SOUND 299 



building, or he could not be heard very far. Why? 

 Sometimes auditoriums are built in such a form that the 

 walls reflect the sound back and forth, and the sound 

 waves from the speaker are so disturbed that he cannot 

 be heard distinctly. This defect of auditoriums can be 

 remedied sometimes by stretching wires across the hall 

 or by hanging draperies, or by moving the stage toward 

 the audience. 



Sometimes an echo is produced when a sound wave 

 passes from one layer of air to another layer of greater 

 density. Sound will also be reflected when it passes from 

 one current of air into another current going in the op- 

 posite direction. The rumbling noise following a thunder- 

 clap is due to the reflection of the sound by the clouds 

 and air. 



QUESTIONS AND EXERCISES 



1. Recall all your experiences with sound and write what 

 you know about it. 



2. How is sound made? 



3. How is sound carried? How fast? 



4. Can sound be reflected? 



5. Explain the echo. 



6. What practical use is made of sound? 



