Document v66ab7y4520161zQJ8ma6Q8Qw

40--Noise and Hearing Conservation Hearing loss is an affliction which has ex isted throughout the history of mankind It occurs in all walks of life and is due to many causes Until the development of the audi ometer in recent years, there was no means of measuring the degree of hearing loss with appreciable accuracy Now partial losses are easily measurable by use of commercially available instruments The growing intei est of mdustual manage ment in hearing loss due to noise has been stimulated by (a) the trend toward coverage of partial loss of hearing under state work men's compensation laws, and (b) the passage of noise regulations bv the Occupational Safety and Health Administi ation in the U S Depaitment of Labor These developments have had consideiable impact upon the opera tions and profitability of many companies That workers in noisy occupations develop a greatei than aveiage degiee of hearing loss has been known for more than 100 yeais Prior to 1948, hearing loss was not regaided as a significant factor in workmen's compensa tion Some claims occuned, but these weie primaillv due to tiaumatic injuries fiom such causes as blasts, concussions, blows to the head, and foreign objects oi infections m the ears Since 1948, a numbei of states have passed hearing loss legislation oi levised existing laws to include noise-induced healing loss as a basis foi workmen's compensation benefits Now it is icpoited that theie are at least 16 states with workmen's compensation laws providing compensation m vaiying degiees foi partial hearing loss due to mdustual noise exposure What is noise? Technically defined, noise is unwanted sound It is a foim of vibration which may be conducted thiough solids, liquids, oi gases It is a form of eneigv m an, consisting of invisible vibrations that enter the ear and cieate a sensation Almost eveiy con ceivable kind of sound can, at one time, be welcome--at another time abhoued It fol lows that befoie attacking a noise ptoblem it must be asked Who doesn't want it? When? And why? When these questions aie answeied, steps can be taken to solve the prob lem In the ojiei ation of industrial machines, un balanced forces aie applied to paits of the machine and cause displacement or move ment of these parts, or the whole machine These displacements or motions are vibrations which set up air-bome sound waves When people come into direct contact with noise, undesired effects may lesult Properties of Noise Some of the properties of noise are intensity (or piessure), fiequency, and duration All of these are important factois m evaluating the effects of noise on man's heaiing The loudei the noise, the highei its intensity, highfrequenev (high-pitch) noises are more dam aging to healing than low-fiequencv noises, and the longer the noise exposure, the greatei the damage to the human heanng mechanism By examining these piopeities of noise caiefully, one immediately recognizes approaches to noise control or to heanng conseivation (Fig 40-1) Sound piessure or intensity follows the inveise-squaie law that is, as the distance fiom the souice increases the sound level deci eases as the square of the distance High-fiequenev sound waves aie mine leadily absoibed by filnous, spongy, and soft matenals of constiuction Finally, by limiting exposure time, woikeis' heanng conseivation can be ac complished The decibel The eai is a lemaikahle oigan, lesponding to sound piessuies fiom 0 0002 to 2000 dynes per squaie centimetei (Table 40-A) To avoid woiking with unwieldy huge numbeis in evaluating sound intensity, howevei, a loga rithmic scale is used with the decibel as the unit of measuie Because decibels aie loganthmic units, they cannot be added oi sub tracted anthmetically In fact, if the intensity of a sound is doubled, theie will be a coilesponding increase of only thiee decibels, not double the numbei Foi example, if one machine caused an exposuie of 90 dB, a sec ond identical machine placed adjacent to the fiist would lesult in a noise exposuie of 93 dB, not 180 dB On the decibel scale, zeio (deci bels) is the thieshold of healing, and 120 decibels is the thieshold of pain Frequency Fiequency is the number of vanations in 1242