THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

11Acoustics of Enclosed Spaces:Architectural Acoustics11.1 IntroductionAlthough people have gathered in large auditoriums and places of worship sincethe advent of civilization, architectural acoustics did not exist on a scientific basisuntil a young professor of physics at Harvard University accepted an assignmentfrom Harvard’s Board of Overseers in 1895 to correct the abominable acousticsof the newly constructed Fogg Lecture Hall. Through careful (but by present-daystandards, rather crude) measurements with the use of a Gemshorn organ pipeof 512 Hz, a stopwatch, and the aid of a few able-bodied assistants who luggedabsorbent materials in and out of the lecture hall, Wallace Sabine establishedthat the reverberation characteristics of a room determined the acoustical natureof that room and that a relationship exists between quality of the acoustics, thesize of the chamber and the amount of absorption surfaces present. He defined areverberation time T as the number of seconds required for the intensity of thesound to drop from a level of audibility 60 dB above the threshold of hearing to thethreshold of inaudibility. To this day reverberation time still constitutes the mostimportant parameter for gauging the acoustical quality of a room. The originalSabine equationT = ∑0.049ViSi α iis deceptively simple, as effects such as interference or diffraction and behaviorof sound waves as affected by the shape of the room, presence of standing waves,normal modes of vibration, are not embodied in that equation. Here V is the roomvolume in cubic feet, S i the component surface area and α i the correspondingabsorption coefficient. On the basis of his measurements Sabine was able to cutdown the reverberation time of the lecture hall from 5.6 s through the strategicdeployment of absorbing materials throughout the room. This accomplishmentfirmly established Sabine’s reputation, and he became the acoustical consultantfor Boston Symphony Hall, the first auditorium to be designed on the basis ofquantitative acoustics.243