Page:Popular Science Monthly Volume 90.djvu/257

 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.

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