232 



PHYSIOLOGY 



CHAP. 



summated algebraically in the form of a single complex vibration, 

 viz. that our capacity for distinguishing and perceiving them 



depends on the analytical capacity of the organ 

 of Corti, is given by the phonograph. By 

 letting the needle glide over the indentations 

 of the phonograph disc the same vibrations are 

 reproduced on the membrane as were recorded 

 on the disc; and these suffice to reproduce dis- 

 tinctly the whole of the tones, noises, and voices 

 which had previously aroused them. 



Physicists and physiologists have made 

 extensive studies of certain fundamental 

 phenomena that may be observed during the 

 synchronous production of a number of tones. 



The simplest of these phenomena is the 

 interference of sound-waves. When the waves 

 of two tones are superposed, and summate 

 algebraically, there may be increase, diminution, 

 or even extinction of the wave-movement and 

 1 tins ufauditory sensation, according to whether 

 the two waves at their meeting-point are in 

 i he same, or in a more or less different, or in 

 an opposing phase. When the two interfering 

 \\aves have the same piteh, the increase or 

 diminution of intensity, or the extinction of 

 the sound, remains constant, as is easily verified 

 with Helmholtz' double syren. 



When, on the contrary, two tones of not 

 quite equal piteh meet simultaneously, that is, 

 when they contain a slightly different number 

 of vibrations, then the two waves do not always 

 meet in the same phase, but there is alternate 

 coincidence and interference of the waves, 

 with a periodic rise and fall in sensation 

 (Fig. 94). 



These periodic increments and decrements 

 of auditory sensation are known as beats. If 

 there is a difference of one vibration per second 

 between the two simultaneous tones, then 

 during that time they will be once in the equal 

 and once in the opposite phase ; in every 

 second there will be a reinforcement and a 

 diminution of intensity, i.e. a beat. If the 

 difference between' the two tones is two vibra- 

 tions per second, two beats will be perceptible 

 in each second. Generally speaking, it may be held that two 

 coincident, non-identical tones give rise at each second to a 





