PAPERS ON PHYSICS 299 



The two auditoriums described indicate the degree of 

 development of the acoustical design of auditoriums. The 

 theory and practice have been tried in many cases with 

 success. There appears to be some range of latitude in the 

 acceptable time of reverberation and in the intensity of 

 the resulting sound so that the amount of sound absorbing 

 material recommended for a room may be varied within 

 certain limits without prejudicing seriously the successful 

 outcome. 



In addition to the acoustical design of auditoriums, an- 

 other problem in buildings presents itself — namely, the 

 sound proofing of rooms. Great annoyance and inconveni- 

 ence are suffered because of the unwelcome intrusions in a 

 room of sounds coming from other parts of the building. 

 The noise of a piano in an adjoining apartment, the hum 

 of a motor, the click of typewriters, etc., are familiar in- 

 stances of sounds that annoy people and reduce their ef- 

 ficiency in the performance of work. 



The sound proofing of rooms to reduce this defect is not 

 a simple matter, nor is it attended with the same cer- 

 tainty of success as in the acoustical design of auditoriums. 

 Sound progresses along unsuspected paths so that prac- 

 tical attempts to stop it, even if based on the suitable theory 

 and in accord with other successful insulations, are not al- 

 ways effective. 



In this connection, two kinds of sound should be con- 

 sidered. First, those generated by the voice, a violin, etc., 

 which originate in the air and proceed through the air. 

 These are reflected in large proportion when they strike a 

 continuous wall of some rigidity. Another tj'pe of sound 

 originates in the vibrations of a piano, cello, motor, etc. — 

 instruments which make an intimate contact with the 

 building structure. These vibrations travel with ease 

 through the continuity of structure and are converted into 

 sound in air, even at distant points in the building, when a 

 wall or some structural member responds in a resonant way 

 to the vibrations. The insulation for this latter type of 

 vibration lies in the interposition of some medium varying 

 in elasticity or density from the medium in which the vi- 

 brations are traveling. Thus, an air space inserted in 



