758 



THE SENSE OF HEARING 



regularity. A true or musical sound arises in consequence of a sus- 

 tained vibration, and possesses an euphonious character by reason of 

 its relatively fixed and uniform rate. The difference between noises 

 and true sounds may be well illustrated by means of sirenes placed upon 

 a rotating disc. If the openings through which the air is blown, are 

 placed at regular distances from one another, the result is a sound of 

 definite pitch, quality and loudness, while if they are arranged in an 

 irregular manner, the result is a noise. 



FIG. 376. FORM OF WAVE MADE BY TUNING FORK. 



Musical sounds result in consequence of the vibration of such 

 bodies as strings, rods, plates, bells, membranes and reeds. The waves 

 produced by them, however, do not affect our organ of hearing in a 

 like manner, because they differ from one another in their pitch, in- 

 tensity or loudness, and quality or timbre. 



(a) The pitch or tone of a sound is determined by the rapidity of vibration of 

 the sonorous body and the number of undulations produced by it. The greater 

 their number, the shorter must be their wave length and hence, the higher the 

 pitch of the sound. Thus, if these oscillations recur at the rate of 500 in a second, 

 their time of vibration is ^00 of a second. 



FIG. 377. To ILLUSTRATE THE CONCEPTION OF DIFFERENCES IN PITCH AND IN AMPLITUDE 



OR INTENSITY. 



In A, three pendular or sinus curves of the same period or pitch, but with different 

 amplitudes. In B, three pendular or sinus curves of the same amplitude, but with 

 different periods. (After Auerbach.) 



(b) The intensity or loudness of a sound is referable first of all to the amplitude 

 of the vibrations of the sonorous body. Thus, if the bass string of a piano is 

 struck with slight force, it will be seen to execute a series of vibrations of small 

 amplitude, which give rise to a sound of low audibility. If this same string is 

 then struck more vigorously, the amplitude of the vibrations will be much greater 

 and the sound much louder. These changes in the intensity of a sound may also 

 be noted as the vibrating body gradually returns into its position of absolute rest. 

 In the second place, the loudness of a sound is determined by the striking force of 

 the waves, because the latter is inversely proportional to the square of the distance 



