458 Lord Rayleigh's Acoustical Observations. 



ment of the sound. It might be supposed, for example, that 

 we are able to recognize which ear is first affected. On trial, 

 however, it appeared that the power of discrimination was not 

 weakened, although the observer stopped his ears during the 

 establishment of the sound. 



When one ear is stopped, mistakes are made between forks 

 right and left ; but the direction of other sounds, such as those 

 produced by clapping hands or by the voice, is often told 

 much better than might have been expected. 



The Head as an Obstacle to Sound. 



The perfection of the shadow thrown by the head depends 

 on the pitch of the sound. I have already mentioned that it 

 appears to make but little difference in the audibility of a pure 

 tone with a frequency of 256, whether the ear used be turned 

 towards or from the source. But the case is very different 

 with sounds of higher pitch, such as that of an ordinary 

 whistle. The one that I employed was blown from a loaded 

 gas-bag, and gave a very steady note of pitch / IV . 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 difference when the further ear is closed, but a great dif- 

 ference when the nearer ear is closed. A similar observation 

 may be made on the sound of running water. 



For the same reason a hiss or whisper, coming from- a 

 person whose face is averted, is badly heard. Under these 

 circumstances even ordinary speech is difficult to understand, 

 though the mere intensity of sound does not seem deficient. 



Reflection of Sound. ' 



In many cases sound-shadows appear much less perfect than 

 theory would lead us to expect. The anomaly is due in great 

 measure, I believe, to an error of judgment, depending on the 

 enormous range of intensity with which the ear is capable of 

 dealing. The whistle of a locomotive is very loud at a distance 

 of ten yards. At a mile off the intensity must be 30,000 

 times less ; but the sound still appears rather loud, and would 

 probably be audible under favourable circumstances even when 

 enfeebled in the ratio of a million to one. For this reason it 

 is not easy to obtain complete shadows ; but another difficulty 

 arises from the fact that there are generally obstacles capable 

 of reflecting a more or less feeble sound into what might other- 

 wise be a nearly complete shadow. An attempt to examine 

 this point led me to a few simple experiments on the reflection 

 of sound, which may be worth recording. 



