SOUND 



169 



no reflecting surfaces are near, it is much more difficult to 

 hear the speaker. This explains the difficulty one has in 

 understanding a speaker in the open air. When sound is 

 reflected from several surfaces at different distances, a suc- 

 cession of echoes following close upon one another may be pro- 

 duced. These we call reverberations. The roll of thunder is 

 thus produced by the reflection of the original sound from 

 different clouds. 



141. Sympathetic Vibrations. The time required for one 

 vibration of a body is called its period. If a vibrating body be 

 brought near another with the same period, 



the latter will soon begin to vibrate in har- 

 mony with it. Vibrations of this kind are 

 called sympathetic vibrations. Other bodies 

 may be forced to vibrate out of their natural 

 period when brought into contact with a 

 vibrating body. Thus the body of a violin 

 vibrates in harmony with the strings stretched 

 across it. Since the volume of sound given 

 out by the vibrating body is proportional to 

 the surface vibrating, forced vibrations of this 

 kind largely increase the original sound. F . I< \ 6L ~rA T n t 



e venient method of 



The property of a body which enables it to changing the length 

 vibrate in harmony with another is called of . a vibratin g air 



* column. 



resonance. 



142. Distinguishing Sounds. When a musical note is 

 sounded, there is produced, in addition to the fundamental 

 tone, a variety of others, known as overtones, that give char- 

 acter to the different musical sounds and enable us to distin- 

 guish the notes of different instruments in an orchestra, to 

 recognize the voices of our friends over the telephone, and the 

 like. Should these overtones be suppressed, the ear would 

 be unable to distinguish the notes produced by the piano or 

 flute from those of the human voice. 



