Sound 



175 



vocal cords vibrate. These and your tongue and lips 

 make the air in front of you vibrate. 



When you talk into a dictaphone horn, the vibrating 

 air causes the needle at the small end of the horn to 

 vibrate so that it traces a wavy line in the soft wax of 

 the cylinder as the cylinder turns. Then when you run 

 the needle over the line again it follows the identical 

 track made when you talked into the horn, and it 

 vibrates back and forth just as at first ; this makes the 

 air in the horn vibrate exactly as when you talked into 

 the horn, and you have the same sound. 



All this goes back to the fundamental principle that 

 sound is vibrations of air; different kinds of sounds 

 are simply different kinds of vibrations. The next 

 experiments will prove this. 



Experiment 54. Turn the rotator rapidly, holding the 

 corner of a piece of stiff paper against the holes in the disk. 

 As you turn faster, does the sound become higher or lower? 

 Keep turning at a steady rate and move your paper from 

 the inner row of holes to the outer row and back again. 

 Which row has the most holes in it? Which makes the 

 highest sound? Hold your paper against the teeth at the 

 edge of the disk. Is the pitch higher or lower than before? 

 Blow through a blowpipe against the different rows of holes 

 while the disk is being whirled. As the holes make the air 

 vibrate do you get any sound? 



This experiment shows that by making the air vibrate 

 you get a sound. 



The next experiment will show that when you hu\v 

 sound you are getting vibrations. 



Experiment 55. Tap a tuning fork against the desk, then 

 hold the prongs lightly against your lips. Can you feel 



