122 APPLIED SCIENCE 



127. Characteristics of Sound Intensity. When a ham- 

 mer strikes a piece of metal a noise is produced. The sound 

 is caused by the particles of the two separate metals vibrating. 

 The vibrations are transmitted through the air in a series of 

 waves. The presence of the vibrations can be detected by 

 pressing a stiff piece of cardboard against the surface or side 

 of the metal when it is struck. 



Sound possesses three properties intensity or loudness, 

 pitch, and quality by which one sound may be distinguished 

 from another. The intensity of a sound depends upon the 

 density of the medium through which the sound is trans- 

 mitted and upon the amplitude of the sound waves which 

 reach the ear. The intensity varies inversely as the square 

 of the distance. The waves become smaller and smaller as 

 they leave the point at which the sound is produced, because 

 the quantity of air through which the sound is conveyed be- 

 comes greater and greater. In other words, the intensity of 

 sound decreases as the distance from the source of sound increases. 



All forms of speaking tubes are based upon the principle 

 that the sound waves set in motion in the tube are confined 

 to the air space of the tube. Therefore the sound is trans- 

 mitted without any decrease in intensity. 



128. Pitch and Quality. Pitch is the property of sound 

 which determines whether the sound is high or low. Pitch 

 is determined by the number of vibrations per second made by 

 the sounding body. Comparatively slow vibrations produce 

 a low sound, while rapidly vibrating substances produce a 

 high-pitched sound. This difference may be quickly ob- 

 served by entering a machine shop where the machines are 

 running at a low speed and comparing the low drill sound 

 to the high-pitched sound produced in a woodmill where 



