HEARING. 



183 



Waves may differ in intensity, in the distance from zero line to crest or trough; 

 the greater the intensity, the louder the note. (3) The waves may be musical, 

 that is, each single wave may be a repetition of the last, and still not have the 

 simple form shown for 1 and 2 in Fig. 52. These curves are such as would be 

 made by a swinging pendulum, writing on an evenly moving surface and they are 

 known as pendular or sine curves. The sound waves which a tuning fork gives 

 have this form. Waves 3 and 4 in Figure 52 on the other hand, are musical ones, 

 but not of this simple shape. They are each made by adding, algebraically, waves 

 1 and 2 at different relative phases. Curve 3 is got by adding 2 at a' to 1 at A. 



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FIG. 51. No. 1: To show the compression and rarefaction in the air, of which a 

 sound waveis composed; No. 2: Diagram to express the conditions in No. 1 in form 

 of a curve. 



For curve 4, 2 at b' was added to 1 at A. Obviously, the number of such composite 

 curves which one might make is unlimited, since one can also choose components 

 of different relative rates. For instance the period of one might be to the period 

 of the other, as one to three, or one to four, or two to five, and so on. Further, 

 one might add three or four waves together instead of only two. The period of 

 the composite is always that of the slowest component wave which it contains. 

 The low note is known as the fundamental. It is the shape of the wave which 



