STANDARD VOLUME INDICATOR AND REFERENCE LEVEL 97 



readings of volume have been customarily expressed in terms of decibels 

 with respect to some volume level chosen as the "reference" level. 



In the past, because of a lack of complete understanding of the 

 matter, there has been little uniformity in the design and use of volume 

 indicators, although attempts have been made by some organizations 

 toward standardization. The devices used were of the r-m-s and peak- 

 reading types having slow, medium or high pointer speeds; half- or 

 full-wave rectifiers; critically to lightly damped movements and refer- 

 ence levels based on calibrations with 10~^, 1, 6, 10, 12^ or 50 milliwatts 

 in 500 or 600 ohms. This great array of variables led to considerable 

 confusion and lack of understanding, especially when an attempt was 

 made to correlate the measurements and results of one group with 

 those of another. 



To remedy this situation, the Bell Telephone Laboratories, the 

 Columbia Broadcasting System and the National Broadcasting Com- 

 pany entered upon a joint development effort during January 1938, 

 with the object of pooling their knowledge and problems, of pursuing 

 a coordinated development program, and of arriving at a uniform 

 practice of measuring volume levels. The outcome of this work is a 

 new volume indicator, a new reference volume level, and new ter- 

 minology for expressing measurements of volume level. The results 

 of this development work have been discussed with, and approved by, 

 more than twenty-four other organizations, and were presented at an 

 open round table conference at the Annual Convention of the Institute 

 of Radio Engineers on June 17, 1938. During May 1939, it was 

 adopted as standard practice by the above two broadcasting companies 

 and the Bell System, and it is hoped that they will be joined by others. 

 It is the purpose of this paper to describe the new standards and the 

 considerations which led to their adoption. 



Early History of Volume Indicators 



As a background for understanding the present development, it 

 will be helpful to review briefly the early history of volume indicators. 

 The particular occasion for the development of the first volume indi- 

 cator was the setting up of the public address system which enabled 

 the ceremonies attendant upon the burial of the Unknown Soldier on 

 Armistice Day 1921, to be heard by large audiences at Arlington, New 

 York and San Francisco. ^ It was noted in some of the preliminary 

 tests that distortion due to overloading of an amplifier was more ob- 

 jectionable when heard in a loud speaker than when heard in an ordi- 



1 "Use of Public Address System with Telephone Lines," W. H. Martin and A. B. 

 Clark, Transactions A. I. E. E., February' 1923. 



