Document 8VDoExykzz40nwg13Re1qkpqZ

662 JEROME R. COX, JR. TABLE 5 Frequency (c.p.s.) Reference pressure for Bound pressure level (pbar) Reference pressure for hearing loss" (pbar) Sound pressure1,j' level" correspondini to 0 db hearing Iobs (db) 125 250 500 1000 2000 4000 8000 .0002 .0002 .0002 .0002 .0002 .0002 .0002 .106 .0191 .00347 .00137 .00141 .00114 .00221 " These pressures are those that Bhould be produced by a Western Electric 705-A earpK measured in a coupler built and used according to the specifications set forth in American StajjJ for Audiometers for General Diagnostic Purposes.'* 1 audiometers designed for diagnostic purposes are 125, 250, 500, 1000, 2000, and 8000 c.p.s.15 For industrial programs a so-called "screening audionf having only four test frequencies, 500, 1000, 2000, and .4000 c.p.s., may be us Many of the "screening audiometers" being produced today have additional' frequencies at 3000 and 6000 c.p.s. because these test tones are very usej_ industrial hearing conservation programs. ^ In general, the test frequencies are related in a simple manner. 'Eachnsl sive tone is an octave (a multiple of 2) above the previous tone. This riifg may be applied to the series of test tones that were originally used on audiomj A frequency of 256 c.p.s., which is almost equal to that of middle C on thejpa was used instead of 250 c.p.s. The remaining test tones of the older sequenced 128, 512, 1024, 2048, 4006, and 8192 c.p.s. The slight differences from thej| frequency series are, for all practical purposes, unimportant. F. SOUND INTENSITY PRODUCED BY THE EARPHONES The intensity of a tone produced in the ear of the person being teste not always be indicated exactly by the setting of the hearing-loss dialT .frequencies any air leiiE'aTmTnaTKe'cuihibn oftKe'earphb'nE^wfirbe ___ tant in determining the intensity of'the tone actually produced in the .earXraW Since these leaks cannot be completely eliminated with present cushions,.'yjfrTM a significant amount of uncertainty in audiometer measurements at freq\^^^^ of the hearing-loss dial becomes. At 250 c.p.s. the range of variation in' S^ff tensity of the tone produced is about 15 db, and at 125 c.p.s. the range oft ar-iaui may be as much as 20 db. There are similar limitations on the high-ffefiy||. test tones produced by the earphones. Their exact placement on the ear ahclTMB American Standard for Audiometers for General Diagnostic Purposes, Z2l|| American Standards Association, New York, 1951. a? NOISE AND THE CONSERVATION OF HEARINO 663 nSipbali&d size of the ear canal will cause variations in the sound intensity pro- liBo the problems involved in the calibration of an earphone become more these frequencies. The range of variation in the intensity of the tone is 10 db at 4000 .c.p.s. and is considerably greater at 8000 c.p.s. and IP ' isit^iil'reasonable care hearing measurements at 500 to 2000 c.p.s. inclusive H^f'fi'IM^eliable within a range of 5 db on all except the most unusual ears. At gifer and lower frequencies the intensity generated in the ear canal for a "*|mg of the hearing-loss dial may be significantly different from person to These differences should be borne in mind when audiometer measurejfextreme frequencies are undertaken. lln'tsr the electronic components of the audiometer itself nor the earphones BSsdlutely stable. Over long periods of time, changes may be observed in 1 intensity produced by the earphones for given settings of the hearingprequency dials. In recent years, however, the seriousness of this problem flhsjgreatly reduced by advances in electronic and earphone design. It is still nevertheless, to maintain some sort of check on the calibration of the Bltem. Otherwise one can never be sure that the sound intensity produced Sarphones corresponds as closely as it should to that indicated by the Toss dial. G. PHYSICAL METHODS OF CALIBRATION Jit-physical method for calibration of the audiometer utilizes a microphone 'Small cavity, both of which are described in detail in recent American |^|jj8s.111,19 The earphone is placed on the cavity and the intensity generated ^^a|ie measured by the microphone. The sound pressure level that should be ^iclby a Western Electric 705-A earphone at each of the. test frequencies for jgsetlihg of the hearing-loss dial is given in Table 5. Similar requirements for Iglphones are given in American Standard Specification for Pure-Tone pters for Screening Purposes.16 fipmeters are calibrated routinely by this method by the manufacturer, Others it is usually a complex and expensive procedure. An experienced fesl engineer can accomplish these measurements without error, but others fajp^difficulty. Therefore, it seems clear that most audiometersji.iUincit-bave v ^eSimf'frequenFcalibration by physical methods. Most organizations have ^Suiuufjie necessary funds nor personnel. jK-ifChd'electrical portion of the audiometer can be checked by simpler measure'Upon receipt of a new audiometer the voltage at both earphones is B^^nrcdr7mcnrAteIW3ndaaec'otdedgfor-ea&h-test--freattehcvr~Peripd'lcgll?~tR'is~test IroralR- (repeated to determine if any electrical changes have taken place. Devia- fein^joffHmore than 2 db indicate that the instrument is in need of repair. Sg^jjyWraerican Standard Specification for Pure-Tone Audiometers for Screening Purposes, |Z2i''12-1952. American Standards Association, New York, 1952. BflHSntWSURi'. . ,