Document 3d4wpq47yOQNK901yvy2w1VD

672 JEROME R. COX, JR. used to bridge this gap in information. It seems certain that eventually there'.be standardization in this area, and it is probable that a method relating cp pensation to the lack of ability to understand everyday speech will be agreed u||> R. THE SEVERITY OF A NOISE EXPOSURE The severity of a noise exposure depends upon a number of factors. In ass ing any noise exposure, each of these factors should be evaluated as careful!. possible. ** The intensity level of the noise is probably the most important single iaSi in determining the severity of the exposure, but by itself can never be used'p' criterion for safety or as evidence of a causal relationship between a noisi, a person's hearing impairment. For instance, peak sound pressures up to O.iff (a pure tone with a level of 160 db would have the. same peak pressure) c_ tolerated without permanent damage to the hearing apparatus under cip stances such as short exposures to artillery fire and heavy explosions.21 ; The duration of a noise exposure is a tremendously important facto|f longer the exposure to intense noise, the greater and more permanent can B[ resulting impairment. An exposure that continues for eight hours a day throug a working lifetime is many times more serious than one of similar intensify occurs only once and lasts for a few minutes. A|- The temporal pattern of the noise must be taken into consideration. E* to a noise for one hour out of eight per day may have a completely differfefi1 than exposure to'the same noise'for "one minute'out'of'every ei'ghlrthrmigln day. Impact noises, such as those produced by drop hammers, are momeL of very high intensity, yet the average intensity may be relatively low. data give us almost no information that allows comparison of the effects' intermittent noises with the somewhat better-known effects of continuous:`ifr The frequency distribution of the noise must be examined becauS'dSf to the hair cells is not uniformly probable nor is it equally significant thro the audible frequency range. Hearing loss occurs at other frequencies thaw present in the stimulating noise. In fact, no matter what the 'speetrUroWif noise, the irreversible part of the hearing loss is usually most severe in 'tlfc to 6000 c.p.s. range. However, when the energy of the stimulating noise isapri^ in'the~mWFfre'qrB'cy~ran'ge7there''isiikely''to''toe'mOTe-h'eariffg~liosKbel^ c.p.s. than when-the energy of the stimulating noise is primarily;i'ffif||feMijg| quency range. Since hearing loss at high frequencies has little effect oS 'lliuS to understand everyday speech, noise concentrated in the mid-frequisic^ is-a-more-imnortanyhazard-than a noise of similar intensity.-at--higli-ff.eV:i Hearing loss is relatively less probable when the stimulating noise CR^pp trated at low frequencies. Thus the mid-frequency range (300 to 24003<"`pif' "BENOX Report, An exploratory study of the biological effects of noise, !0.\/ffj NR 144079, 1953, p. 32. '* NOISE AND THE CONSERVATION OF HEARING 673 I ^importance in noise control for the conservation of hearing. In fact, Intid^ta20 indicate that the 300 to 600 c.p.s. octave band should be weighted ||yily.in estimating the effects of noise on the hearing loss for speech. wH"oand width of the noise should be known, at least approximately. Nar- 5ahd|,,and pure-tone sounds appear to be somewhat more damaging than , `1 noises of the same intensity. 5R!ic. susceptibility to noise of exposed persons may vary over a wide range, to develop a significant hearing loss after long exposure to intense ffi|?iitf.^|v:'.ereas others may develop a severe impairment. The urfusually susceptible nal`,can be tentatively identified only through repeated audiograms taken '~^ie1pxppsed group. He will suffer significantly greater changes in hearing seni^mKy'jfhan. his co-workers in the same noise exposure. Also, upon leaving the `^i^iie-j^jless likely to show rapid improvement in his hearing. llfftOW MUCH NOISE EXPOSURE CAUSES HEARING DISABILITY? Spllpririations in individual susceptibility make the task of answering the lllpiow much noise is too much?" extremely difficult. A small percentage |||mi]}.of people will incur hearing loss from age and other causes no matter leir.hoise environment. Some will incur presbycusis (the natural loss of result of aging) before middle age, while others may have normal ^gfSj.wcll mto old'age. The symptoms of these hearing losses will be, in some ^nditfifiguishable from the symptoms of hearing loss caused by noise. Theref|||!||fter how quiet the environment, a certain percentage of the employees rf"lEtS'''have''ar''noise-md'uc'ed~iihpairmen7"As_indicated above, these im- jmay actually result from diseases of the central nervous system, cerSlflnesses, hereditary causes, head injuries, explosions, nonoccupational jPp^, and, above all, presbycusis. "^gn'these factors influence only a very small percentage, of the group, ^jfnfcA.makes an accurate definition of the "maximum nonhazardous noise ^^pmost impossible. That is, when the number of people in the group who usability following repeated noise exposure is about.the same as SsMgfeVho may be expected to incur similar disability during the same TMc||||me?as the result of some other cause, it becomes statistically imprac- Blilafita reliably the effect, of noise. .In such,a. situation,,>the'.uncertainty whether or notnoise has actually produced damage, to tfeat an unreasonably quiet condition would have to be estab- the possibility of hearing impairment to the most susceptible t*h|group. The cost of noise control to this end would, in most eases, i.ate-^additionr-ibecmlae~Jif''^he-BtatisticH.l_d.ifJfiicfiici.l.,--tieVRii_-_._dj.*iQ___a__H__S___H__P_ ah<)^bwkr |..(;jm||pl-may actually be sufficient in many cases. It seems impractical, Mjattempt complete elimination of all industrial noiBe exposures,. A approach is the reduction of any noise that impairs the hearing significant percentage -of the people exposed. 11