EXPLANATION OF SOUND 



93 



sound. In other words, sound is the sensation made by 

 waves in the air striking the ear. In order to get used to 

 this idea, let us study it a bit further. 



104. Sound Waves. Once more let us consider how 

 small a motion in still water will cause ripples to spread 

 far away over its surface. Now, recalling the fact that 

 air is perfectly elastic, it should not be hard to see that 

 waves may likewise be caused in the atmosphere by 

 the many motions which are always disturbing it ; and, 

 as in water these waves may be large or small, so in 

 the air there are long ones and short ones, according to 

 the motion which caused them. Not all of these waves 

 can affect the ear to produce sound, some ^ 

 being too long and some too short. Those a <~ 

 waves which can produce sound in the ear ' I 

 are called sound waves. \ 



105. The Cause of Sound Waves. A 



tuning fork (Fig. 70) may be used in show- 

 ing how sound waves are started, for its 

 vibrations can be easily seen. 



Experiment 81. Strike a tuning fork sharply 

 on a desk and at once look for any vibrating 

 (buzzing) of its prongs. Again strike the fork ; 

 then hold its prongs downward so that they lightly dip into water. 

 Do you see anything to show that they vibrate? 



Let us see how this motion causes sound waves in 

 the air. The prong / (Fig. 70), in vibrating, moves 

 rapidly to and fro between a and b. As it moves toward 

 a the air in front of it is condensed, but is quickly 

 rarefied as the prong flies toward b. Thus the prong, 



FIG. 70 



