298 



GENERAL SCIENCE 



reflect light. The objects which diffusely reflect light we 

 can see, while those with smooth surfaces, like a mirror, 

 reflect light so perfectly that we see images of objects 

 instead of the mirror itself. In a sense the opposite is 

 true with sound. The objects which are so rough that 

 they reflect sound diffusely do not make an echo, be- 

 cause the sound wave is broken up and reflected in all 



directions to such an ex- 

 tent that no distinct 

 wave returns to our ears. 

 Buildings and most rock 

 cliffs and steep hillsides 

 are smooth enough to 

 reflect sound and make 

 an echo. If we make a 

 loud sound about one- 

 eighth of a mile from a 

 large building or hill, 

 the echo will be heard 



in about a second. If we are only fifty feet from the re- 

 flector, we will not get a good echo because the sound 

 wave will return in less than a tenth of a second. The 

 human ear cannot distinguish more than about ten dis- 

 tinct sounds per second. If eleven or twelve sounds per 

 second are made, they will seem to the human ear like 

 one continuous sound. 



In a hall or large auditorium the speaker's voice is 

 reinforced by the reflections of the sound from the walls. 

 Since the reflected sound and the sound direct from the 

 speaker's voice strike our ears so nearly at the same time, 

 we are unable to distinguish any echo, but we get a louder 

 sound than if there were no reflection. A speaker in the 

 open air must talk with a much louder voice than in a 



SOUND REFLECTOR FOR AN ORCHESTRA 



