Perception of Sound Direction. 22\ 



from the interior of which a composition (gas) pipe led the 

 sound through a hole in a thick wall to the observer in a 

 third room. With the aid o£ closed doors and various other 

 precautions of an ohvious character, the sounds were fairly 

 well isolated. But each resonator emits a rather loud sound 

 into its neighbourhood, a little of which might eventually 

 reach the other. To eliminate this cause of disturbance more 

 completely a second resonator * of like pitch was employed 

 in association with each fork, so situated that the phases of 

 vibration in the two resonators were opposed. By a little 

 adjustment it was possible to provide that but little sound 

 radiated externally from the combination, though the internal 

 vibrations might be as vigorous or more vigorous than when 

 one resonator was employed alone. These arrangements 

 Ave re so successful that when one fork singly was in action, 

 the sound was imperceptible from the tube belonging to the 

 other, even though the open end were pressed firmly into 

 the ear, by which the effect is enormously increased. The 

 open ends of the two pipes may thus be regarded as sources 

 of sound of constant intensity. 



In the greater number of experiments the observer, leaning 

 over a table for the sake of steadiness, placed his head between 

 the pipes, which were at such a distance apart that one or two 

 inches separated the open ends from the adjacent ears. At 

 the very first trial on July 31, the period of the cycle being 

 5 seconds, Lady Rayleigh and I at once experienced a distinct 

 right and left effect, the sound appearing to transfer itself 

 alternately from the one side to the other. When the effect 

 was at its best, the sound seemed to lie entirely on the one side 

 or on the other. 



The beat may be slowed down until it occupies 40 or even 

 70 seconds, thus giving opportunity for more leisurely 

 observation. The position of the head should be so chosen 

 that the right and left effects are equally distinct. Under 

 these circumstances it is found that the sound seems to be 

 jn'edominantly on the right or on the left for almost the 

 whole of the cycle, the transitions occupying only small 

 fractions of the whole time. The observations may be made 

 with the ears continuously open, or, as in some of the out- 

 door experiments, the ears may be opened and closed simul- 

 taneously at short intervals. It is perhaps better still, keeping 

 the ears open, to close periodically with the thumbs the open 

 ends of the tubes from which the sounds issue. When the 

 tubes are closed, no sound is audible. It should be said that 



* Three out of the four resonators cousisted of " Winchester " bottles 

 from which the necks had been removed. 



Phil. Mag. S. 6. Vol. 13. Xo. 7-i. Feb. 1907. li 



