Curing an Auditorium of Echoes 



Hairfelt padding mounted 

 on thin furring strips 

 over the dome and walls 

 solved the problem for 

 the University of Illinois 



Diagram showing 

 the floor plan of 

 the auditorium of 

 the University of 

 Illinois and the 

 concentration of 

 sounds under 

 the balcony walls 



THE auditori- 

 um of the 

 University 

 of Illinois was a 

 veritable sound- 

 ing board before 

 its acoustic faults 

 were remedied. 

 Echoes and rever- 

 berations were so 

 pronounced as to 

 distress an audi- 

 ence. Various methods of cure were con- 

 sidered — the effect of padding and paneling 

 the walls, the possible advantages of instal- 

 ling wires and sounding boards, and finally, 

 the action of the ventilating system. 

 These were all discarded in favor of padding 

 the walls with sound-absorbing hairfelt. 



Before the effective cure was found 

 several methods of tracing sound were tried. 

 One test was made by talking through a 

 megaphone toward different walls. The 

 sound was generated inside a small house 

 and its direction controlled by two mega- 

 phones, one being pointed toward a 

 listener and the other toward a wall which 

 gave echoes. No distinct echo could be 

 obtained by speaking simultaneously into 

 the two megaphones. The ticks of a 

 metronome (a clock-moved pendulum for 

 marking exact time in music) produced 

 very little additional effect, but when a 

 sharp intense metallic sound was tried, 

 echoes were obtained from unpadded walls 



One method employed for locating echoes. Sound 

 was generated for the purpose inside the little house 

 and its direction controlled by two megaphones 



Diagram showing 

 how sounds di- 

 rected toward an 

 unpadded pen- 

 dentive in the rear 

 wall were reflect- 

 ed to different 

 parts of the hall 



but only faint re- 

 sponses from pad- 

 ded walls. The 

 intense hissing 

 sound of an arc 

 light backed by a 

 reflector gave 

 more pronounced 

 results. It showed 

 that the padded 

 walls produced a 

 marked effect in 

 reducing the intensity- of the sound. 



Before it was decided to use hairfelt as an 

 absorbing material, curtains and draperies 

 were hung at critical points suggested by 

 the reverberating sounds. Four large pieces 

 of canvas were suspended in the dome. 

 From an acoustic standpoint the audi- 

 torium was in a much improved condition 

 as soon as the curtains and draperies were 

 suspended. Of course the architectural 

 features of the auditorium forbad the con- 

 tinued use of curtains. Hairfelt was finally 

 adopted. 



Accordingly, one large cur\ed wall was 

 covered with strips of one-inch hairfelt 

 thirty inches wide, placed vertically and 

 thirty inches apart so as to leave bare spaces 

 between them. Following this the dome 

 above the arches and the double curved 

 side walls and single curved rear wall above 

 the balcony were padded. The felt was 

 mounted on thin furring strips which were 

 bent to fit the curvature of the surfaces. 



