ACOUSTICS. 



musical tone, bat is called a noise, an, for instance, when wo 

 pieces of metal in a box, or let a weight fall to the ground, 

 then, is the difference between the two '( Wli.-n Un- 

 ions Buooeed one another in a regular and uniform ni.uni.-r, 

 t lie case first mentioned, a perfect sound or tone is pro- 

 duced ; but when the vibrations are not isochronous, or when 

 le explosive disturbance of the air is produced at), for 

 instance, by a sudden blow or tho report of a pistol or when 

 several sounds interfere with ono another so aa to produce con- 

 fused waves in the air, in any such oases a noise is the result. 



If we examine a few sounding bodies we can easily satisfy 

 ourselves that in every case their particles are thrown into a 

 state of rapid vibration. 



In a sounding cord (Fig. 1), or the wire of a piano, theso vibra- 

 tions are easily seen by the eye, and in cases where a flat surface 

 ir. made to sound they may be rendered manifest by sprinkling 

 a little light powder, as, for instance, lycopodium, on it. The 

 motion will thus be at once rendered visible by the agitation of 

 the dust. 



A much more elegant plan of showing the same effect is by 

 means of the apparatus represented in Fig. 2. A hemispherical 

 bowl of thin glass is fixed to a stand, and directly over it is 

 suspended a small frame of six arms, from which hang as many 

 threads, each carrying a small ivory ball. This is so arranged 

 that the balls shall just rest against the rim of the glass. 



Now let a violin bow be rubbed with a lump of resin, and 

 then drawn steadily over the edge of the glass. A clear 

 musical note will be produced, but the vibrations of the glass 

 will scarcely be perceived by the eye. The ivory balls will, 

 however, at once act as tell-tales, for they will be violently 

 agitated and swing away from the glass, and the louder the 

 note produced, the greater will be the amount of their oscil- 

 lation. 



We must now see in what way the vibrations which are thus 

 produced are propagated through the air, so as to reach our 

 auditory nerves. The particles of air immediately around the 

 vibrating body are not driven right away so as to strike the tym- 

 panum of the ear. Each one is moved a slight distance from 

 its original position, to which, however, it immediately returns, 

 and then recedes almost as far in the other direction. These 

 particles, however, impart a similar oscillating movement to 

 those lying beyond them, which in their turn communicate the 

 movement, and thus the waves produced are conveyed from 

 particle to particle, and travel widely and rapidly. 



If we fix one end of a long rope or cord to a staple in a wall, 

 and holding the other end in the hand, shake it, waves will 

 appear to travel from the hand to the staple and back again. 

 We know, however, that in reality each portion of the cord 

 merely moves up and down in an almost straight line, and the 

 successive movements of the single portions produce the appear- 

 ance of a wave. This affords a good idea of the mode in which 

 sound-waves are propagated by the oscillations of different 

 layers of air. 



By standing at the head of a pier, and watching the waves 

 rise and fall in the sea, we get a further illustration of the same 

 fact. They appear to be travelling along and coming ashore in 

 rapid succession ; but if wo drop a piece of wood on the surface 

 in a part where it is not affected by the breaking of the waves 

 against the pier, we shall find that it scarcely moves along at 

 all, but merely rises and falls on their surface. 



So, too, if we drop a stone into the middle of a pond of water 

 whose surface is quite calm, we shall see the waves produced by 

 it gradually enlarging and spreading in all directions towards 

 the sides. As, however, they recede and become wider, they 

 diminish in height, till in a large pond they are quite lost. In 

 just the same way a bell or any sounding body produces waves 

 in the air around it, which extend further and further, diminish- 

 ing in intensity as they travel, till at last they become too faint 

 to affect the ear, or else are overpowered by the multitudes of 

 other vibrations which exist in the air. 



By taking a shallow rectangular vessel of water, and watching 

 the waves produced in it when we touch its surface, we shall be 

 able to understand many things in connection with the diffusion 

 and reflection of sound that would otherwise appear difficult. 



A moment's consideration will easily show us why it is that 

 a sound diminishes so rapidly in intensity as we recede from the 

 sounding body. Since the waves are propagated equally in all 

 directions, it is clear that tho mass of air set in vibration in- 



very rapidly ; the original vibration ha* therefore to be 

 spread over a much larger area, and its intensity is <<imi n j f hfd 



in tin; Hiuiii) proportion. 



