4 WALLACE CLEMENT WARE SABINE— HALL tMEM ""^vifu'xxi, 



All the cushions from the seats in Sanders Theatre were then brought over and stored in the lobby. On bringing 

 into the lecture room a number of cushions having a total length of 8.2 meters, the duration of audibility fell 

 to 5.33 seconds. On bringing in 17 meters the sound in the room after the organ pipe ceased was audible but 

 4.94 seconds. Evidently, the cushions were strong absorbents and rapidly improving the room, at least to the 

 extent of diminishing the reverberation. The result was interesting and the process was continued. Little 

 by little the cushions were brought into the room, and each time the duration of audibility was measured. When 

 all the seats (436 in number) were covered, the sound was audible for 2.03 seconds. ... In this lecture room 

 felt was finally placed permanently on particular walls, and the room was rendered not excellent, but 

 entirely serviceable. . . . 



Let us note the extreme simplicity of the apparatus and method finally used by Sabine for 

 studying the reverberation of a room. A standard horn or organ pipe was blown by means of 

 a certain air pressure till the room was as full of sound as this source could make it. The action 

 of the horn was then stopped by the push of a button which simultaneously recorded itself on 

 the cylinder of a chronograph ; a good observer 5 placed somewhere in the room listened intently 

 but with unassisted ear, till the reverberation became inaudible and at this instant pushed an- 

 other button and thus made another mark on the chronograph cylinder. The interval of time 

 between the two records on the chronograph — that is, the duration of audible reverberation 

 after the sound supply was cut off — could be measured to about 0.01 second. 



Extremely simple in theory, but hard enough in practice; successfully carried out because 

 the man born to do this thing and bound to do it, to break through the armor of difficulties 

 hiding the secret of acoustics, had been found and had found his place. The work must be 

 done "during the most quiet part of the night, between half-past twelve and five." 



To secure accuracy, ... it was necessary to suspend work on the approach of a street car within 

 two blocks, or the passing of a train a mile distant. In Cambridge these interruptions were not serious; in 

 Boston and in New York it was necessary to snatch observations in very brief intervals of quiet. In every case 

 a single determination of the duration of the residual sound was based on the average of a large number of 

 observations. 



Three general conclusions which may be rather surprising to most people were derived 

 from these observations, as applying to rooms of fairly regular shape: 



1. The duration of audibility of the residual sound is nearly the same in all parts of an audi- 

 torium. 



2. The duration of audibility is nearly independent of the, position of the source (the horn) . 



3. The efficiency of an absorbent in reducing the duration of a residual sound is, under ordi- 

 nary circumstances, nearly independent of its position. 



All this comes from the fact that a sound wave emitted by the source is reflected back and 

 forth across an ordinary room many times a second and in many directions, so that, when a 

 horn has been sounding for a few seconds and has then stopped suddenly, all parts of the air of 

 the room are about equally full of sound energy. 



At first the unit of absorptive capacity employed by Sabine was that of a running meter of the 

 Sanders Theater cushions mentioned in one of the preceding quotations. Thus, the absorbing 

 power of the walls, floor, windows, etc., of a certain bare room was found to be equal to that 

 of 146 running meters of these cushions. But presently it was found that the exposed vertical 

 edge of a cushion counts as much per unit area as the exposed horizontal top. 6 So the square 



» Mr. Oifford LeClear and Mr. E. D. Densmore assisted in this labor, each making his own observations of the duration of reverberation, 

 for comparison with those of Sabine. This is an example of the latter's power, already mentioned, to enlist the interest of capable young men. On 

 the other hand, he was able to give full recompense to LeClear and Densmore by getting professional work for them at the beginning of their career, 

 now become distinguished, as building engineers. Another who should be mentioned here is Mr. John Connors, head janitor of the Jefferson 

 Physical Laboratory, a man of remarkable resourcefulness and practical good sense. He came from Ireland about the time Sabine came from 

 Ohio. They were nearly of the same age and though, naturally, very unlike in many respects, were close friends. When Sabine died, "John" 

 knew more about his experiments and his plans than did any one of the teaching staff. 



• This fact was brought out in striking fashion after experiments made with cushions having one edge pushed against the backs of settees had 

 given anomalous results. " It was then recalled that about two years before, at the beginning of an evening's work, the first lot of cushions brought 

 into the room were placed on the floor, side by side, with edges touching, but that after a few observations had been taken the cushions were scat- 

 tered about the room, and the work was repeated. This was done not at all to uncover the edges, but in the primitive uncertainty as to whether 

 near cushions would draw from each other's supply of sound, as it were, and thus diminish each other's efficiency. No further thought was then 

 given to these discarded observations until recalled by the above-mentioned discrepancy. They were sought out from the notes of that period, 

 and it was found that, as suspected, the absorbing power of the cushions when touching edges was less than when separated. Eight cushions had 

 been used and, therefore, fourteen edges had been touching. A record was found of the length and the breadth of the cushions used and, assuming 

 that the absorbing power was proportional to the area exposod, it was possible to calculate their thickness by comparing the audible duration of the 

 residual sound in the two sets of observations; it was thus calculated to be 7.4 centimeters. On stacking up the same cushions and measuring their 

 total thickness, the average thickness was found to be 7.2 centimeters, in very close agreement with the thickness estimated from their absorp- 

 tion of sound. Therefore, the measurements of the cushions should be, not in running meters of cushion, but in square meters of exposed surface." 



