2/8 SOUND 



260. Motion does not always produce Sound. While we 

 know that all sound can be traced to motion, we know equally 

 well that motion does not always produce sound. The ham- 

 mock swinging in the breeze does not give forth a sound ; 

 the flag floating in the air does not give forth a sound unless 

 blown violently by the wind ; . a card moved slowly through 

 the air does not produce sound, but if the card is moved 

 rapidly back and forth, a sound becomes audible. 



Motion, in order to produce sound, must be rapid ; a ball 

 attached to a string and moved slowly through the air produces 

 no sound, but the same ball, whirled rapidly, produces a dis- 

 tinct buzz, which becomes stronger and stronger the faster 

 the ball is whirled. 



261. Noise and Music. When the rapid motions which 

 produce sound are irregular, we hear noise ; when the motions 

 are regular and definite, we have a musical tone ; the rattling 

 of carriage wheels on stones, the roar of waves, the rustling 

 of leaves are noise, not music. In all these illustrations we 

 have rapid but irregular motion ; no two stones strike the 

 wheel in exactly the same way, no two waves produce pulses 

 in the air of exactly the same character, no two leaves rustle 

 in precisely the same way. The disturbances which reach 

 the air from carriage, waves, and leaves are irregular both in 

 time and strength, and irritate the ear, causing the sensa- 

 tion which we call noise. 



The tuning fork is musical. Here we have rapid, regular 

 motion ; vibrations follow each other at perfectly definite 

 intervals, and the air disturbance produced by one vibration is 

 exactly like the disturbance produced by a later vibration. 

 The sound waves which reach the ear are regular in time and 

 kind and strength, and we call the sensation music. 



To produce noise a body must vibrate in such a way as to 

 give short, quick shocks to the air ; to produce music a body 



