THE CAUSE AND CHARACTER OF THE SOUND WAVES 759 



of the vibrator from the ear and to the density or elastic quality of the medium. 

 Thus, the voice becomes remarkably feeble on top of a mountain and is much 

 stronger in a calm atmosphere. 



(c) The quality, timbre, stamp or color of a sound is the product of a variety of 

 factors; primarily, however, of the form of the movement of the sonorous body 

 and of the form of the waves produced by it. Thus, a sound of a certain pitch and 

 intensity emitted by a piano, is quite different from that of a violin or of the phonat- 

 ing organs of man. If these sound waves are examined more closely, it will be 

 found that they appear in two distinct forms, namely, as a simple or pendular 

 and as a compound or non-pendular type. If we permit the pointed end of a simple 

 reed vibrator to record its excursions upon the smoked paper of a kymograph, the 

 record so obtained will show perfectly symmetrical deviations from the line of 

 rest, because the pointer has swung back and forth across the midline in a uniform 

 manner. A compound wave, on the other hand, presents asymmetrical deviations, 

 which, however, may be perfectly periodic. 



FiQ. 378. — Schema by Helmholtz to Illustrate the Formation of a Compound Wave 

 FROM Two Pendular Waves. 

 A and B, pendular vibrations, B being the octave of A. If superposed so that e 

 coincides with d° and the ordinates are added algebraically, the non-pendular curve C 

 is produced. If superposed so that e coincides with d' the non-pendular curve D is 

 produced. {Howell.) 



Fundamental Tones and Overtones. — If the string of a musical 

 instrument is set into transverse vibration by plucking it, a certain 

 sound will be emitted, the pitch, quality and loudness of which will 

 depend not only upon the length and the thickness of this vibrator, 

 but also upon the force with which it is displaced. If the string is 

 now firmly held midpoint between its two ends, the vibrations of 

 each half per unit of time will be doubled. Furthermore, if the 

 string is divided in this way into three segments, each division will 

 vibrate with a frequency three times greater than that of the entire 



