PUBLIC ADDRESS SYSTEMS 121 



It has been found that in order to transmit speech with entire 

 satisfaction for loud speaker purposes, that is, sufficiently well so 

 that the audience is not aware of the contribution of the mechanical 

 equipment, it is necessary for the system to operate with essentially 

 uniform amplification over a range of frequencies from 200 to 4000 

 cycles. While there are, in speech, frequencies slightly outside of this 

 range, the loss in naturalness and intelligibility by the system's 

 failure to reproduce them, is slight. 8 



While no such frequency range is required for intelligibility only, 

 it has been found that systems not covering substantially this fre- 

 quency range, sound unnatural. W 7 hen the lower frequencies are 

 missing, the reproduction sounds "tinny." When the higher fre- 

 quencies are missing it sounds heavy and muffled. The requirements 

 for thoroughly natural reproduction of music are probably more 

 severe, particularly in the low-frequency region, than are the similar 

 requirements for speech, but, at the present time, complete data are 

 not available to indicate the contribution of these frequencies to 

 naturalness. 



In connection with the flexibility of the system, it is interesting to 

 note that the speech input equipment has been designed to raise the 

 power delivered by the transmitter to such an extent that it is suffi- 

 cient for long distance telephone transmission or for the operation of 

 a radio transmitting set. The power amplifier is designed to receive 

 power at approximately this level and deliver it to the projector units 

 sufficiently amplified to operate them satisfactorily. 



Future Development 



In viewing the loud speaker field from the point of view of future 

 development there are two lines of attack along which work is being 

 done, and which give promise of success. These are the improve- 

 ments in frequency characteristics and increase in the range of loud- 

 ness which the system can accommodate satisfactorily. 



The improvement to be expected from a more uniform frequency 

 characteristic is mainly an increase in naturalness, especially where 

 music is being reproduced. A slight increase in intelligibility may 

 be hoped for, although this factor is of little importance, as the present 

 system is satisfactory in this respect. 



The other improvement mentioned, namely, the volume range, is 

 probably the more difficult, but is necessary before music can be re- 



8 The Nature of Speech and Its Interpretation. Harvey Fletcher, Journal of the 

 Franklin Institute, Vol. 193, No. 6, June, 1922. 



