The Bell System Technical Journal 



January, 1929 



Decibel — The Name for the Transmission Unit 



By W. H. MARTIN 



IN 1923 the "mile of standard cable" was replaced in the Bell System 

 by a new unit for expressing telephone transmission efficiencies and 

 levels. At that time the generic term "transmission unit" was taken 

 to designate this new unit, since it was considered desirable to defer 

 the adoption of a more distinctive name until this unit had been given 

 further consideration by others who would have use for a unit of this 

 type. This new unit is defined by the statement that two amounts of 

 power differ by one transmission unit when they are in the ratio of 

 10-\ and any two amounts of power differ by N transmission units 

 when they are in the ratio of 10^^- ^^ In accordance with this, the 

 number of transmission units corresponding to the ratio of any two 

 powers is ten times the common logarithm of that ratio. 



For a unit of this kind, it is evidently desirable to have universal 

 use. Accordingly, the Bell System, prior to its adoption of the 

 transmission unit, discussed this matter with various foreign telephone 

 administrations, and suggested their consideration of the use of the 

 proposed "transmission unit." A number of these administrations 

 expressed a favorable attitude towards this unit. 



In 1924 there was organized the International Advisory Committee 

 on Long Distance Telephony in Europe. The purpose of this commit- 

 tee, which is composed of representatives of the various telephone 

 administrations of Europe, is to recommend standards and practices 

 for the development of telephone service between the European coun- 

 tries. One of the early considerations of this committee was this 

 proposal of the universal standardization of a unit for telephone trans- 

 mission work. This brought forth a difference of v'\ev7, since some of 

 the countries represented on this committee w^ished to continue their 

 use of a unit based on naperian or natural logarithms, for which the 

 basic power ratio is e~. The characteristics of the unit based on decimal 

 logarithms and that based on natural logarithms and their relative 

 merits were discussed in a number of papers which were published at 

 that time * and so need not be rehearsed here. 



At the request of the International Advisory Committee, representa- 

 tives of the Bell System attended some of their meetings at which 

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