PHYSICS OF SOUND 723 



The sound, as the waves recede from the sonorous body, becomes dis- 

 tributed over an increased area. The propagation of sound has been 

 reduced also to the formula, that the intensity diminishes in proportion 

 to the square of the distance. 



Sonorous vibrations are subject to many of the laws of reflection of 

 light. Sound may be absorbed by soft and non-vibrating surfaces in the 

 same way that certain surfaces absorb the rays of light. By carefully 

 arranged convex surfaces, the waves of sound may readily be collected 

 to a focus. These laws of the reflection of sonorous waves explain echoes 

 and the conduction of sound by confined strata of air, as in tubes. To 

 extend the parallel between sonorous and luminous transmission, it has 

 been ascertained that the waves of sound may be refracted to a focus by 

 being made to pass through an acoustic lens, as a balloon filled with car- 

 bon dioxide. The waves of sound also may be deflected around solid 

 bodies, when they produce what have been called shadows of sound. 



Observing the sound produced by the blow of an axe, it is seen that 

 sound is transmitted with much less rapidity than light. At a short dis- 

 tance the view of the blow is practically instantaneous ; but there is a 

 considerable interval between this and the sound. This interval rep- 

 resents the velocity of sonorous conduction. This fact is also illus- 

 trated by the interval between a flash of lightning and the sound of 

 thunder. The velocity of sound depends on the density and elasticity 

 of the conducting medium. The rate of conduction of sound by atmos- 

 pheric air at the freezing-point of water is about 1118 feet (340 meters) 

 per second. This rate presents comparatively slight variations for the 

 different gases, but it is much more rapid in liquids and in solids. 



Noise and Musical Sounds. There is a well-defined physical as well 

 as an aesthetic distinction between noise and music. Taking as examples, 

 single sounds, a sound becomes noise when the air is thrown into con- 

 fused and irregular vibrations. A noise may be composed of musical 

 sounds when these are not in accord with each other, and sounds called 

 musical are not always free from discordant vibrations. A noise pos- 

 sesses intensity, varying with the amplitude of the vibrations, and it may 

 have different qualities depending on the form of its vibrations. A noise 

 may be dull, sharp, ringing, metallic, hollow etc., these terms express- 

 ing qualities that are readily understood. A noise also may be called 

 sharp or low in pitch, as the rapid or slow vibrations predominate, with- 

 out answering the requirements of musical sounds. 



A musical sound consists of vibrations following each other at regular 

 intervals, provided that the succession of waves be not too slow or too 

 rapid. When the vibrations are too slow, there is an appreciable suc- 

 cession of impulses and the sound is not musical. When they are too 



