large forces and are often done in congested urban areas. Some piles can be sunk with less noisy methods, but sometimes the noise and vibration must simply be tolerated; however, these effects can be minimized and the working hours adjusted to cause the least disturbance.
Blasting for such construction can be controlled by the size of charges and protective mats. Some steel mill operations and scrap-handling operations are equally noisy and produce vibration but are normally not found near residential areas.
Sonic booms from aircraft extend over wide areas and affect many people. These noises are impulsive, and people respond to them as to other impulsive noises; the pressure levels are not high enough to be especially hazardous to hearing (maximums usually one to three pounds per square foot), but they can produce large total forces on large areas, for example, flat roofs.
Some air-bag protectors, designed to protect passengers in automobiles, can produce noise levels of 140 to 160 dB inside a closed car and, at the same time, sudden increases in atmospheric pressure at the ear. Teenagers who use headphones to listen to music often impose hazardous sound levels on their ears; they do not realize it, and no one else hears it. Hearing aids have been known to produce levels so high that in an attempt to gain intelligibility, actual harm is done. In these instances, the frequency response of the unit should produce the high levels only in the frequency range where they are needed.
In construction work, explosive-actuated devices can produce high noise levels at the operator's ear. Chain saws and other portable gasoline-powered tools are used close to the operator and, thus, their noise readily reaches the ear.
The dentist's drill, used several hours per day close to the ear, is a hearing hazard. Even musicians (especially in military bands and in amplified rock-type music groups) have shown hearing losses after some time. Usually, of course, music is not continuous, and the intervals of rest for the ear are helpful.
Some special industrial processes, such as explosion-forming, shot-peening, and flame-coating, are so noisy that they must be performed in remote locations or behind walls. Many mining, ore-dressing, and other mineral-processing operations are performed in remote locations; but those employees who must be present must be protected.
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