ACOUSTICS. 21 
ACOUSTICS. 
Acoustics! is the science which treats of sound. Sound is the sen- 
sation produced when the vibrations of some sounding body are conveyed 
directly or indirectly to the organ of hearing. That a body producing 
sound is in a state of vibration at the time is seen in any stringed 
musical instrument, the prongs of a tuning-fork, the lip of a bell, &c. 
These vibrations must be communicated to the ear through some 
medium connecting the sounding body with the ear, the ordinary 
medium being the air. This is proved by the fact, that when a bell is 
struck in a chamber from which the air has been withdrawn, no sound is. 
produced, and that the less dense the air becomes, the fainter are the 
sounds heard in it, so that on high mountains, where the air is very rare, 
what would elsewhere be loud talking is heard like whispers. Air, how- 
ever, is not the only medium, the vibrations being conveyed through. 
water, wood, and other substances. If the head be held under water, any 
noise at a distance, such as that caused by two stones being knocked, 
against each other, is heard with great distinctness. Or if the ear 
be placed at one end of a log of wood, the slightest scratch at the. 
other end is heard very distinctly, the vibrations being transmitted. 
through the particles of the wood. Bodies denser than air are better: 
conductors of sound, but, as air is the ordinary medium, the general, 
principles of sound have reference to it. Vibrations are the wave-like 
motion which is communicated to the air by any shock, such as a shot, 
the blow of a hammer, or an explosion. ,The air is not made to change 
its place, but the compression of its particles caused by the shock is passed 
along like the undulations of a rope held at one end and moved up and 
down. These undulations pass outward from the body producing them, 
in all directions, exactly like those caused on the surface of water by a 
stone thrown into it, and, like them too, they diminish in force as they 
proceed outward. : 
Loudness of Sound.—The loudness of any sound depends upon the force 
of the concussion of the air, and the force with which this concussion is 
conveyed to the ear; so that, from the decreasing force of the sound-waves, 
in a certain ratio, the intensity or loudness of sound diminishes with the 
distance of the hearer from the object causing it. But a great many other 
things besides the distance have to be taken into account as affecting the 
intensity of sound. Thus, sounds at a distance are heard much better at 
one time than another: this arises from various causes. As was men- 
tioned above, the denser the air is the better are sounds heard; a dense 
state of the atmosphere would therefore account for sounds being heard 
1From Greek akoustikos, relating to hearing or sound, from akoud, to hear. 
