CHAPTER XXIII. 

 HEARING. 



Almost all that is known about the physiology of hearing has been got by 

 interpreting the structure of the ear. The subject lends itself hardly at all to 

 experimental investigation and on this account no attempt has been made to have 

 this chapter similar in treatment to the others. It is inserted merely in order to 

 round out for the student the outline of the physiology of the special senses. 



THE PHYSICAL BASIS OF SOUND. 



Preliminary to the consideration of the ear itself the student is reminded of 

 the following facts in the physics of sound. Sound, like light, is carried by waves, 

 but while light waves are strains in an intangible ether and have nothing to do 

 with the molecules of the medium in which they travel, sound waves are vibra- 

 tions of the medium which carries them and they do not pass across a vacuum. 

 The vibrations of the air molecules by means of which a sound wave is carried are 

 in the direction in which the wave itself is travelling, each particle swinging to and 

 fro alternately in front of and behind its mean position. The movement of the 

 particles close to the source of the sound is started by the vibrations of the source 

 itself. Those molecules in turn set those swinging which are a little farther away, 

 these communicate the movement to the next, and so the wave is carried. It 

 follows that in air in which a sound wave is travelling there must be some place 

 where the number of air molecules is greater than the average and somewhere 

 where it is less, alternate places of compression and of rarefaction (Fig. 51. No. 1). 

 The condition in the column of air at any one time may be shown diagrammatically 

 by a curve such as in Fig. 51, No. 2. Here, lengths along the abscissa represent 

 distances from the source along the line of the wave; lengths along the ordinate 

 show the number of molecules per unit area. Zero represents the average num- 

 ber, lengths above, numbers per unit area greater than the average (compression), 

 and those below, numbers less than the average (rarefaction). If the sound 

 is musical the wave will be a regular one and the distance from one crest to 

 another, or from one trough to another, always the same. Unmusical sounds, or 

 noises, are made by waves which have no regular shape. 



Between musical sounds, which will chiefly occupy us, it will be seen that 

 there are several possible differences. (1) They may differ in rate, that is, in their 

 distance from one trough of the wave to the next, or from one crest to the succeed- 

 ing one. The rate of the wave determines the pitch of the note; the faster the 

 wave the higher the note. The note of the second wave in Figure 52, for instance, 

 whose period is half that of the first, is an octave higher than the first one. (2) 



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