CHAPTER XII 

 SOUND 



Sound is one of our greatest blessings. By means of 

 sounds we talk with one another when together and even 

 when hundreds or thousands of miles apart. Sounds warn 

 us of dangers. Sound-signals control many of our acts. 

 Sound in the form of music brings real enjoyment to most 

 of us. 



We all know that sound travels. We see a distant wood 

 chopper swing his axe against a log, but the blade often is 

 high in air again before the sound of the blow comes to us. 

 How sound is produced, how the air and other substances 

 aid in carrying the sound message, and how the wonderful 

 mechanism of the ear receives the message and trans- 

 mits it to the brain, should all be part of our scientific 

 knowledge. 



We are all familiar with the fact that if we toss a stone into 

 still water, or stir it with a stick, tiny waves, or ripples, are caused. 

 This disturbance of the water spreads out in all directions for a 

 considerable distance. In the same way, the motion of any object 

 in the air about us causes air waves which spread out in all direc- 

 tions. If the object causing the air motion is vibrating, or quiver- 

 ing, that is, moving very rapidly back and forth, the air waves 

 it sets in motion cause the sensation of sound when striking our 

 ears. 



Sound is always the result of motion. Energy is necessary to 

 produce motion. So we may say that sound is the result of a 

 transformation of energy. 



Experiments to Show How Sound Is Carried. That sound 

 is usually transmitted to our ears through the air from a vibrating 

 body may be shown by a simple experiment. Place a metronome, 

 an instrument used to keep time in music, on a piece of felt on 

 the receiver of an air pump. Set the metronome in motion and 

 cover it with a bell jar, being careful that the metronome does 

 not touch the glass. Notice the sound. Pump the air from the 



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