48 ILLINOIS ACADEMY OF SCIENCE 



From the acoustical standpoint the Auditorium was then in 

 fairly satisfactory shape. The canvas curtains, however, were 

 unsightly and did not accord with the architectural features 

 of the room. Steps were taken to find an arrangement that 

 would satisfy both the acoustical and architectural require- 

 ments. Calculations were made by Sabine's method to deter- 

 mine the amount of hairfelt necessary to cure the reverbera- 

 tion. 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 much absorbing material 

 and make the Auditorium too dead for sound. 



In the face of this difficulty it was decided to carry on fur- 

 ther 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 felt strips 

 were strongly absorbed and changed in phase. The results 

 obtained were encouraging, though not as marked in diminish- 

 ing the echoes as anticipated. Another wall was therefore 

 padded in a similar way except that the felt strips were in- 

 stalled 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 sky- 

 light and gradually nearing the wall until it touched at the 

 crown of the arches. 



Other changes were made in the Auditorium. A pipe organ 

 was installed, the lighting system was changed and the interior 

 was redecorated. All of these modfications affected the acous- 

 tics and were considered when calculating the amount of hair- 

 felt to be used. 



The results obtained have been generally satisfactory. The 

 remodeled Auditorium has been used almost continuously 

 under varied conditions for music and speaking and has been 

 four.- 1 to have acceptable acoustics. A speaker with a mod- 



