ACADEMY OF SCIENCES] BIOGRAPHY 3 



some writers, 1:1:2, and others, 2:3:4; it is probable that the basis of these suggestions is the ratios of the har- 

 monic intervals in music, but the connection is untraced and remote. Moreover, such advice is rather 

 difficult to apply; should one measure the length to the back or to the front of the galleries, to the back or 

 the front of the stage recess? Few rooms have a flat roof, where should the height be measured? One writer, 

 who had seen the Mormon Temple, recommended that all auditoriums be elliptical. Sanders Theatre is by far 

 the best auditorium in Cambridge and is semicircular in general shape, but with a recess that makes it almost 

 anything; and, on the other hand, the lecture room in the Fogg Art Museum is also semicircular, indeed, was 

 modeled after Sanders Theatre, and it was the worst. But Sanders Theatre is in wood and the Fogg Lecture 

 room is plaster on tile; one seizes on this only to be immediately reminded that Sayles Hall in Providence 

 is largely lined with wood and is bad. . . . 



In order that hearing may be good in any auditorium, it is necessary that the sound should be sufficiently 

 loud; that the simultaneous components of a complex sound should maintain their proper relative intensities; 

 and that the successive sounds in rapidly moving articulation, either of speech or music, should be clear and 

 distinct, free from each other and from extraneous noises. These three are the necessary, as they are the entirely 

 sufficient, conditions for good hearing. . . . Within the three fields thus defined is comprised without exception 

 the whole of architectural acoustics. i 



Starting with the simplest conceivable auditorium — a level and open plain, with the ground bare and hard, 

 a single person for an audience — it is clear that the sound spreads in a hemispherical wave diminishing in intensity 

 as it increases in size, proportionally. If, instead of being bare, the ground is occupied by a large audience, the 

 sound diminishes in intensity even more rapidly, being now absorbed. The upper part of the sound-wave 

 escapes unaffected, but the lower edge — the only part that is of service to an audience on a plain — is rapidly lost. 

 The first and most obvious improvement is to raise the speaker above the level of the audience; the second is to 

 raise the seats at the rear; and the third is to place a wall behind the speaker. The result is most attractively 

 illustrated in the Greek theatre. These changes being made, still all the sound rising at any considerable angle 

 is lost through the opening above, and only part of the speaker's efforts serve the audience. When to this 

 auditorium a roof is added the average intensity of sound throughout the room is greatly increased. . . . 



In discussing the subject of loudness the direct and reflected sounds have been spoken of as if always 

 reenforcing each other when they come together. A moment's consideration of the nature of sound will show 

 that, as a matter of fact, it is entirely possible for them to oppose each other. The sounding body in its forward 

 motion sends off a wave of condensation, which is immediately followed through the air by a wave of rarefaction 

 produced by the vibrating body as it moves back. These two waves of opposite character taken together con- 

 stitute a sound wave. The source continuing to vibrate, these waves follow each other in a train. Bearing 

 in mind this alternating character of sound, it is evident that should the sound travelling by different paths — ■ 

 by reflection from different walls — come together again, the paths being equal in length, condensation will 

 arrive at the same time as condensation, and reenforce it, and rarefaction will, similarly, reenforce rarefaction. 

 But should one path be a little shorter than the other, rarefaction by one and condensation by the other may 

 arrive at the same time, and at this point there will be comparative silence. . . . When the note changes in 

 pitch the interference system is entirely altered in character. A single incident will serve to illustrate this point. 

 There is a room in the Jefferson Physical Laboratory, known as the constant-temperature room. . . . While 

 working in this room with a treble c . . . organ pipe blown by a steady wind-pressure, it was observed that 

 the pitch of the pipe apparently changed an octave when the observer straightened up in his chair from a position 

 in which he was leaning forward. The explanation is this: The organ pipe did not give a single pure note, but 

 gave a fundamental treble c accompanied by several overtones, of which the strongest was in this case the 

 octave above. Each note in the whole complex sound had its own interference system, which, as long as the 

 sound remained constant, remained fixed in position. It so happened that at these two points the region of 

 silence for one note coincided with the region of reenforcement in the other, and vice versa. 



Sound, being energy, once produced in a confined space, will continue until it is either transmitted by the 

 boundary walls, or is transformed into some other kind of energy, generally heat. This process of decay is 

 called absorption. Thus, in the lecture room of Harvard University, in which, and in behalf of which, this 

 investigation was begun, the rate of absorption was so small that a word spoken in an ordinary tone of voice was 

 audible for five and a half seconds afterwards. During this time even a very deliberate speaker would have 

 uttered the twelve or fifteen succeeding syllables. Thus the successive enunciations blended into a loud sound, 

 through which and above which it was necessary to hear and distinguish the orderly progression of the speech. 

 Across the room this could not be done. . . . This [disturbance] may be regarded, if one so chooses, as a process 

 of multiple reflection from walls, from ceiling and from floor, first from one and then another, losing a little at 

 each reflection until ultimately inaudible. This phenomenon will be called reverberation, including as a special 

 case the echo. 



So much from Sabine by way of analysis and illustration of the conditions to be dealt with 

 in architectural acoustics. His final method of experimental attack on the particular difficulty 

 presented by the lecture room in question, that of the Fogg Art Museum, now appears, and, 

 since his work in this room gives us the first known instance of the rational and successful 

 treatment of such a difficulty, the story should be told in his own words. 



With an organ pipe as a constant source of sound, and a suitable chronograph for recording, the duration 

 of audibility of a sound after the source had ceased in this room when empty was found to be 5.62 seconds. 



