350 



NOISE 



laboratory experiments (45, 48,) in general 

 agree that the high frequencies are more 

 annoying, other things being equal, than 

 middle frequencies (500 to 1000 c.p.s.). 

 This laboratory finding has in a sense been 

 verified by field studies dealing with the de- 

 gree of acoustical comfort provided by sound- 

 absorbing materials installed on walls and 

 ceihngs. Although such installations in 

 areas of high noise levels reduce the overall 

 noise levels only one, two, or three db, people 

 working in these areas generally report a 

 marked decrease in noise annoyance (4, 63). 

 Such materials absorb relatively more of the 

 high frequency energy than low frequency 

 energy and consequently produce a noise 

 spectrirai in which the high frequencies are 

 markedly reduced. 



Interrupted or discontinuous noise has 

 been found to be more annoying than steady 

 noises (47, 69). A number of writers have 

 referred to other factors, none of which have 

 been subjected to systematic investigation. 

 Among these are the unexpectedness of noise, 

 the spreading effect, or reverberation, and the 

 degree to which noise is unnecessary or indi- 

 cates malfunctioning of equipment. 



Noise Protection 



Efforts to deal constructively with the 

 noise problem may be directed to (a) the 

 suppression of noise at its source by redesign 

 of equipment, (b) the reduction of its trans- 

 mission through structures, (c) the reduc- 

 tion of reverberation by placing low density 

 materials on walls or ceilings, or (d) the use 

 of protective devices apphed to the ear mech- 

 anism. It is beyond the scope of this report 

 to deal with the first three of these methods, 

 although the Army Signal Corps recently 

 completed studies (75) directed to this end 

 on small Diesel engines. 



Investigations of various types of ear pro- 

 tectors has been pubhshed (79, 100), showing 

 that V-51R type ear wardens are in over-all 

 performance superior. These provide 25 to 

 30 db acoustic insulation at low frequencies 

 and 40 db or more at the high end of the 



audio-frequency range. Furthermore, speech 

 communication in the presence of high level 

 continuous noise is not importantly hindered 

 and may be definitely improved by the use 

 of these wardens. Service reports from field 

 tests indicate protection thus afforded against 

 gun blast is "adequate." 



Comparative acoustic insulation tests were 

 performed with other types of ear defenders 

 and showed that the V-51R gave generally 

 greater acoustic insulation with least distor- 

 tion over the audio-frequency range. The 

 author concluded, "A signal that is audible 

 to the open ear in a noise higher than 75 db 

 is equally audible or more audible when ear 

 wardens (V-51R) are worn. Noise levels in 

 submarine engine rooms, tanks and air- 

 planes usually range from 100 to 130 db; 

 hence, in these environments ear wardens 

 should improve speech communication" 

 (79). To obtain maximum acoustic insula- 

 tion, the ear wardens must be properly fitted 

 and those people who wear them must be 

 properly instructed. 



Other attempts at providing ear protec- 

 tion have used wax, artificial silk, cotton 

 impregnated with various semi-soft com- 

 pounds, and even a hood that apparently 

 covered the forehead and cheek bones as well 

 as the ears. Swedish sources (39, 61) re- 

 ported favorably on an isoHte ear plug that 

 was tested principally for protection against 

 gun blast. No audiometric data are avail- 

 able on the effectiveness of this plug. 



Selection 



In most of the laboratory studies as well 

 as many of the chnical observations reported 

 above, a frequent reference has been made to 

 marked individual differences in susceptibil- 

 ity to auditory fatigue and disturbances of 

 functions traceable to high noise levels. It 

 has been suggested that the detrimental ef- 

 fects of noise would be less serious if person- 

 nel could be selected who were least suscep- 

 tible to the adverse effects of noise. In spite 

 of this suggestion, which has appeared in 

 numerous places, there has been httle or no 



