SECTION XXI 

 THE SENSE OF HEARING 



CHAPTER LXII 

 THE CAUSE AND CHARACTER OF THE SOUND WAVES 



The Cause of Sound Waves. Sound waves arise in consequence 

 of the vibration of elastic bodies. If a metal plate is suspended in 

 space and its central area is struck with the end of a rod, it suffers a 

 displacement of its constituents which permit it to deviate in the 

 direction of the stroke. Having attained its extreme position in this 

 direction, it immediately swings back toward the opposite side, and 

 so on until it has again attained its equilibrium. These deviations 

 of the plate in turn give rise to a vibration of the air surrounding it, 

 because those molecules which lie directly in its path will be alternately 

 condensed and rarefied. In this way, the vibrations of the sonorous 

 body are transferred into undulations of an elastic medium, formed 

 toy the air. The first are stationary and the second progressive in 

 their nature. 



Vibrations of a sonorous body may be either transverse, as in a 

 string, or longitudinal, as in a rod. The undulations in a medium, 

 however, must of necessity be longitudinal, because only forward im- 

 pulses or pushes can be communicated from one molecule to another. 

 Thus, sound is conveyed onward by an undulatory or wave-like 

 motion in air, similar to that exhibited by particles of water during 

 the translation of a wave. In water, however, the different particles 

 move in a circle, while in air they move in a straight line, backward 

 and forward, in the direction in which the sound is projected. 



The initial energy of the undulations in air is gradually reduced as 

 they pass away from the sonorous body, so that the sound diminishes 

 constantly until it becomes completely neutralized. This reduction, 

 however, takes place at a more rapid rate than is theoretically sug- 

 gested by the law of inverse squares. The reason for this discrepancy 

 is that vibration leads to friction and friction to heat, generated, of 

 course, at the expense of the initial energy. 



Sound waves may also be propagated by media other than air, in 

 fact, in many instances with much better results. Thus, they pass 

 along rods of wood with the greatest ease, and also along cords and 

 wires. Practical use has been made of this fact in the construction 



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