Document GKorEZLwJgORJ4yLLXbDg6grr

660 JEROME R. COX, JR. Thus, the method for measurement of the sound pressure, the kind of sponse that the listener is requested to give and, of course, the nature and mai of presentation of the sound are all factors that can influence the thresho) hearing. Therefore, one must use standard audiometric techniques and make di| comparisons of thresholds only under .like circumstances. D. NORMAL HEARING Normal hearing is the threshold of hearing found in young people wilt evidence or history of infection or other diseases of the ears. As with heig weight, their thresholds of hearing are not all the same, but are distributed 6. range of sound pressure levels of about 25 db. This is the "range of normaHh ing."ls The thresholds of these young people are distributed throughout range in a nonuniform manner, as are their heights and weights. Many arep the center of the range and only a few are at the extremes. For convenience in comparing thresholds of hearing sensitivity, a dec; scale is used. The reference level for most hearing scales is set arbitrarily art- commonest (modal) threshold sound pressure level (not the lowest sound pre~= level) for a group of listeners with normal hearing. The difference between^h'l arbitrary reference level and the sound pressure level at the listener's threi Hol'd called his "hearing loss.'' The use of the word "loss" here may "be somewh'aSmi leading because the smaller "hearing losses" lie within the "range of : no hearing." Another common cause of confusion is the existence' of "negativAvlfl^pi losses." Since the reference level is the commonest threshold for- normal Usks there will be many whose threshold sound pressure level is less than the1 r|||ra level. Thus listeners with "negative hearing losses" should be expected whenfa' group of normal listeners are tested, under appropriate conditions. . .- Because "hearing loss" is a term reserved for a particular use, it shonl be applied to the amount of hearing lost by an individual between two audiom?r examinations. Such changes in an individual's hearing are called "thro-' shifts." For example, a man has a 30 db "hearing loss" as the result of a""tlfi old shift" of 20 db. His original 10 db "hearing loss" may have.been quit'e#nfouma for him contrary to the misleading implication of the words "hearing loss." ...................... -......................E. PUR&TONE-AUTHOMETRY--------------------- The instrument of choice in hearing-conservation programs is the'k'iffiSdnd ^ tion, pure-tone audiometer. This device presents a tone like that from a(ftggjjp fork to the ear of the listener. The hearing loss for pure tones is usuaily?fS| _mind_at_a_numb.eiL.oi_req.uenaiesr.and_a_graph_of_hearjng-loss-v.sP-f-fe:(j&'n called an audiogram (Figure 24). Note that reductions in hearing spsi'twto ''For a clear statement of the concept "range of normal hearing," see Council dsMl-hfHi Medicine and Rehabilitation, Principles for evaluating hearing loss, J. Am. Med. Assoi^l.JSjjjj 1408 (1955). NOISE AND THE CONSERVATION OF HEARING 661 llffarinfflosses, are plotted downward. Negative hearing losses are plotted upward, JBmfeSfi&refcrence level indicated by the double line. '** Shafedial'of the audiometer that controls the intensity of the tone presented to iigjliS|!ner's ear is called the hearing-loss dial. The 0 db setting of this dial correqpulsW a reference level for the threshold for pure tones that was derived from FREQUENCY (CPS) Figure 24. The audiogram, a method for recording audiometric data. commonest value of the thresholds of a large number of listeners with preMwnormal ears measured in a survey carried out by the United States Public fflService in 1935.14 It should be emphasized that the reference level does not erflfthe best human hearing, nor the boundary between normal and abnormal he reference level does represent a threshold that is a few decibels more j|e'than a threshold in the middle of the "range of normal lfekring." "host audiometers, the markings on the hearing-loss diaTextend from --10 Sfextremely faint tone) to 100 db (a very intense tone). The "range of Sf ' extends from --10 db to about --f-15 db, but the limits are not clearly 'pit is important to note that these values of hearing loss do not correspond Itllpund pressure levetoorintensitv.Ieyels'gyen-dtoigh-tJiftMRtt^afca.a^i'fe'gtii-.. g|FA,uge1thb'-rSferehce: for the hearing-loss scale is different from that: for lgffiFthe two level scales and, in fact, varies from frequency to frequency as Table 5. 'second maj or control on the audiometer is the frequency dial, which' allows ''wstP^tS^sW-itcB-:tb^;hy;:6ne;;oFa-'p-oup-of--test-turies"rThose required on -ationaJ Health Survey: 1935-1936, Normal hearing by air and bone conduction, <flirWAreaUk Surv?v, Prelim. Rept. Hearing Study Series, Bull. 5, 1 (1938). ......