TUB HUMAN EAK. 1 



U ADULATORY FORCES A CO USTICS. 



external curves, 6 6 and c c, with the exception, that 



Jhe intensity of the sound diminishes as the distance 



Fig. 30. 



b a S tt 6 



from the speaker increases ; but still every person on 

 the same curve has the same amount of sound arriving 

 at his ears. This is a necessary consequence of the 

 geometrical law that the circumference of a circle is 

 equi-distant in every part from its centre. 



But this law is not applicable to the case of the oblong 

 building ; for we will supposa two persons placed, one at 

 g 1 and another at g. The first one is nearer the speaker, 

 and at the circumference of the first curve of sound a a, 

 whose radius is S, tf; whereas, the other person at g is 

 at the extremity of the third curve, c c, although level 

 with him at g' ; but because the curve c c is three times 

 the distance from S, the speaker, than is a a it follows, 

 .ing to the law of radial forces, that he at g hears 

 < 'f only one-ninth the intensity of the listener at 

 g'. The same reasoning holds good with the successive 

 positions of d, e, and/, the parallel lines of which touch 

 the curves at d', e, and/'. Many of our readers will have 

 noticed how much more difficult it is to hear a speaker 

 when they are placed at the sides instead of the middle 

 of a building, although both positions may be in a 

 similar parallel line with the speaker ; yet perhaps few 

 would expect that so great a difference existed between 

 the chances of hearing at each place. But we will 

 now transfer our attention to persons situated in the 

 recesses on either side, and placed at h and i. Those 

 on h are on the same parallel with others situated 

 at e ; but being on a curve, the circumference of which ia 

 double the distance from N, they will only hear one- 

 fourth as well. The same reasoning holds good with 

 respect to persons situated at i, <tc. Now, all the rays 

 of sound impinging on the walls c h, on each side, 

 are either lost or reflected back : and the same oc- 

 curs with all the glass surfaces of the oblong building, 

 which can be easily traced by our readers. The 

 effect of this is twofold ; for an echo is produced, and 

 the interference and destruction of sound also ensues. 



It will be therefore readily perceived, that considerable 

 loss of acoustic effect is experienced in all buildings of 

 the oblong form ; and that the remedy which presents 

 itself, is that of constructing new edifices in obedience 

 to the principles we have been indicating. We have 

 confined our attention solely to circular erections; but 

 those of the elliptic or parabolic form may easily be 

 studied in the manner already mentioned. 



The faults of the internal arrangement of halls, &c. , 

 may often be remedied by the most simple means. 

 They chiefly arise from the echoes produced by the 

 reflection of sound from plane surfaces in oblong or 



Xare buildings, and especially from glass windows, 

 ch, having a very level surface, act as good reflectors. 

 If the seats of a building are arranged in concentric 

 circles, rising one above the other, the echo from the 

 wall is at once destroyed, because the audience is 

 arranged between them and the speaker ; and this plan 

 conduces highly to the comfort of both parties. The 

 echo produced by windows, or other plane surfaces, is 

 at once destroyed by covering them with cloth ; indeed, 

 ordinary cotton blinds will, generally speaking, answer 

 the purpose perfectly well. If the walls of a hall are 

 papered, instead of being left bare, or painted, the echo 

 is almost destroyed. Those who have not had the 

 opportunity of soeaking in a building in which echoes 



abound, can have no idea of the painful effect experienced 

 by the public speaker. Each word is reverberated to his 

 ear, as its successor passes from his mouth ; and the 

 effect is most confusing to the mind if an extempora- 

 neous address is being delivered by him. 



Sounding-boards are a contrivance by means of which 

 the voice of the speaker can be reflected towards the 

 audience ; and here again we perceive an analogy existing 

 between sound and light. By means of a reflector, we 

 can send the rays of light from theii source in any 

 direction we please ; and the sounding-board answers the 

 same purpose. In some buildings, the wall before 

 which a speaker may stand, is made of a curved shape ; 

 and if the roof be contrived so that a regular geometrical 

 figure be formed, such becomes an excellent reflector, 

 and adds much to the acoustic character of the place. 

 Besides acting as a reflector, it also serves as a diffuser 

 of sound,* and, as such, tends to spread the vibrations in 

 all directions. The acoustic effects of a solid reflector, 

 and a sounding-board which is made hollow, are very 

 different : for, in the former case, the sonorous waves 

 are merely sent forward from the reflecting surface ; but, 

 in the latter, the body itself partakes of the vibrations, and 

 so assists in their general diffusion. That such arrange- 

 ments themselves vibrate, is easily proved by throwing a 

 little sand on a drum-head, or placing on a piano a sheet 

 of glass, over which some sand has been strewn. If any 

 instrument be played near the drum, the sand will 

 speedily arrange itself in various forms. The same will 

 occur with the glass on the piano, if that instrument be 

 performed on; and what are termed acoustic figures are 

 produced. Another mode of obtaining the same ap- 

 pearances, is that of placing fine sand on a piece of thin 

 sheet glass, and drawing over one of its edges the bow 

 of a violin ; the glass will vibrate, and the sand will 

 assume star-like and other figures, owing to its partaking 

 of the vibration. To so great an extent can these 

 effects be produced, that a solid body, such as a thin 

 glass cylinder, may be fractured by producing near it 

 more powerful sounds in unison with the note which it 

 affords on being struck. 



THE EAR. 



WE shall now give a short description of the human ear, 

 as the organ by means of which sounds are perceived. 

 Its external part is of such a form as to receive the 

 vibrations of the air, as they proceed from any sounding 

 body. From the outer part there is a tortuous passage, 

 by which the sounds arrive at the tympanum or drum. 

 This is a thin membrane ; and it is stretched over bones 

 in a similar manner to that of the parchment covering of 

 the musical instrument of the same name. The vibra- 

 tions which havi arrived thus far, set the drum in 

 motion : and here we may mention a peculiar provision 

 that is made, by which the intensity of sound is modified. 

 When describing the eye,t we mentioned that the iris 

 could be either opened or closed, so as to admit or 

 restrain the rays of light, and prevent injury to the 

 organ of sight. Now, similarly, the drum of the ear 

 may either be tightened or extended, so as to increase or 

 diminish the force of communicated vibrations ; and by 

 this beautiful arrangement, the ear itself is preserved 

 from the injurious effect of too powerful a sound, or is 

 enabled to catch the slightest whisper. From the drum 

 of the ear, a channel, called the Eustachian tube, passes 

 towards the mouth. This tube serves as an exit, by 

 means of the air it contains, for the vibrations which 

 have been communicated by the drum. It is, in fact, 

 analogous in its office to the holes in the sounding-board 

 of the piano, the harp, violin, <fcc. ; and deafness always 

 ensues when, by any means, this tube is closed, which 

 occurs from cold, the deposition of wax, (fee. 



A cavity, called the vestibule of the ear, also covered 

 with a membrane, is provided with nerves ; and these, 

 like the optic nerve, J connect the external vibrations of 

 matter with the sensorium of the brain; and thus the 

 sense of hearing is produced. The entrance of the ear 



See ante, p. 27*. t See ante, p. 48. t See ante, p. 48. 



2Q 



