62 BELL SYSTEM TECHNICAL JOURNAL 



pure tone that it is with difficulty that the patient is convinced that 

 the stimulation is the same pure tone to which he has been listening 

 at the other intensities. The subject of these tests is a violinist and 

 capable of better descriptions and finer distinctions than average. 



Since all speech sounds may be considered as stimuli composed of 

 various frequency components of certain intensities, the sensation 

 caused by such a sound may be represented on this plot by points, 

 or by a line provided the sound has a band spectrum. If the points 

 or line, falls within the sensation area the sound is audible. It is 

 easy to see that if the points or the part of the line which represent 

 those frequency components most essential to interpretation of the 

 sound, fall within any of these abnormal areas, the sound is very 

 likely to be misinterpreted. This adds a further source of loss in 

 intelligibility to that already observed due to a narrowing of the 

 sensation range. When an amplifying deaf set is designed, due care 

 should be taken to raise the principle speech region in such a way 

 as to cause a minimum overlapping with the abnormal areas. 



Many practically normal ears have verv small abnormal areas. 

 They have always been found near minimum audibility and "f this is 

 always true would, therefore, have little influence on the hearing of 

 the individual. They seem to be associated with "catarrhal" con- 

 ditions although this cannot be stated positively. 



8. Binaural Sense. The normal individual has learned to interpret 

 the differential sensations of the two ears to advantage. It helps 

 him to locate the direction from which sounds come, to have a sort of 

 sense of orientation with respect to sounds approaching from different 

 directions, and whether for physical or for purely psychological 

 reasons to assist in focusing of the attention on one sound of a large 

 number. Two ears also assist the individual in perceiving equally 

 well sounds coming from different directions. 



When one ear becomes less sensitive, even though the loss is 

 small, the use of the binaural sense disappears an] after a time is not 

 missed, the subject depending upon other means of locating sounds. 

 For the binaural sense to be most effectively utilized it is necessary 

 that the ears be very nearly alike. When a binaural deaf set is made 

 and fitted to a person with compensating sensitivity for the two ears 

 so that both hear the sounds equally loud, the sensation is usually 

 so novel, that if the patient is actually able to experience a binaural 

 sensation he is very much pleased. Usually, however, he has not 

 used his binaural sense for so long a time that it takes a considerable 

 amount of practice before he is able to have binaural experiences. It 

 may be noted in this connection that the same experience is en- 



