276 



NATURE OF SOUND. 



and the air particles will move forward and backward through a 

 greater distance. In other words, the amplitude of vibration has 

 been increased. But this change in the aerial wave produces a 

 change in the sensation. We still recognize the pitch to be the 

 same as before ; the tone is neither higher nor lower. We even 

 recognize it still as being produced by a guitar string. The only 

 difference is that the sensation is more intense ; we say that the 

 sound is louder. 



432. Intensity and Distance. The intensity 

 of sound varies inversely as the square of the dis- 

 tance from the sounding body. Hence, the distance 

 to which a sound may be heard depends upon its intensity. 



I 



FIG. 217. 



433 Acoustic Tubes. If the sound wave be not 

 allowed to expand as a spherical shell, the energy of the 

 wave cannot be diffused. This means that its intensity 

 will be maintained. In acoustic tubes (Fig. 217) this 

 diffusion is prevented ; the waves are propagated in 



