178 Our Surroundings 



sound and being reflected back to the ear. Megaphones and 

 hearing trumpets are so constructed that they use the reflection 

 of sound to intensify it. 



SUMMARY 



Sound is caused by the striking on the ear 'of air waves from 

 a vibrating body. 



Sound may be transmitted through all solids and liquids as 

 well as through air and other gases. 



The intensity of sound depends upon the density of the 

 medium through which it travels. 



Sound travels equally well in all directions in the same sub- 

 stance, provided there are no objects that divert the sound waves. 



Sound waves may be started in such ways as by blowing a 

 whistle, snapping a taut wire, or ringing a bell. 



Voice is produced by the vibration of the vocal cords in the 

 larynx. 



The phonograph is an instrument capable of reproducing the 

 human voice or other sounds. 



The ear is the organ of hearing. It has three main divisions. 



Sound waves in the air are reflected when they strike against 

 walls or other barriers. 



An echo is a reproduction of a sound caused by the reflection 

 of sound waves back to the ear. 



FACT AND THOUGHT QUESTIONS 



1. What is sound? 



2. If plaster fell from a ceiling in a vacant room and there were no 

 ears there to hear it, would there be any sound? 



3. Suggest several ways in which sounds are used as warnings. 



4. Name sounds about the home and neighborhood, other than the 

 speaking voice, that convey information to you. 



5. What use do automobile repair men make of sound? 



6. Through what substances will sound pass? 



7. How does the density of a medium affect sound? Illustrate. 



8. Describe an experiment to show the production of sound. 



9. Describe the action of a piano player. 



10. Explain the production of the human voice. 



11. Describe the ear. 



