684 BELL SYSTEM TECHNICAL JOURNAL 



density modulator utilizing a three ribbon push-pull valve is described. 

 The entire valve is sealed by the force of the Alnico V permanent magnet on 

 the Permendur pole pieces. Signal is applied to the center ribbon and noise- 

 reduction currents are applied to the outer ribbons. True class A push-pull 

 operation is obtained from the two component single ribbon valves by the 

 use of an inverter prism which aligns the modulating and noise-reduction 

 edges of each aperture. 



An anamorphote condenser lens is used to eliminate lamp filament stria- 

 tions at the valve ribbon plane. An anamorphote objective lens gives a 4: 1 

 reduction of the valve aperture in the vertical plane at the film and a 2:1 

 reduction along the length of the sound track. A meter is supplied to meas- 

 ure exposure as well as setting up "bias." A photocell monitor is supplied 

 and a "blooping" light for indicating synchronous start marks. 



Mathematical analysis of the exposure produced by the modulating ribbon 

 is appended as well as a similar analysis of the four ribbon push-pull valve 

 which the new valve supersedes. 



Factors Governing the Intelligibility of Speech So^mds^ N. R. French 

 and J. C. Steinberg. The characteristics of speech, hearing, and noise are 

 discussed in relation to the recognition of speech sounds by the ear. It is 

 shown that the intelligibility of these sounds is related to a quantity called 

 articulation index which can be computed from the intensities of speech and 

 unwanted sounds received by the ear, both as a function of frequency. 

 Relationships developed for this purpose are presented. Results calculated 

 from these relations are compared with the results of tests of the subjective 

 effects on intelligibility of varymg the intensity of the received speech, alter- 

 ing its normal intensity-frequency relations and adding noise. 



Short Duration Auditory Fatigue as a Method of Classifying Hearing Im- 

 pairment.'^ Mark B. Gardner. Earlier studies have classified deafness 

 cases into two general groups, those having functional disorders of the middle 

 ear and those having impairments resulting from atrophy of the nerve fibers 

 terminating along the basilar membrane (conductive and nerve deafness 

 types, respectively). Such classifications have been made using bone con- 

 duction threshold measurements and unilateral loudness balance results as 

 the basis for differentiation. Bone conduction results, however, are often 

 subject to considerable error while the unilateral loufbiess balance technique 

 can only be applied to individuals ha\'ing one normal and one impaired ear. 

 These limitations introduce a need for a completely independent monaural 

 method of classifying deafness types. This is particularly true for the selec- 



^Jour. Acoiis. Soc. Amer., January 1947. 

 'Jour. Aeons. Soc. Amer., January 1947. 



