SECTION IV 

 AUDITORY SENSATIONS 



BY means of our auditory sensations we are made aware of such changes 

 in our environments as are capable of giving rise to a disturbance which can 

 be propagated through the surrounding elastic medium, the air, to our 

 ears. Any sudden jar given to a solid body sets up vibrations which are 

 propagated to the surrounding air as sound waves, i.e. a series of acts of 

 condensation and rarefaction spreading out from the centre of disturbance, 

 like the waves which are caused on the surface of a pond by throwing a stone 

 into its middle. With a delicate tambour we can record these changes of 

 pressure and convert them, by means of a lever writing on a blackened sur- 

 face, into movements at right angles to the direction of movement of the 

 surface. The amplitude of vibration of the membrane will be proportional 

 to the amount of compression and expansion occurring at each wave. 

 These waves travel through the air at the rate of 1100 feet per second, 

 their wave length varying with the number of vibrations per second. It is 

 of course possible to get vibrations of almost any number per second. Only 

 when the number of vibrations fall within distinct limits are they effective 

 in producing a sensation of sound. Before we can discuss the physiological 

 mechanism of hearing we must have a clear idea of the character of the 

 physical change the stimulus which is effective in evoking an auditory 

 sensation and determining its quality. 



Sounds may be divided into noises and musical tones. If the vibrations 

 or series of vibrations arriving at the ear are irregular in character, such 

 as those produced by striking the table or the floor with a stick, we speak 

 of the resultant sensation as a noise. If, on the other hand, the vibrations 

 follow one another at a regular sequence and possess a rhythm if, for 

 instance, a series of vibrations be imparted to the air by a tuning-fork vibra- 

 ting at 100 times per second the effect on consciousness is that of a musical 

 tone. Of course there is no hard-and-fast line between the two kinds 

 of sound ; even when the prevalent impression is that of a noise it is often 

 possible to pick out some series of vibrations which predominate among the 

 irregular ones with which they are accompanied. When we strike a single 

 stick with a hammer the effect is that of a noise. If, however, we take a series 

 of sticks of different lengths and strike them in succession, it will be noticed 

 that the sound produced by each stick corresponds to a distinct note, and 

 tunes may be played on such a collection of sticks. On the other hand, 



505 



