SCIENTIFIC RESEARCH APPLIED TO THE TELEPHONE 267 



we can analyze this apparatus as electromechanical structures. 

 Further, in the design of telephone transmission apparatus we are 

 concerned with a power transmission system in which the design 

 engineer has no control over the power source, the human voice, nor 

 over the receiving agency, the human ear. His control is limited to 

 the conveyance of power from speaker to listener. In the Laboratories, 

 therefore, we have recognized that it is necessary, at least without 

 present knowledge, to supplement physical measurements by measure- 

 ments involving speech sounds and the human ear. Some years ago, 

 these tests consisted of comparisons between different instruments or 

 transmission elements made by the process of talking first over a 

 circuit containing, for example, one instrument, and then over the 

 same circuit containing a different instrument. Dependence was 

 placed wholly upon the listener's skill to detect differences in volume, 

 quality and intelligibility. It was recognized that this method of 

 testing left much to be desired. Owing to the limitations of the 

 human ear, small volume differences could not be detected, but even 

 more important, this simple test furnished no very accurate measure 

 of speech distortion affecting intelligibility, and obviously no definite 

 information as to the relation between volume, various types of 

 distortion, and overall effectiveness. 



Dr. George A. Campbell, in 1910, proposed a method of testing 

 which has been highly developed in our Laboratories. This method, 

 termed "articulation testing," measures the relation between the 

 reproduced and impressed sounds from the standpoint of effects on 

 intelligibility of different kinds of distortion. This method has been 

 described in a number of publications. Briefly, in this method, lists 

 of syllables chosen at random and usually meaningless monosyllables 

 are called over the circuits to be rated, and the percentage of syllables 

 correctly understood gives a measure of the circuit performance. 

 Further, the method has been extended to give quantitative measures 

 in terms of the recognizability of reproduced speech sounds, of the 

 effects of loudness of these sounds, and of the noise which may be 

 present. 



While various physical tests and the articulation test method are 

 exceedingly useful tools in the hands of the research and development 

 engineer, they do not give a direct measure of the transmission service 

 performance of a circuit in terms of the ability of the user to carry 

 on a conversation under actual commercial conditions. This ability 

 of the user to carry on what may be termed a successful telephone 

 conversation depends not only upon the performance of the telephone 

 instruments and circuits but also, to a substantial extent, upon the 



