352 COORDINATION AND SENSATION 



bodies against which they strike. 1 Though the force exerted 

 by most sound waves is, indeed, very slight, it is sufficient 

 to enable them to act as stimuli to the nervous system. 



How Sounds Differ. Three distinct effects are produced by sound 

 waves upon the nerves of hearing, and through them upon the mind. 

 These are known as pitch, intensity, and quality, and they are depend- 

 ent upon the vibrations of the sound-producing bodies. 



Pitch, which has reference to the height, or degree of sharpness, of 

 tones, is determined by the rapidity of the vibrations of the vibrating 

 body. The more rapid the vibrations, the higher the pitch, the 

 number of vibrations doubling for each musical interval known as 

 the octave. 



Intensity is the energy, or force, of the sound waves. This is recog- 

 nized by the strength of the sensation and is expressed by the term 

 loudness. Intensity is governed mainly by the width of the vibrations 

 of the vibrating body, and the width depends upon the force applied 

 to the body to make it vibrate. 



Quality is that peculiarity of sound that enables tones from different 

 instruments to sound differently, although they may have the same 

 pitch and intensity. Quality depends upon the fact that most tones are 

 complex in nature and result from the blending together of simple tones 

 of different pitch. 



Reinforcement of Sound Waves. The sound vibrations from 

 small bodies are not infrequently reenforced by surrounding conditions 

 so that their outgoing waves reach farther and are more effective than 

 waves from larger bodies. This is true of the sound waves produced 

 by most musical instruments and also those produced by the human 

 larynx. Such reinforcement is effected in two general ways by 

 sounding boards and by inclosed columns of air. Stringed instru- 

 ments violin, guitar, piano, etc. employ sounding boards, while 

 wind instruments, as the flute, pipe organ, and the various kinds of 

 horns, employ air columns for reenforcing their vibrations. In the use 

 of the sounding board, the vibrations are communicated to a larger sur- 

 face, and in the use of the air column the vibrations are communi- 

 cated to the inclosed air. (See Practical Work.) 



1 Somewhat as the waves on a body of water impart motion to the sticks and 

 weeds along the shore, sound waves are able to cause bodies that are small or that 

 are delicately poised to vibrate. 



