Air within an Auditorium upon its Acoustic Qualities. 113 



of being wholly lost, is interrupted in its backward course by 

 the first current of air and reflected along the path of the 

 primary wave, but following it at an interval of time depend- 

 ing upon the thickness of the current of air. Each reflection 

 being thus again and again reflected and divided, we have, fol- 

 lowing close upon the primary wave, a multitude of secondary 

 waves, which, falling upon the ear, greatly mask the distinct- 

 ness of the original sound. Currents of air of varying density 

 then cause, first, a decrease in intensity of sound, and secondly, 

 an indistinctness or confusion of the sound. 



That currents of air, which we have thus studied in the labo- 

 ratory, act in the same way in an auditorium may be shown 

 by the following experiment, in which the sound-waves may 

 be actually traced out in space, and their confusion, conse- 

 quent upon the introduction of currents of air, likewise shown. 



Near the middle of a lecture-hall, 92 feet long and 65 feet 

 wide (the hall of the Massachusetts Institute of Technology in 

 Boston), a heavy plank, 6 feet wide and 12 feet long, was set 

 on one end and firmly fixed. Eight feet from its middle point, 

 on one side, was placed a B 4-stopped lead organ-pipe, which 

 was so connected with a gasometer as to be blown with a con- 

 stant pressure of air. 



On the other side of the plank and within the sound-shadow 

 a system of coordinates, in a plane parallel with the floor, was 

 established by means of light wooden rods, which ran parallel 

 and perpendicular to the board, and their length divided into 

 centimetres. 



Now it has been shown by the author (Proc. Amer. Acad. 

 Arts and Sciences, May 10, 1876) that rays of sound diver- 

 ging from such a source, and being diffracted around the edges 

 of the board, will, when they meet each other, after having 

 passed over paths differing by a half wave-length, neutralize 

 each other and produce comparative silence. By moving a 

 B 4 resonator, connected by means of a rubber tube with the 

 ear, along these coordinates the points of interference are easily 

 detected, and are found to be situated as predicted by formulae 

 similar to those used in the diffraction of light. In fact the 

 cases are entirely analogous. 



We have here, then, a means of mapping out the positions 

 of the sound-waves in space, and can say that at one point 

 there is silence because two sound-waves have met, crest upon 

 the trough, and neutralized; at another the sound is loud, 

 because two waves have met, crest and crest, and trough and 

 trough, and have doubled. All this is true so long as the air 

 of the hall is at rest ; and these experiments have been made 

 with doors, windows, and registers carefully closed. 



