TRANSMISSION OF SOUND WAVES 



FIG. 74 



and sympathetic. To understand the difference, it must 

 be known that every body lias its own natural rate of 

 vibration, at which it vibrates when free to do so. 

 When the motion of one 

 body causes another to vi- 

 brate at a rate which is not its 

 own, these vibrations are said 

 to be forced. When the natu- 

 ral rate of the second is the 

 same as that at which the first 

 is vibrating, its vibration is 

 said to be sympathetic. 



Experiment 86. Cause a 

 tuning fork to vibrate. Can you easily hear its tone ? Now strike 

 it, and at once hold it to a table, as in Fig. 74. Is the sound any 

 louder? The table is forced to vibrate by the motion of the fork. 

 Experiment 87. Put water in a tall jar and hold 

 a vibrating fork over it, as in Fig. 75. Vary the 

 amount of water till the sound is the loudest. What 

 body adds its vibrations to those of the fork ? If the 

 air column were in forced vibration, would its length 

 make the difference that you now find ? Is its vibra- 

 tion forced or sympathetic ? 



112. Resonance. In these experiments the 

 sound seemed louder. The vibrations of the 

 larger body were added to those of the fork, 

 increasing the energy of the sound waves. 

 In such cases the waves are said to be re'en- 

 forced. The ability of a body to reenforce 

 sound waves is called resonance, and the body itself 

 is a resonator. Bodies of this sort are very useful and 

 are employed particularly in musical instruments. 



. 



FIG. 75 



