114 BELL SYSTEM TECHNICAL JOURNAL 



has been directed toward, obtaining these two results under the various 

 conditions which surround the operation of these systems. 



The faithful and natural reproduction of sound depends upon many 

 factors, of which the following are some of the more important: The 

 acoustics of the space in which the sound originates, the characteristic 

 of the loud speaking system itself and the acoustics of the space in 

 which the sound is reproduced. Where the sound is picked up and 

 reproduced in the same space, as is the case when the speaker is using 

 one of these systems to address a large audience locally, there is a 

 reaction between the horns and the transmitter or pick-up mechan- 

 ism which is controlled by the acoustic conditions under which the 

 system is operated. 



Acoustics of the Space 



In connection with the acoustics of the space in which the sound 

 originates, or in which it is reproduced, four factors stand out. These 

 concern the effects of reverberation, of echo, of resonance, and of 

 diffraction. In the specific cases where the sound is reproduced in 

 the same space or room in which it originates, another effect is en- 

 countered, which has generally been termed "singing," and is evi- 

 denced if sufficiently great by the emission of a continuous note from 

 the equipment. 



Reverberation is caused by reflection and is evidenced by the 

 persistence of the sound after its source has ceased emitting. When 

 the reverberation in the space in which the initial sound is being 

 picked up is sufficient to cause one sound to hang over and become 

 mingled with succeeding sounds, in other words, so that the sound 

 from one syllable interferes with that of the succeeding syllable, it is 

 practically impossible to improve the acoustic conditions solely by 

 the use of the public address system. In such a case, the first pro- 

 cedure is to place material which absorbs sound in the space. The 

 purpose of this absorbing material is to lower the time required for the 

 sound to die away after the source ceases to emit. The amount which 

 any given material lowers the time of reverberation depends not only 

 upon the amount of material introduced, but also upon its disposition 

 within the space. 



The term "echo" applies to a similar phenomenon, but is gener- 

 ally used where there is sufficient time lag between the reflected sound 

 and that originally emitted, so that two distinct impressions reach 

 the ear. 2 



Collected Papers on Acoustics, Wallace Clement Sabine, Harvard University 

 Prets, 1922. 



