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 dis- 

 turbance 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 surface, into move- 

 inents 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 vibrating 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 



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