60 BELL SYSTEM TECHNICAL JOURNAL 



characteristic of the normal ear. Any point within this area repre- 

 sents a definite auditory sensation in frequency and intensity. The 

 area of sensation is analogous to the field of vision of the eye. The 

 part of this area which is most utilized in the interpretation of speech 

 is represented approximately by the shaded area in Figs. 1 and 2 

 and corresponds in a way to the center of the field of vision. A 

 normal listener tries, by keeping at a certain distance from a speaker, 

 to bring this part of his sensation area into play in the same way that 

 when examining an object he directs his eyes so that it falls in the 

 center of the field of vision. 



An abnormal ear may be regarded as having an area of sensation 

 which is smaller than the normal area but included within it. Fig. 2 

 is a plot of the minimum audibility of the right and left ears for a 

 man (CHK) having a "catarrhal" deafness. The areas between 

 these curves of minimum audibility and the curve of feeling are his 

 areas of sensation. It will be seen that CHK retains about 50 or 60 

 per cent of the normal amount of sensation. He hears and interprets 

 conversation with some difficulty. 



Since the CHK curves pass through the speech region, part of it ' 

 is entirely inaudible and the remainder is near minimum audibility' 

 for him. In order to make him hear well, the speech area must be' 

 raised to a higher level of intensity or loudness as indicated by the 

 dotted curve. 



In general it takes a loss of about 20 per cent of the sensation area 

 to become noticeable and much more is disagreeable. A loss of 50 

 per cent requires the use of deaf apparatus. A loss of 75 per cent 

 can be aided considerably by the use of high powered amplifying 

 apparatus. 



6. Importance of Various Intensities in Speech. It is interesting 

 to speculate on how CHK interprets speech. It has been shown ' 

 that the intensity of speech may be varied over perhaps 70-80 per 

 cent of the range of sensation without serious loss of intelligibility 

 to the normal ear. As the sound intensity is decreased, the in- 

 telligibility drops very suddenly to zero at minimum audibility. A 

 similar drop is to be expected at an intensity so loud as to be painful. 

 It is evident, therefore, that the range in which speech is intelligible for 

 CHK is very considerably limited as compared to normal. It is 

 possible to design a deaf set which raises the intensity of the principal 

 speech region to any desired place within the abnormal sensation area 

 and so in a measure, compensate for this narrowed range. The 

 region in Fig. 1 "Region of Lesser Importance in Speech," corre- 

 ' H. Fletcher, Journal of the Franklin Institute, June, 1922. 



