140 



BELL SYSTEM TECHNICAL JOURNAL 



We will now pass to a consideration of the effect of distortion 

 upon the articulation of the sounds. 



Description of Electrical Filters Used to 

 Produce Distortion 



In order to investigate distortion we would like to be able to take 

 the train of speech waves going from the mouth to the ear and oper- 

 ate upon it in various ways such as eliminating frequencies in certain 

 regions without marring or disturbing other frequencies. For ex- 



Loiv Pass Filter 

 TTTinnp- 



. '"nm'a'rr 1 — 



High Pass Filter 



q|:>-r^!p- 



F"iG. 6. 



ample, if all frequencies above 1000 were eliminated, it would be 

 possible to determine what intelligibility is carried by this range of 

 frequencies. 



Fortunately one of the recent electrical inventions is admirably 

 adapted for this purpose, namely, the electrical wave filter invented 

 by Dr. G. A. Campbell. This device was used extensively in this 

 investigation. 



The schematic circuit diagrams of the two types of filters which 

 were used are given in Fig. 6. 



This arrangement of coils and condensers produces an electrical 

 conductor with the unusual properties that it transmits without 

 appreciable diminution in amplitude any frequency between certain 

 limits and reduces the amplitude of all frequencies outside these 

 limits to less than 1/1000 of their original value. By varying the 

 numerical values of the inductances and capacities this transmitted 

 range can be placed at any desired position. In the arrangement 

 which was used in the investigation these coils and condensers were 



