Document mb980X3KkmO2V3R1DmM7vm79O

664 JEROME R. COX, JR. Changes in the sensitivity of the earphones cannot, of course, be detected by tin1 method. ' ]Jf H. PSYCHOPHYSICAL METHODS OF CALIBRATION A useful method for the calibration of the entire audiometer involves :i measurement of the hearing of a number of individuals who have no histor^ noise exposure or other hearing difficulties (see Section III.O). The average a\|| gram of this group will usually give a reliable indication of the calibration oti audiometer. ' ;i The audiometric procedure for the test group should be the same ast standard procedure adopted for all tests (see, for instance, Hirsh17). Aboil,, young people should be tested in both ears. This number of audiograms appj to be sufficiently large to give a reliable average at the important audiohfi frequencies, 500, 1000, 2000, and 4000 c.p.s. If measurements at either high|f lower frequencies are contemplated, it may be necessary to test a larger $r of jieople. If the average of the audiograms at the important frequencies is Wit 5 db of the zero setting, it may be assumed that the audiometer is calibr;. correctly. If larger deviations are observed audit is reasonably certain ths!f group used in the test is normal, the instrument should be returned to the hi facturer or his representative for repair and recalibration. If the normalcy hi test group is subject to question, comparison measurements can be made1 another audiometer. A simpler psychophysical procedure may be used to detect -changes-jW calibration of the audiometer. A small group of practiced listeners (perhaps. 5) should be tested regularly. Any systematic change in their thresholds 'iS* ally an indication of a change in the calibration. Of course, the operator sf be suspicious of individual threshold shifts. They may be caused by wax if ear canal or a mifd respiratory infection. Since this procedure does not attem give an absolute calibration of the instrument, the listeners need not be . nor have normal hearing provided that their hearing loss is relatively stable; cause this procedure is quite simple to carry out, it is recommended thatn repeated with the same listeners at least once for each full week of operati ' the audiometer. " I. MASKING Acuity of hearing is influenced strongly by the ambient noise in the tl room. If this ambient or background noise is excessive, the quiet tones canrf heard and-the-individual will show an apparent.hearing .loss. .This , apparent^ ing loss is the result of an auditory phenomenon called masking. We are fa||' with other aspects of this phenomenon, such as the difficulty in hearing ou^. mobile radios as a result of road or wind noise. Masking means the inabiliS "1. J. Hirsh, The Measurement oj Hearing. McGraw-Hill, New York, 1953, p. 364,. NOISE AND THE CONSERVATION OF HEARING 665 Idesired signal in the presence of an undesired sound or background noise, ^measurement of hearing, background noise will reduce the ability of the |?to hear the test signal and, therefore, produce an apparent hearing loss. it? j, HOW.QUIET MUST IT BE TO MEASURE NORMAL HEARING? li&rder to design satisfactory yet practical rooms for the pure-tone testing Sgwe should know the allowable background noise level for each of the l|tric test tones. The noise at frequencies near that of a test tone is most Sin masking that tone and, therefore, the criteria for the background noise I'diometer room will depend directly on the test frequencies that are to be JSRthe hearing measurements. For example, only the criterion shown in wlor the 300 to 600 c.p.s. octave band need be observed if correct audiomeasurements are to be made at ,500 c.p.s. The appropriate octave band Iffor other audiometric test frequencies are also listed in Table 6. If the Sband levels of the background noise in the test room exceed any of these "^masking .of the corresponding test tones is likely for a 0 db setting of the floss dial. This condition would cause some listeners to have an apparent floss that would disappear were the measurement made in a room with Jfiind noise less than that specified in Table 6. H # TABLE 6 Maximum Allowable Sound Pressure Levels for No Masking ____ at Zero Hearing Losa.on~Standard Audiometers - lj. Audiometric test ug a*i- frequency (c.p.s.) 500 1000 2000 4000 Important octave band (c.p.s.) 300-600 600-1200 1200-2400 2400-4800 Sound pressure level in the octave band (db) 57 JWaBW ---- ------- j----------- w* vAwooivc uuiac. ai uetJllU aUQlOIDGtnC o'be kept, it is mandatory that the measurements be madelin a I'cjuiet environment. \ -? -GENERAL;BRINGU`-LER,QPATIDTQMT.TK'R_'RQQM-t>F,SIR-N- ickground noise in an audiometer room is usually most intense at. low lithe control of low frequency noise is particularly difficult. Smaftllaks jltsand windows and the openings of ventilating systems provide exceljpfor the transmission of these sounds. Vibrations travelling through A'