252 



BELL SYSTEM TECHNICAL JOURNAL 



80 100 

 ANGLE IN DEGREES 



180 



Fig. 3 — Variation in loudness as a speech source is rotated in a horizontal plane 



around the head. 



As may be inferred from the varying shapes of the curves of Fig. 2, 

 the directive effects of hearing introduce a frequency distortion more 

 or less characteristic of the direction from which the sound comes. 

 Thus the character or quaUty of complex sounds varies with the angle 

 of the source. There are quality differences at each ear for various 

 angles of source, and quality differences between the two ears for a 

 given angle of source. In Fig. 4 is shown the frequency distortion at 

 the right ear when a source of sound is moved from a position on the 

 right to one on the left of an observer. It is a graph of the "difference" 

 values of Fig. 2 for an angle of 90 degrees. Frequencies above 4,000 

 cycles per second are reduced by as much as 15 to 30 decibels. This 

 amount of distortion is sufficient to affect materially the quality of 

 speech, particularly as regards the loudness of the sibilant sounds. 



Reference to the difference curve of Fig. 3 shows that if, for example, a 

 source of speech is 20 degrees to the right of the median plane the speec h 

 heard by the right ear is 3 db louder than that heard by the left ear. 

 A similar difference exists when the angle is 167 degrees. Presumably, 

 when the right ear hears speech 3 db louder than the left, the observer 

 localizes the sound as coming from a position 20 degrees or 167 degrees 

 to the right, depending upon the quality of the speech. If this be 

 assumed to be true, even though the difference is caused by the com- 

 bination of sounds of similar quality from several sources, it should be 

 possible to calculate the apparent angle. 



