ECHO 277 



sound would be heard, because there would be no external 

 forces to set into vibration the air within the shell. 



Tuning forks do not produce strong tones unless mounted 

 on hollow wooden boxes (Fig. 175), whose size and shape 

 are so adjusted that resonance occurs and strengthens' the 

 sound. When a human being talks or sings, the air within 

 the mouth cavity is thrown into sympathetic vibration and 

 strengthens the otherwise feeble tone of the speaker. 



259. Echo. If one shouts in a forest, the sound is some- 

 times heard a second time a second or two later. This is be- 

 cause sound is reflected when it strikes a large obstructing 

 surface. If the sound waves resulting from the shout meet a 

 cliff or a mountain, they are reflected back, and on reaching 

 the ear produce a later sensation of sound. 



By observation it has been found that the ear cannot dis- 

 tinguish sounds which are less than one tenth of a second 

 apart ; that is, if two sounds follow each other at an interval 

 less than one tenth of a second, the ear recognizes not two 

 sounds, but one. This explains why a speaker can be heard 

 better indoors than in the open air. In the average building, 

 the walls are so close that the reflected waves have but a short 

 distance to travel, and hence reach the ear at practically the 

 same time as those which come directly from the speaker. 

 In the open, there are no reflecting walls or surfaces, and the 

 original sound has no reinforcement from reflection. 



If the reflected waves reach the ear too late to blend with the 

 original sound, that is, come later than one tenth of a second 

 after the first impression, an echo is heard. What we call 

 the rolling of thunder is really the reflection and re-reflection , 

 of the original thunder from cloud and cliff. 



Some halls are so large that the reflected sounds cause a 

 confusion of echoes, but this difficulty can be lessened by 

 hanging draperies, which break the reflection. 



