LAWS OP SOtTKD VIBKATION 489 



drawn forward and to one side and the vocal cords are 

 thus separated. In this position they do not vibrate, and 

 hence produce no sound. In whispering they are in the 

 same position and the sounds produced are in this case 

 due entirely to the position of the lips and tongue. 



In talking, shouting, singing, however, these cords are 

 drawn inward and backward by the cartilage muscles, and 

 the expired air is thus forced through the narrower slit 

 between their edges. This sets the edges in vibration 

 producing sound. How this sound is varied in pitch, 

 quality, and loudness requires some knowledge of the laws 

 of vibrating bodies. 



Laws of sound vibration. — The production of sound depends 

 upon two factors. The first is a vibrating body. The second is a 

 layer of air between this body and the sense organs which produce 

 the impulses. Under these conditions the vibrations of the body 

 are transmitted by the air in the form of waves. If the body 

 which vibrates does so regularly and continuously, a musical tone 

 is produced. If irregularly or at irregular intervals, the effect is 

 called noise. Sound sensations differ in three characteristics, 

 pitch or tone, loudness, and quality. This variation in sensation 

 finds its explanation in corresponding variations in the manner 

 of vibration of the sounding body. 



(a) Loudness. Of two vibrating bodies vibrating at the same 

 rate, the one whose vibrations are widest are most audible. Thus, 

 a body struck hard produces wider vibrations and hence a louder 

 sound than when struck lightly. 



(6) Pitch. The number of vjbrations a body makes in a given 

 time determines the pitch or tone of the sound. Thus, a tuning 

 fork whose prongs vibrate two hundred and fifty-six times a second 

 produces the pitch called middle C of the piano, while the next 

 note above it in the musical scale requires two hundred and 

 eighty-eight vibrations per second. Vibrating bodies differ in size 

 and elasticity, and this difference determines their rate of vibra- 

 tion. A large-sized string vibrates slower, and thus gives a lower 



