50 BELL SYSTEM TECHNICAL JOfRX.^L 



of the telephone system as shown in Fig. 4. Essentially this consists 

 of the transmitter, the line and the receiver. The oldest method 

 of measurement is to compare one transmitter against a standard 

 in the following way. An observer calls first in the standard and 

 tlien in the test transmitter, while another observer at the receiving 

 eiui judges the faithfulness of reproduction. The pressure wave 

 striking the transmitter diaphragm varies with the observer and also 

 with the degree of mechanical coupling hctwci-n the sound source 



Fig. 4 





\ 



and the diaphragm of the instrument. The judgment of the observer 

 at the receiving end in influenced by physiological and psychological 

 causes. Obviously it is desirable that such a method be supplanted 

 by a machine test which will eliminate the \ariabilitics in the sound 

 source and in the human ear. I'p to the present time the nature of 

 speech and the characteristics of the human ear are not known suffi- 

 ciently well to establish either an ideal sound source or an electrical 

 meter to replace the human voice and car respectively. The best that 

 can be done is to approximate this condition. Even though the 

 meter readings may be the same, the simultaneous observations 

 made with the ear in general will be different. A calibration of the 

 machine must, therefore, depend upon a study of the degree of correla- 

 tion between the average measure gi\en 1)\- tin- machine and that 

 given by the older method of test. 



Thus, we see how special problems arise in the fields of both physics 

 and engineering wherein it is im[)ossible to control the variations. 

 In what way, if any, are these jiroblems relateil, or is it necessary to 

 attack each one in a different manner? We shall see that all of 

 these problems arc in a wa\- fuii<lamentalK' the same and that the 

 same method of soluiinn lan be applied to all of them. This is true 

 because it is necessary to determine in every instance the law of 

 distribution of the variable about some mean value. 



