22 THE STUDY OF SPEECH CURVES. 



a perfectly stiff piston in an air-tight box with no friction, produce a sinu- 

 soid movement of the air and a smooth tone. The diaphragm of the sound 

 box, however, bends so that there is more or less of yielding and side 

 motion to the air behind it; this alters the form of the wave transmitted 

 to the air. In both gramophone and phonograph the wave is distorted 

 by the bending of the diaphragm in the manner just explained. It is 

 also distorted by the friction in the wax, which, being proportional to 

 the velocity, will diminish as the point departs from the position of 

 equilibrium. 



The reason for the difference between the phonograph twang and the 

 gramophone twang lies apparently in the difference in tension of the dia- 

 phragms at the position of rest, and in the differences in friction in the 

 two instruments. In the phonograph one diaphragm is already bent 

 upward when at rest; as explained above (p. 15), the vibration is unequal 

 on the two sides. In the gramophone the diaphragm is held without 

 tension at rest; it vibrates equally to both sides. In the phonograph the 

 friction in the wax increases as the point moves downward. In the 

 gramophone recorder the needle vibrates always in the same thickness of 

 wax; the element of friction which affects the vibration is that which 

 opposes the deviation of the needle from its position of equilibrium; this 

 is equal on both sides. 



It must be added that the highly developed technique of the present 

 day makes it possible occasionally to produce both phonograph and gramo- 

 phone records that for the ear have absolutely no twang. 



