244 JUDGING DIRECTION OF -SOUNDS. [BOOK in. 



who is absolutely deaf of one ear has great difficulty in recognizing 

 the direction of sounds. 



Further, when we desire to judge particularly as to the 

 direction of a sound, we listen to it, and in the act move the head 

 into the position in which we hear the sound most distinctly. In 

 this way the movements of the head in hearing play a part 

 somewhat analogous to the movements of the eyes in vision. 



Even in the case of ourselves, and still more in the case of some 

 animals, the form of the external ear favours the entrance into the 

 meatus, and hence the access to the tympanic membrane, of sounds 

 travelling in a particular direction ; this also assists our judgment 

 of the direction of sounds. Hence, by tying back the ears and 

 affixing artificial ears, differing in shape or position from the 

 natural ones, we may make false judgments in this matter. 



Moreover, in forming a judgment as to the direction of sounds 

 we appear to be guided by something more than the mere relative 

 intensity of the sounds falling on the two ears. When a complex 

 sound emanates from a body on one side of us, the constituent 

 vibrations do not travel equally and uniformly over and around 

 the head ; some are refracted more than others, so that they do 

 not reach the two ears equally ; and besides when they reach them 

 are not equally reflected by the two pinnse. In this way partial 

 tones of different pitch, and this applies especially to high tones, 

 reach the two tympanic membranes in unequal intensities, and 

 the sound of which they form part appears as heard by the one 

 ear of a quality slightly different from that heard by the other 

 ear; this difference of quality, like the difference in mere 

 intensity of the sound as a whole, serves as a basis for re- 

 cognizing the direction of the sound. Such a difference will be 

 more marked in the complex sounds which we call noises than in 

 purer and more simple musical sounds ; and, as a matter of fact, 

 our appreciation of direction is more accurate in the case of noises 

 than of musical sounds. An exception to this rule is met with 

 in the case of the human voice, the direction of which, though it 

 is as a whole a musical sound, can be judged better than even 

 that of a noise ; but noises enter largely into the human voice, 

 and besides we are much more practised in relation to it than in 

 relation to any other kind of sound. All our judgments of the 

 direction of sounds are however at the best imperfect. 



856. Our judgment of the distance of sounds is even still 

 more limited. A sound whose characters we know appears to. us 

 near when it is loud, and far off when it is faint. A blindfold 

 person will be unable to distinguish between the difference of 

 intensity produced on the one hand by a tuning-fork being held 

 before him, first with the broad edge of the fork toward him and 

 then with the narrow edge, and the difference on the other hand 

 caused by the removal of the tuning-fork to a distance. And our 

 judgments in this respect may be false, as is seen in the effects 



