Sound 



175 



tense, and currents of air coming through the glottis from the 

 lungs cause them to vibrate. The pitch of the voice depends 

 upon the greater or less tension of the vocal cords. Its volume 

 depends largely upon the force with which the air is expelled 

 from the lungs. 



The Phonograph. A phonograph is a machine for repro- 

 ducing sounds. These sounds must first foe recorded. In doing 

 this, the sound waves are allowed to strike against a thin disc 

 which they cause to vibrate. This vibration is transmitted to a 

 needle attached to the disc. The point of the needle rests upon 

 a flat, circular wax plate which revolves at a regular rate. As the 

 needle vibrates it cuts a groove of irregular depth in the wax. 

 Later this wax record is copper plated, and from it duplicates are 

 made on a rubber-like substance which becomes very hard when 

 it cools. 



These records are played on a machine much like that used 

 to make them. As the record revolves, a needle which rests in 

 the groove is caused to vibrate in just the same manner as the 

 needle which cut the groove. This vibration is transmitted to a 

 thin disc, which sends it forth as sound waves in the air. Thus, 

 sound waves which are recorded on the record are reproduced 

 when the record is played. 



The phonograph principle is 

 applied to dictating machines. 

 The person dictating speaks into 

 a receiver and his message is 

 recorded on a cylindrical wax 

 record. It can then be repro- 

 duced from the same record. 



The Ear. The ear is the 

 organ of hearing. It is wonder- 

 fully adapted for receiving 

 sounds. The ear has three main 

 divisions, the outer ear, the mid- 

 dle ear and the inner ear. The 

 outer ear is oval and somewhat 

 funnel shaped, and therefore 



THE EAR 



Hammer, Anvil and Stirrup. 



Auditory Canal. 



Ear Drum. 



Auditory Nerve. 



Cochlea. 



Locate the outer, middle, and inner ear. 



