Repository: UIHistories Project: Board of Trustees Minutes - 1916 [PAGE 241]

EXTRACTED TEXT FROM PAGE:

PROCEEDINGS OE THE BOAED OE TRUSTEES. 241 ./ On motion of Mr. Carr, President James was authorized to grant leave of absence on half-pay to Prof. W. J. Fraser for the year beginning September 1, 1915, provided suitable arrangements can be made for his work.

ACOUSTICS OF T H E AUDITORIUM. URBANA, I I I . , , March 16, 1915.

1915]

(9) A report from Prof. F. R. Watson on curing the acoustic defects in the Auditorium: Prof. J. M. White, Supervising Architect. DEAR SIR: The following report on the acoustics of the University Auditorium gives an account not only of the final cure but also a summary of the entire investigation. The Auditorium is a large structure with its inner surface made up almost entirely of curved walls. Because of its large size and concreted walls with small absorbing power, it was afflicted with both a reverberation and echoes. This unfortunate condition proved beneficial in the respect that it allowed tests of faulty acoustics to be made under exceptionally good conditions. A systematic investigation, avoiding "cut and try" methods of cure, was inaugurated first to ascertain what the acoustical defects were, and then to investigate the methods of cure and apply them to correct the troubles. Sabine has shown in his classical experiments on architectural acoustics that a reverberation may be corrected by installing sound absorbing materials. Calculations and experiments were made by Sabine's method to determine the amount of material necessary to reduce the reverberation in the Auditorium to a satisfactory point. The main problem, however, was to find the echoes and eliminate them. The location of walls producing echoes was attempted by a number of methods and finally solved by using an arc light reflector as source of sound. The arc gave forth an intense hissing sound that traveled with the light so that an observer could see where the sound struck and thus locate the echoes. A complete diagnosis of the action of sound was made by this apparatus. Experiments to improve the acoustics were then carried on in accordance with the results of the diagnosis. Sounding boards of various kinds were tested, the most successful one being of parabolic shape. This was not satisfactory as a permanent arrangement since it was suited for only a single speaker in a fixed position. Canvases were hung in various positions to determine the effect of cutting off certain walls from the action of sound. Absorbing materials we're also hung at critical points suggested by the analysis. The provisional cure was brought about when .four large canvases were hung in the dome. For the first time speakers could talk with comparative ease without suffering great annoyance from echoes. From the acoustical standpoint the Auditorium was then in fairly satisfactory shape. The canvas curtains, however, were very unsightly and did not accord with the architectural features of the room. Steps were then taken to find an arrangement that would satisfy both the acoustical and architectural requirements. Calculations were made by Sabine's method to determine the amount of hairfelt necessary to cure the reverberation. Unfortunately this amount was not sufficient to cover all the walls producing echoes. It was desirable to eliminate the echoes but it was risky to install too much absorbing material and make the Auditorium too dead for sound. In the face of this difficulty it was decided to carry on further experiments before attempting the final cure. One of the large curved walls was covered with vertical strips of hairfelt 30 inches wide placed 30 inches apart with bare wall space between them. This arrangement had several advantages. It maintained the curvature of the wall and used only half the material necessary to cover the surface completely. Also, it was theoretically more effective in breaking up the incident sound because the portions of the waves striking the felt1 strips were strongly absorbed and changed in phase. The results obtained were encouraging though not as marked in diminishing the echoes as anticipated. Another wall was therefore padded in a similar way except that the felt strips were installed one foot out from the surface. This would allow the felt to act on both the incident and reflected waves and thus more thoroughly modify the regularity of the sound. The dome surface was also treated, the felt being mounted in radial strips placed 18 inches from the ceiling at the edge of the skylight and gradually nearing the wall until it touched at the crown of the arches. The material used was Akustikos felt developed by the Johns-Manville Company under the direction of Prof. Sabine. The felt has excellent sound absorbing qualities and, in addition, is fireproof and vermin proof. It is installed by a special process and covered with a rep of suitable color so that it presents a final satisfactory appearance. The Johns-Manville Company agreed to install their felt at cost price because of the advertisement that would come to them in curing with their material the notoriously bad acoustics of the Auditorium. I have been interested in making an estimate of the amount of money saved on the installation. According to Sabine's method we should have installed 6,300 square feet of the Akustikos felt. The usual charge made .by the Johns-Manville Company for this felt is from 40 cents to $1.00 or more per square foot depending on the difficulty of installing the material and the amount of expert investigation of the problem. A conservative estimate for the Auditorium, which was not only an unusual case acoustically but also one in which the installation of material would be difficult, would be 75 cen+« pe^ soua^e -Foo+. or a. total- cost of $4,725. Now we saved in two ways. We obtained the felt and expert labor of installation a/t cost price, and also, by mounting the material out from the walls so that

—16 U