100 SOUND 



113. Resonators. Thin boards, metal tubes, and 

 columns of air are very commonly used as resonators. 

 Fig. 76 shows how organ pipes make use of air 

 columns as resonators. Air ent'ering an opening 

 at the bottom strikes a reed at r, making it 

 vibrate ; this causes air in the pipe to vibrate, 

 giving a loud tone. In the same way all horns, 

 cornets, and other wind instruments are only 

 tubes or pipes full of air; vibration is caused 

 by the lips and a mouthpiece, but most of the 

 sound waves come from the tube and the air 

 PIG. 76 w ithin, which act as resonators. A violin, guitar, 

 or mandolin would be useless without the thin wood 

 body and the air it incloses, both of which reenforce 

 the vibrations of the strings. Pianos have large sound- 

 ing boards of thin wood which serve as resonators. 



FIG. 77 



Megaphones (Fig. 77) or speaking trumpets partly 

 reflect the sound waves which would otherwise escape 

 sidewise, and partly serve as resonators to increase the 

 energy of the waves. By their use sounds may be 

 heard at much greater distances from their sources 

 than usual ; sailors, firemen, and others find them very 

 necessary. 



