46 ILLINOIS ACADEMY OF SCIENCE 



to ascertain what the acoustical defects were and then to in- 

 vestigate the methods of cure and apply them to correct the 

 trouble." 



The usual acoustical faults in auditoriums, as pointed out 

 by Professor Sabine 3 in his classical experiments on this sub- 

 ject, are a reverberation, or undue prolongation of sound, and 

 echoes ; both of these faults being due to reflection of sound 

 from the walls. Sabine has shown definitely how the rever- 

 beration can be corrected by installing sound-absorbing materi- 

 als. Other defects, such as interference and resonance, may 

 also be present, but they are usually of small consequence com- 

 pared with the first two mentioned. 



The reverberation in the Auditorium at the University of 

 Illinois could therefore have been cured by installing hairfelt 

 on the walls. Experimental tests on the reverberation were 

 conducted by Sabine's method and calculations made to deter- 

 mine the amount of absorbing material needed to reduce the 

 reverberation to a satisfactory point. The greatest annoy- 

 ance, how ever, appeared to be due to echoes, so that the main 

 purpose of the investigation was to find the echoes and elimi- 

 nate them. 



If an observer stood on the stage and clapped his hands a 

 veritable chaos of sound resulted and echoes were heard from 

 every direction.This action was too complex to lead to a defi- 

 nite analysis of the trouble, so a simpler method was adopted 

 by which a small beam of sound was to be sent successively in 

 different directions and its paths traced after reflection. A 

 difficulty then arose to find a suitable arrangement of appa- 

 ratus to carry out the method. A ticking watch backed by a 

 reflector gave definite data, as did also a metronome enclosed 

 in a box so that its sound could escape only through a directed 

 horn. The results were not entirely conclusive. A satisfac- 

 tory method was finally found by using an arc light at the focus 

 of a parabolic reflector. 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 walls that 



^"Echoes in an Auditorium," Physical Review, Vol. 32, p. 231 1911. ''Air Cur- 

 rents and the Acoustics of Auditoriums," Engineering Record, Vol. 67, p. 265, 1913. 

 "Acoustical Effect of Fireproofed Cotton-Flannel Sound Absorbers,' Engineering 

 News, Vol. 71, p. 261, Jan. 29, 1914. 



'See articles oa "Architectural Acoustics," American Architect, 1900. 



