2 , DYNAMICAL .THEOKY OF SOUND 



pressure on one side or other of the mean, the only difference 

 being that the horizontal and vertical scales are now enormously 

 magnified. 



The variety of such curves is of course endless, and it is 

 impossible to suppose that a distinct provision is made in the 

 ear for the recognition of each, or even of each of the numerous 

 classes into which they might conceivably be grouped. It is 

 therefore necessary to analyse, as far as possible, both the 

 vibration- forms and the resulting sensations into simpler 

 elements which shall correspond each to each. 



As regards the vibration-forms, there is one mode of 

 resolution which at once claims consideration on dynamical 

 grounds. The fundamental type of vibration in Mechanics is 

 that known as "simple-harmonic," which is represented graphic- 

 ally by a curve of sines (Fig. 3, p. 10). This is met with in 

 the pendulum, and in all other cases of a freely vibrating body 

 or mechanical system having only one degree of freedom. It 

 can moreover be shewn that the most complicated oscillation of 

 any system whatever may, so far as friction can be neglected, be 

 regarded as made up of a series of vibrations of this kind, each 

 of which might be excited separately by suitable precautions. 

 The reason for the preeminent position which the simple- 

 harmonic type occupies in Mechanics is that it is the only type 

 which retains its character absolutely unchanged whenever it 

 is transmitted from one system to another. This will be ex- 

 plained more fully in the following chapter. 



The analysis of sensations is a much more delicate matter, 

 and it was a great step in Acoustics when Ohm* in 1843 

 definitely propounded the doctrine that the simplest and 

 fundamental type of sound-sensation is that which corresponds 

 to a simple-harmonic vibration. This implies that all other 

 sound-sensations are in reality complex, being made up of 

 elementary sensations corresponding to the various simple- 

 harmonic constituents into which the vibration-form can be 

 resolved. The statement is subject to some qualifications, in 

 particular as to the degree of independence of elementary 



* G. S. Ohm (17871854), professor of physics at Munich 184954, known 

 also as the author of " Ohm's Law " of electric conduction. 



