134 AMERICAN ASSOCIATION FOE THE 



divide this distance into the velocity of sound, Ave ascertain the limit of perceptibil- 

 ity in time. 



In the experiment just made with the Avail, a source of error was discovered, 

 in the fact that a portion of the sound returned Avas reflected from the cornice 

 under the eaves, and as this Avas at a greater distance than the part of the Avail im- 

 mediately perpendicular to the observer, the moment of the cessation of the echo 

 Avas less distinct. In subsequent experiments with a louder noise the reflection 

 was observed from a perpendicular surface of about twelve feet square, and from 

 this more definite results were obtained. The limit of the distance in this case 

 was about thirty-five feet, varying slightly, perhaps, Avith the intensity of the sound 

 and the acuteness of different ears. This Avill give about one-sixteenth of a se- 

 cond as the limit of time at which the ear can separately distinguish tAVO similar 

 sounds. From this experiment we learn that the reflected sound may tend to 

 strengthen the impression or to confine it, according as the difference of time be- 

 tween 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 im- 

 pulse from the edge of a forest of trees, each leaf properly situated within a range 

 of thirty 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 a greater distance every 

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

 tion of the sound. 



On the same principle we may at once assert that the panneiling of a room, or 

 even the introduction of reflecting surfaces at different distances, will not prevent 

 the echo, provided they are parallel to each other, and situated relatively to each 

 other within the limit of perceptibility. 



Important advantage may be taken of the principle of the 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 sesthetical point of view, it is certainly at variance with correct 

 acoustic arrangements — the great object of which should be to husband every 

 articulation of the voice, and to transmit it unmingled with their impulses, and 

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



Another effect ofthe 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 perception of the articulations of the 

 speaker than the single echo. 



Reverberation is produced by repeated reflection of a sound from the walls of 

 the apartment. If, for example, a single detonation takes place in the middle of 

 along hull, with naked and perpendicular Avails, an impulse will pass in each di- 

 rection, will be reflected from the walls, cross each other again at the point of 

 origin, be again reflected, and so on until the original impulse is entirely absorbed 

 by the soiid materials which confine it. The impression Avill 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 thirty-five feet of the ear. If a series of impulses, such as 

 those produced by the rapid snaps of the teeth of a wheel against a quill, be 



