COMPOSITION OF SOUND WAVES. 



289 



direct and reflected waves in like phases. Under such 

 circumstances, one set of waves is said to reinforce the 

 other. The phenomenon is spoken of as the reinforce- 

 ment of sound. 



45O. Resonance. Resonance is a variety of the 

 reinforcement of sound due to sympathetic vibrations. 

 'The resonant effects of solids were shown in 444. 

 The resonance of an air column is well shown by the 

 following experiments: 



(a.) When a sounding tuning-fork is held over the mouth of a 

 glass jar, 18 or 20 inches 

 deep, a feeble sound is 

 heard. By carefully pour- 

 ing in water, we notice 

 that when the liquid 

 reaches a certain level, 

 the sound suddenly be- 

 comes much louder. The 

 water has shortened the 

 air column until it is able 

 to vibrate in unison with 

 the fork. If more water 

 be now poured in, the in- 

 tensity of the sound is 

 lessened. If a fork of dif- 

 ferent vibration be used, 

 the column of air that 

 gives the maximum reso- 

 nance will vary, the air 

 column becoming shorter 

 as the rate of vibration of 

 the fork increases. The 

 length of the air column 

 is one-fourth the length of the wave produced by the fork. 



FIG. 226. 



Why? 



(6.) Fig. 227 represents Savart's bell and resonator. The bell, 



on being rubbed with the bow, produces a loud tone. The resonator 



is a tube with a movable bottom. The length of the resonant air 



column is changed by means of this movable bottom. The point 



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