From this wo see that it is necessary to have some substance 

 to convey the vibrations from the vibrating body to the ear. If 

 the atmosphere were entirely removed, no sound would ever teach 

 us ; all would bo continual unbroken silenoe. We can easily 

 obtain an experimental illustration of this fact. An alarum 

 (Fig. 3), made so as to continue striking for some little time, is 

 placed under the receiver of an air-pump, a layer of wadding 

 being placed between it and the pump-plate to prevent the 

 vibrations being communicated to tho air in that way. It is 

 now set in action, and the pump rapidly worked; as the air 

 under the receiver becomes more and more rarefied the sound 

 becomes feebler and feebler, till at last it almost entirely rfmso, 

 though we can see by the eye that the hammer still continues 

 to strike on the bell. A better way of performing the experiment 

 is to suspend the alarum by means of threads from four supports, 

 as in this way all the vibrations are kept from the pump-plate. 

 A rod is then made to pass air-tight through the top of the 

 receiver, and by pressing this down a detent can be moved, sc 

 as to stop or start the bell at pleasure. When a nearly perfect 

 vacuum is attained, no sound whatever will be heard even when 

 the ear is applied closely. 



Now admit hydrogen gas into the receiver in place of common 

 air, and allow the alarum to strike as before ; the sound will be 

 heard, but it will be faint and peculiar in tone. If we inhale 

 hydrogen gas (which for this purpose must be quite pure), and 

 then attempt to speak, the voice likewise will be found greatly 

 changed in character, having become hollow and thin, at the 

 same time being considerably higher than usual, so as to re- 

 semble a squeak. We see then that the intensity of any sound 

 depends upon the density of the air in which it is generated 

 and not of that in which it is heard. 



When at great elevations on the sides of mountains, all sounds 

 are wonderfully diminished in intensity in consequence of the 

 rarefied state of the air. Saussure says that on the summit of 

 Mont Blanc the report of a pistol was not louder than that of 

 an ordinary cracker, and the travellers were obliged to speak in 

 a louder tone than usual in order to be heard. 



The rate at which the sound-wave travels through the air 

 does not depend at all upon the intensity or the pitch. If it 

 did, music when heard at a little distance would be quite 

 changed into disicord, since the louder notes would outstrip the 

 others. 



In the case, however, of extremely loud sounds, such as, for 

 instance, the report of a heavy piece of ordnance, there seems 

 to be a slight departure from this law. 



Sound is conducted by liquids or solids, as well as by gases. 

 When two stones ore struck together under water, the sound is 

 conveyed a considerable distance. Divers, too, can communicate 

 with those on the surface by striking the sides of the diving-bell 

 with a hammer or stone. If a watch be laid upon one end of a 

 plank, and the ear applied to the other end, the ticking will be 

 heard much further off than it would otherwise be. In a similar 

 way the earth acts as a conductor of sound, for if the ear be ap- 

 plied to its surface, the footsteps of men or horses approaching 

 may be heard at a very great distance. So, too, by laying the 

 ear upon the metal rails, the sound of a train can be heard 

 much further off than it con by any person merely standing up 

 and listening. 



Many very interesting experiments can be tried to illustrate 

 the conduction of sound. One of the simplest is to suspend a 

 common poker by a piece of string or list. Wind the ends of 

 this round the forefinger of each hand, and, having put the 

 fingers in the ears, make the poker swing so as to strike against 

 the fender or some piece of metal. Instead of the sound usually 

 heard we shall now hear one almost resembling that of a church 

 bell. The vibrations are conveyed so much more plentifully 

 along the string than through the air, that tho sound is very 

 greatly increased in intensity, and is heard for a longer period. 



In a similar way we can easily conduct sound from place to 

 place. Let a thin wooden rod some twelve or fifteen feet long 

 be rested on the tips of the fingers of two people, and against one 

 end of it let there be held a thin sounding-board, or a box of 

 thin wood, or, better still, a violin. Now strike a tuning-fork, 

 and place it against the other end of the rod. The sound will 

 at once fill the room, but will appear to proceed, not from the 



