Document b5bEJLYBGOjJkKydREXrpwrdk

NOISE AND THE CONSERVATION OF HEARING 659 J|^ applications, but more complete models are available for diagnostic pur- ipjpached to each audiometer is a set of earphones fitted with rubber cushjjgajsnug fit over the ears. An electrical signal produced by the audiometer nsraitted to the earphones where it is converted to an acoustic signal. Jlustrial programs for the conservation of hearing require knowledge of ^ir simplest aspects of audiometry. The following discussion is limited to ifiiic aspects, but this should not lead the reader to the conclusion that the |ement of hearing is an uncomplicated field. On the contrary, the diagnosis vq|use and nature of a hearing impairment by an otologist may involve the f|8|a dozen tests not described here and, in addition, may require careful ^Stjjbased on years of medical experience. psft;. C. THRESHOLD OF HEARING Abound is at the threshold of hearing when it is just intense enough to evoke a J|j|;e,,from the listener. Since a listener's threshold may vary slightly from mfnijtq moment, the average of the responses to a number of trials is usually W..To describe a threshold completely one must report the nature of the ffue physical measurements made to specify the sound, and, in addition, the 1fa|.ure of the response or series of responses that the listener makes. ijjgg different sounds have been used in threshold measurements. Pure tones, .l^pind noise are the major classifications of useful sounds, but each group ^Subdivided many times. For instance, the pure tone may be presented at fgfguency in the audible ranee, may beintemmted or steady, and may have a Jor. an abrupt beginning. The speech material may be sentences, words, or Ipgoise may be presented with a vast variety of spectra and band widths. Jhrjesholds obtained with the same kind of sounds can be expected to be the sC ' or^most audiometric measurements the . sound is specified in terms of the "pressure level that the earphone of the audiometer produces in; a small afonbae volume is about the same as that of the outer ear,, when closed by Iphone. The sound pressure produced in this cavity js, throughout ,the infrequency range, very nearly that produced in the .ear jCanal.when,it is 'l^y the same earphone. *&pxact nature of the response .elicited ,from.' tha:Iiaten'erIiS-dffen...lfiM-f!areJPglfteH. In measurements, of'the threshdld of intelligibility for speech the ju^fequested may be that the listener repeat words he has heard. In measure- the threshold of detectability the response that may be requested is a ||rom the listener at the insfant that the sound becomes audible as..the ^|g5pxeds-increa8ed1--A-nother-response--that-may_be'T5queste'd is a 'signaF Ifeistener at the instant the sound becomes inaudible as the sound pressure flfSed. Usually these two responses give slightly different thresholds. Aver- jfiei`results of a number of threshold determinations by each of these two s is usually considered good audiometric procedure.