NON-AUDITORY EFFECTS 



347 



the greater its effectiveness in producing 

 hearing loss, other things being equal. How- 

 ever, marked individual differences were 

 noted. This laboratory study confirmed the 

 cUnical observations noted above in demon- 

 strating that hearing loss is produced most 

 rapidly at the beginning of the exposure 

 period and then more and more slowly as 

 exposure is prolonged. Furthermore, recov- 

 ery is usually rapid at first and then proceeds 

 more and more slowly, following approxi- 

 mately an exponential curve. However, 

 hearing loss from exposures from 140 db for 

 two minutes is still considerable after two 

 hours, but recovery is nearly complete within 

 24 hours. This study found no evidence of 

 cumulative permanent effects of 15 or 20 

 exposures (at intervals of several days at in- 

 tensity of 130 or 140 db), provided recovery 

 is complete or nearly so after each exposure. 



Although he reports only chnical observa- 

 tions, J. A. Weiss (93) states "the harmful 

 level of sound intensity is considered to begin 

 between 85 and 100 db for prolonged noises." 

 One of the chnical cases reported in this 

 article is a 37-year-old patient who had a 

 history of duty in a Diesel engine room on a 

 patrol craft for eight hours daily, 24 days per 

 month, for 14 months preceding examina- 

 tion. His hearing loss extended over the en- 

 tire frequency range and varied from 10 to 

 55 db. 



These laboratory and chnical observations 

 of aviation, surface craft, and submarine per- 

 sonnel are in general agreement with com- 

 parable observations made of personnel 

 exposed to manufacturing and construction 

 noises of high intensity. McCoy (53) re- 

 ports audiometric tests of shipyard workers 

 before and after one day's initial assignment 

 as riveters and chippers exposed to rapid im- 

 pact noises of 110 to 135 db. Average hear- 

 ing losses were highest in the 2,000 to 5,000 

 c.p.s. range, varying between 50 and 60 db. 

 Losses in the lower frequencies were 15 to 18 

 db. Comparable results have been reported 

 in the following references: 6, 27, 66, 72, 73, 

 74, 89. 



Individual Differences 



Throughout the reports on stimulation 

 deafness is an almost universal comment 

 that marked individual differences occur 

 both in regard to susceptibiHty to hearing 

 loss and to speed of recovery. Thus, Ullman 

 (90) found that 30 percent of officers assigned 

 to Flying Fortresses or Liberators were found 

 to have a hearing loss of more than 50 db 

 in at least one frequency, although others 

 exposed an equal period of time showed losses 

 not exceeding 10 db. Senturia (78) reports 

 further that his study of 500 pilots before and 

 after exposure to aircraft noise showed, con- 

 trary to popular behef , no tendency for those 

 with greatest initial deafness to be unusually 

 sensitive to auditory fatigue. Furthermore, 

 there seemed to be an improvement in hear- 

 ing in some ears after 70 hours of flight when 

 he compared audiograms at that time with 

 previous measurements made at the end of 

 10 hours of flight, provided that both sets of 

 measures were taken later than 24 hours fol- 

 lowing the last exposure. This apparent 

 improvement in hearing has not been re- 

 ported in other studies, but if substantiated 

 might provide a basis for explaining the ap- 

 parent adaptation to noise to be discussed in 

 greater detail later. 



NoN-AuDiTORY Effects 

 Reports of the effects of noise on vital 

 processes such as blood pressure (25), respi- 

 ration rate (25, 60), metabohsm (32, 46, 47), 

 muscle tension (18, 19, 26), and digestive 

 processes (84), generally agree that the ef- 

 fects are such that, at least during the initial 

 periods of exposure, physiological "costs" in- 

 crease. The amount of this increase per- 

 centage-wise, its significance, and the speed 

 with which the individual adjusts physiologi- 

 cally to the adverse effects of noise are points 

 of disagreement among the various investi- 

 gators. In evaluating these studies it must 

 be noted that the noise levels and periods of 

 exposure employed in these studies did not 

 in any instance equal those characteristic of 

 submarine engine rooms when surfaced. 



