Lord Rayleigh : Acoustical Notes. 283 



Beats of Sounds led to the Two Ears separately. 



When two approximately pure tones, of equal intensity 

 and of approximately equal frequency, are conveyed to one 

 ear, beats are perceived according to a well-known elementary 

 theory, the frequency of the beats being the difference of 

 the frequencies of the tones. When the beats are somewhat 

 slow, the phase of silence is distinctly recognizable, and indeed 

 the moment of the occurrence of this phase is capable of being- 

 fixed with great accuracy. 



The question whether the beats are still audible when one 

 sound is led to one ear alone, and the second sound to the 

 second ear alone, is of great importance. A careful expe- 

 riment of this sort is described by Prof. S. P. Thompson*, 

 in which the sounds were conveyed to the ears by rubber tubes ; 

 and the conclusion was that in spite of all precautions the 

 beats were most distinctly heard, although there was no phase 

 of " silence," such as is perceived when both sounds are 

 conveyed to the same ear. 



I have lately tried a somewhat similar experiment, using 

 telephones and electrical conveyance, by which perhaps the 

 risk of the sounds reaching the wrong ears is reduced to a 

 minimum. Two entirely independent, electrically driven, 

 forks of about 128 vibrations per second were the sources of 

 sound. Near the electromagnet of each fork was placed a 

 small coil of wire in connexion with a telephone. The higher 

 harmonics were greatly moderated by the interposition of 

 thick sheets of copper ; but the sounds were doubtless no 

 more than rough approximations to pure tones. Both forks 

 were placed at a great distance from the observer ; and in one 

 case the doubled connecting wire was passed through a hole 

 in a thick wall specially arranged many years ago for this 

 sort of experimenting. When the telephones were pressed 

 closely to the ears, the utmost possible was done to secure 

 that each sound should have access only to its proper ear. 



The results depended somewhat upon the frequency of the 

 beats. When this exceeded one per second, the beats were 

 very easily audible. When, on the other hand, the frequency 

 was reduced to J or \ beat per second, the beats were not 

 easily perceived at first. After a little while the attention 

 seemed to concentrate itself upon the variable element in the 

 aggregate effect, and the cycle became clear. But even after 

 some practice neither Mr. Gordon nor I could hear slow beats 

 during the first 10 or 15 seconds of observation. 



* Phil. Mag. vol. iv. p. 274 (1877). 



