290 



COMPOSITION OF SOUND WAVES. 



at which the reinforcement of sound is greatest is easily found by 



trial. If, when the sound 

 of the bell has become 

 hardly audible, the tube be 

 brought near, the resonant 

 effect is very marked. 



451. Interference 

 of Sound. If, while 

 a tuning-fork^is vibrat- 

 ing, a second fork be set 



in vibration, the waves from the second must traverse the 

 air set in motion by the former. If the waves from the two 

 forks be of equal length, as will be the case when the two 



FIG. 228. 



forks have the same pitch, and the forks be any number of 

 whole wave lengths apart, the two sets of waves will unite 

 in like phases (Fig. 228), (condensation with condensation, 



FIG. 229. 



etc.), and a reinforcement of sound will ensue. But if the 

 second fork be placed an odd number of half wave lengths 

 behind the other, the two series of waves will meet in 

 opposite phases ; where the first fork requires a condensa- 

 tion, the second will require a rarefaction. The two sets 



