Sound 183 



SECTION 29. Echoes. 



When you put a sea shell to your ear, how is it that you 

 hear a roar in the shell ? 



Why can you sometimes hear an echo and sometimes 

 not? 



If it were not for the fact that sound travels rather 

 slowly, we should have no echoes, for the sound would 

 get back to us practically at the instant we made it. 

 An echo is merely a sound, a series of air vibrations, 

 bounced back to us by something at a distance. It 

 takes time for the vibration which we start to reach the 

 wall or cliff that bounces it back, and it takes as much 

 more time for the returning vibration to reach our ears. 

 So you have plenty of time to finish your shout before 

 the sound bounces back again. The next experiment 

 shows pretty well how the waves, or vibrations, of 

 sound are reflected; only in the experiment we use 

 waves of water because they go more slowly and we 

 can watch them. 



Experiment 59. Fill the long laboratory sink (or the 

 bathtub at home) half full of water and start a wave from 

 one end. Watch it move along the side of the sink. Notice 

 what happens when it reaches the other end. 



Air waves do the same thing ; when they strike against 

 a flat surface, they bounce back like a rubber ball. If 

 you are far enough away from a flat wall or cliff, and 

 shout, the sound (the air vibrations you start) is reflected 

 back to you and you hear the echo. But if you are 

 close to the walls, as in an empty room, the sound 

 reverberates; it bounces back and forth from one wall 



