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XII. On our Perception of Sound Direction*. 

 By Lord Rayleigh, O.M., Pres. P.S.\ 



IT is some thirty years ago since I executed a rather ex- 

 tensive series of experiments in order to ascertain more 

 precisely what are the capabilities of the ears in estimating 

 the direction o£ sounds J. It appeared that when the alter- 

 native was between right and left, the discrimination could 

 be made with certainty and without moving the head, even 

 although the sounds were pure tones. Nor was any difficulty 

 introduced by the requirement that the ears should be stopped 

 at the moment when the sounds commenced. 



On the other hand, if the question was whether a sound 

 were situated in front or behind the observer, no pro- 

 nouncement could be made in the case of pure tones. The 

 impossibility of distinguishing front and back carries with it 

 further confusions relating to cases where the sound may 

 be obliquely situated. But with sounds of other character 

 and notably with the speaking voice, front and back could 

 often be distinguished. It is understood, of course, that the 

 head was kept still. A slight rotation, bringing a pure 

 tone (originally situated exactly in front or exactly behind) 

 to the right or the left, gives the information that was pre- 

 viously lacking. 



The discrimination between right and left is usually sup- 

 posed to be explicable by the greater intensity of sensation 

 experienced by the ear which lies nearer to the sound. When 

 the pitch is pretty high, there is no doubt that this explana- 

 tion is adequate. A whistle of pitch / IV , preferably blown 

 from a gas-bag, is much better heard with the nearer than 

 with the further ear. ;i A hiss is also heard very badly with 

 the averted ear. This observation may be made by first 

 listening with both ears to a steady hiss on the right or left, 

 and then closing one ear. It makes but little d i Her ence when 

 the further ear is closed, but a great difference when the 

 nearer ear is closed. A similar observation may be made 

 upon the sound of running water." In a modified form of 



* This paper formed the substance of the Sidgwick lecture given at 

 Cambridge on November 10, 190(1, and (except the last two or three pages) 

 was written before the delivery of the lecture. I have learned since from 

 Dr. L. More that three years ago at Cincinnati he made experiments which 

 led him to similar conclusions. It is to be hoped that Dr. More will 

 publish an account of his work, the more as it was conducted on lines 

 different from mine. 



t Communicated by the Author. 



t ' Nature,' xiv. p. 32 (1876) 5 Phil. Mag. iii. p. 546 (1877) ; Phil. Map-, 

 xiii. p. 340 (1882). l iScientihc Papers,' i. pp. 277, 314 ; ii. p. 98. 



