MUSIC, AND SOUND INSTRUMENTS 419 



14. Why does the wind whistle? 



15. Why do telegraph wires hum? 



16. How do crickets produce a shrill noise? 



17. Why does a person hear better if he places the small end of 

 a megaphone to his ear? 



MUSIC, AND SOUND INSTRUMENTS 



Resonance. Resonance is caused by sound waves which, in 

 entering a cavity, such as a bottle, cause the air in the cavity so to 

 vibrate that it reenforces these sound waves. 



Experiment. On Simple Resonance. 



Blow a whistle across the mouth of a large bottle, gradually raising the pitch 

 until the tone is struck which will cause the bottle to produce sympathetic 

 vibrations, thus reenforcing the sound waves of the whistle. 



Sometimes a fly or a bee placed in a bottle of the right size gives 

 a good example of resonance. 



Hold a tuning fork over a tall cylindrical jar, slowly adding water until 

 the sound wave given off by the fork is reenforced. A high degree of resonance 

 is obtained. 



Another interesting way to show resonance is to place over a candle or gas 

 jet a long tube about 1 inch in diameter which just fits inside of another tube. 

 Move the outer tube up and down over the inner tube until the proper length 

 is obtained to produce a loud shriek. 



Draw a piece of glass tubing into a fine jet and attach it to the gas fixture 

 with a piece of rubber tubing. Hold a wire screen about 4 inches above the 

 jet and light the gas above the screen. Place over the flame a bottle, and move 

 the screen up and down until a loud noise is obtained in the bottle. 



Sympathetic Resonance. If a slide trombone is sounded in front 

 of a pipe organ, a pipe which will produce the same type of tone will 

 respond sympathetically. If the trombone is sounded from a low 

 pitch to a high pitch the different pipes will respond, each to its own 

 tone, and pipes which can not produce the vibrations of the tone 

 will remain silent. This is also true of a piano. If a person sings 

 in front of a piano, the string which has the same musical pitch 

 as the sound will vibrate, producing a sound of its own. 



Experiment: Sympathetic Resonance: 



1. Fasten two steel wires securely on a bridge at one end of a long board. 

 Into the other end of the board insert two screws about two inches apart. 



