PUBLIC ADDRESS SYSTEMS 117 



sounds acts to lower the intelligibility. It probably causes more 

 trouble in speech transmission than the fact that the sound con- 

 tinues after the source ceases. 



Asymmetric distortion affects one half of the wave differently from 

 its other half. This causes the introduction of frequencies which, in 

 some cases, produce very serious disturbances in the transmission of 

 music and speech. The most noticeable troubles are from the forma- 

 tion of sum and difference tones. 3 Such tones are likely to give rise 

 to dissonances with the other sounds occurring in the music. In the 

 case of speech asymmetric distortion manifests itself by a lower in- 

 telligibility. 



The effect of foreign noises sometimes encountered is twofold. 

 First, they influence the ability of the listener to hear the character- 

 istics of the speech sounds and hence tend to lower the intelligibility. 

 Secondly, the constant attempt of the hearer to sort out the speech 

 sounds or music through the disturbing noises tires him appreciably. 

 In order that this strain shall be inappreciable, it is desirable that the 

 sound delivered by the system shall be at a power level approximately 

 10,000 times that of the noise. 



The second general requirement which is placed on a successful 

 system is that it shall deliver its faithful reproduction loud enough 

 for all the audience to hear it comfortably and enough above noise 

 for good intelligibility. In this connection there have arisen one or 

 two interesting points bearing on the psychology of hearing. One 

 of the most striking of these is concerned with the coordination be- 

 tween hearing and seeing. Although the projectors are usually 

 mounted twenty or more feet above the speaker's head, and in some 

 exceptional cases, slightly to one side of him, the majority of the 

 audience is conscious of only one source of sound, and that appears 

 to be the speaker himself. 



This phenomenon is so marked that in several cases the question 

 has been rasied in the minds of the listeners as to whether the system 

 was functioning. They could only be convinced that it was by having 

 it shut down for a few seconds when their inability to hear made them 

 realize how successfully the system could operate. 



Another of these psychological phenomena deals with the apparent 

 distortion of the voice when its intensity at the ears of the listener 

 is too great or too small. If the speaker is talking in a normal con- 

 versational tone, his voice contains a larger percentage of low fre- 

 quencies than is the case when he is raising his voice to a considerable 



3 Origin of Combination Tones in Microphone-Telephone Circuits. E. Waetz- 

 mann, Annalen der Physik, Vol. 42, 1913. 



