Perception of Sound Direction. 229 



I could not satisfy myself that I experienced any right and 

 left effect. Another observer thought he heard a little. It 

 seems clear that at any rate the limit was being approached. 



It will be understood that some of these observations were 

 not made without difficulty. Probably an experimenter new 

 to the work would feel misgivings with respect to some even 

 of the easier decisions. But all the more important results 

 have the concurrence of at least three observers *. I regard 

 it as established that up to pitch g' phase-differences are 

 attended with marked lateral effects. They are probably the 

 principal basis on which discriminations of right and left are 

 founded — at any rate below c' (250). 



As has already been suggested, it was reasonable to anticipate 

 that phase- difference would cease to avail as an indicator at 

 high pitch. Up to about e' the conditions are favourable. At 

 this pitch the phase-difference at the ears affected by a distant 

 sound increases from zero when the source is in front or 

 behind to a maximum of a quarter period (in one or other 

 direction) when the source is on the right or the left in the 

 line of the ears. This is the phase-difference for which one 

 would expect the lateral sensation to be most intense, so that 

 up to this pitch the lateral sensation would keep step with 

 the true lateralness of the source. At a point somewhat 

 higher in pitch it would seem that complications must enter. 

 The maximum sensation (corresponding to a phase-difference 

 of a quarter period) would occur while the source was still in 

 an oblique position, and the sensation of lateralness would 

 diminish while the true lateralness was still increasing. At a 

 pitch in the neighbourhood of e" (610) the maximum phase- 

 difference would rise to half a period, a phase-difference which 

 could not give rise to lateral sensation at all. Thus, although 

 there might be right and left sensations from sources obliquely 

 situated, these sensations would fail when most needed, that 

 is when the source is really in the line of the ears. In this 

 case a perception of phase-differences would seem to do more 

 harm than good. At a pitch a little higher, ambiguities of a 

 misleading and dangerous kind would necessarilv enter. For 

 example, the same sensations might arise from a sound a little 

 on the left and from another fully on the right. 



On the whole it appears that the sensation of lateralness 

 due to phase-difference disappears in the. region of pitch 

 where there would be danger of its becoming a misleading 

 guide. It is not suggested that there is any precise numerical 

 coincidence. If it were a question of calculating a pitch 

 precisely, it might be necessary to look beyond the size of 

 * Lady Rayleigh, Mr. Enock. and myself. 



