498 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1924 



and higher frequency. It is evident that the processes of evolution 

 have worked out in such a way that conversation usually employs 

 the central part of the area of audition. For clear understanding 

 the weaker consonant sounds must not fall below the threshold of 

 hearing nor must the loudest speech sounds rise to such intensity 

 that the threshold of feeling is reached or nonlinear effects in- 

 troduced. 



Defective hearing is lacking more or less in range of sensation 

 (that is either frequency or intensity), quality of sensation in 

 various parts of the sensation area, or in the binaural sense or the 

 ability to locate the direction from which a sound is received. While 

 space will not permit a discussion of abnormal hearing, Figure 5 

 is presented to illustrate the way in which defective range of sensa- 

 tion may be measured and compared with normal conditions. The 

 area of audition plot is again reproduced together with hearing- 

 threshold curves, or audiograms, for a person whose hearing is 

 subnormal due to catarrhal deafness. The areas between these 

 threshold-of-hearing curves and the threshold of feeling are the 

 diminished areas of sensation for the respective ears. The scales 

 used in plotting the area of audition are such that the area of any 

 part of the diagram represents approximately the number of simple 

 tones which can be distinguished in that region. Hence, the pro- 

 portional part of the whole area in which sensation can still be per- 

 ceived may be taken as a measure of deafness. It is apparent that 

 the subject retains about 50 per cent of the normal range in this 

 case. He hears and interprets conversation with some difficulty. A 

 suitable deaf set amplifying the speech region to the position indi- 

 cated by the dashed lines would be of some assistance. Certain of 

 the weaker consonant sounds would frequently be low enough in 

 intensity to drop below the range of his hearing even with this aid, 

 however. If too great amplification were provided, the energy of 

 some of the vowel sounds would give rise to subjective distortion and 

 might even produce painful sensations. It is of interest to note 

 that an unusual degree of nonlinearity is characteristic of some 

 types of defective hearing. It is evident that in prescribing aids 

 for the deaf, great care must be exercised in order that there may 

 be no danger of injury and in order that the best results may be 

 obtained. 



For the study of the interpretation of speech it is necessary to 

 be able to adjust at will the loudness of the speech sounds and to 

 introduce determinable amounts of distortion. With acoustic ap- 

 paratus this is very difficult and consequently for many years 

 meager results were obtained. The recent development of the 

 vacuum tube and the electric-wave filter make it possible to produce 

 the equivalent of the desired changes in the high quality reproducing 



