SOUND 215 



tions do not follow each other in equal intervals of time, 

 the result is noise. 



Pitch. If the vibrations which produce tones were 

 all of the same frequency, music would be impossible, 

 since we would have but one tone. Happily this is not 

 the case. As the number of vibrations in a second in- 

 creases, the tone becomes higher. By pitch we mean 

 the highness or lowness of a sound. Middle C on the 

 piano is produced by a string vibrating 256 times a 

 second. Any other string vibrating the same number of 

 times a second will produce a tone of the same pitch. If 

 the number of vibrations a second be doubled, the result 

 will be another C an octave higher ; and if the number be 

 decreased to 128 a second, the result is C an octave lower 

 than middle C. 



Vibrating Strings. A large number of musical in- 

 struments employ vibrating strings or wires. Variations 

 in pitch are obtained by varying the length, tension, and 



FIG. 191. A Sonometer. 



mass of the strings. For example, if a string 20 inches 

 long produces a tone of a certain pitch, the pitch will be 

 an octave higher if the length of the string be changed 

 to 10 inches. The shorter the string the higher the pitch. 

 Or we may say the vibration frequencies of strings are 

 inversely proportional to their length. 



If we increase the tension on a string, the pitch will be 

 higher ; and if we decrease the size of the string and keep 



