226 ACOUSTICS APPLIED TO PUBLIC BUILDINGS. 



of the sound and the acuteness of different ears. This will give about 

 the sixteenth part of a second as the limit of time necessary f }r the 

 ear to separately distinguish two similar sounds. From this experi- 

 ment we learn that the reflected sound may tend to strengthen the 

 impression, or to confuse it, according as the difference of time between 

 the two impressions is greater or less than the limit of perceptibility. 

 An application of the same principle gives us the explanation of some 

 phenomena of sound which have been considered mysterious. Thus, 

 in the reflection of an impulse from the edge of a forest of trees, each 

 leaf properly situated within a range of 30 feet of the front plane of 

 reflection will conspire to produce a distinct echo, and these would 

 form the principal part of the reflecting surfaces of a dense forest, for 

 the remainder would be screened; and being at a greater distance, any 

 ray which might come from them would serve to produce merely a 

 low continuation of the sound. 



On the same principle, we may at once assert that the panelling of 

 a room, or even the introduction of reflecting surfaces at different dis- 

 tances, will not prevent the echo, provided they are in parallel planes, 

 and situated, relatively to each other, within the limit of perceptibility. 

 Important advantage may be taken of the principle of reflection of 

 sound by the proper arrangement of the reflecting surfaces behind the 

 speaker. We frequently see in churches, as if to diminish the effect 

 of the voice of the preacher, a mass of drapery placed directly in the 

 rear of the pulpit. However important this may be in an resthetical 

 point of view, it is certainly at variance with correct acoustic arrange- 

 ments — the great object of which should be to husband every articu- 

 lation of the voice, and to transmit it unmingled with other impulses, 

 and with as little loss as possible, to the ears of the audience. 



Another effect of the transmission and reflection of sound is that 

 which is called reverberation, which consists of a prolonged musical 

 sound, and is much more frequently the cause of indistinctness of per- 

 ception of the articulations of the speaker than the simple echo. 



Reverberation is produced by the repeated reflection of a sound from 

 the walls of the apartment. If, for example, a single detonation takes 

 place in the middle of a long hall with naked and perpendicular walls, 

 an impulse will pass in each direction, will be reflected from the walls, 

 cross each other again at the point of origin, be again reflected, and 

 60 on until the original impulse is entirely absorbed by the solid 

 materials which confine it. The impression will be retained upon the 

 ear during the interval of the transmission past it of two successive 

 waves, and thus a continued sound will be kept up, particularly if the 

 walls of any part of the room are within 30 feet of the ear. If a series 

 of impulses, such as that produced by the rapid snaps of a quill aj^ainst 

 the teeth of a wheel, be made in unison with the echoes, a continued 

 musical sound will be the result. Suppose the wheel to be turned 

 with such velocity as to cause a snap at the very instant the return 

 echo passes the point at which the apparatus is placed, the second 

 sound v/ill combine with the first, and thus a loud and sustained 

 vibration will be produced. It will be evident from this that every 

 room has a key-note, and that, to an instrument of the proper pitch, 

 it will resound with great force. It must be apparent, also, that the 



