STANDARD VOLUME INDICATOR AND REFERENCE LEVEL 129 



500 ohms. The value chosen was preferred by a majority of a large 

 number of people who were consulted and in addition was found to be 

 the only value to which all could agree. Some of the reasons for 

 choosing 1 milliwatt (10~' watt) were: (1) It is a simple round number, 

 easy to remember; (2) 10~^ is a preferred number; ^ (3) 1 milliwatt is 

 a much used value for testing power for transmission measurements, 

 especially in the telephone plant, so that choice of this value therefore 

 permits the volume indicators to be used directly for transmission 

 measurements. 



The choice of the standard impedance of 600 ohms was influenced by 

 the fact that, considering all of the plants involved, there is more equip- 

 ment designed to this impedance than to 500 ohms. 



The question may very well be raised why the reference volume has 

 been related to a calibrating power rather than to a calibrating voltage, 

 inasmuch as a volume indicator is generally a high impedance, voltage 

 responsive device. A reference level could conceivably be established 

 based on voltage and the unit of measurement might be termed 

 "volume-volts." However, volume measurements are a part of the 

 general field of transmission measurements, and the same reasons 

 apply here for basing them on power considerations as in the case of 

 ordinary transmission measurements using sine-waves. If the funda- 

 mental concept were voltage, apparent gains or losses would appear 

 wherever impedance transforming devices, such as transformers, occur 

 in a circuit. This difficulty is avoided by adopting the power concept, 

 making suitable corrections in the readings when the impedance is 

 other than 600 ohms. 



Having chosen the zero point to which the new volume readings 

 would be referred, the next question to be decided was the terminology 

 to be employed in describing the measurements. As has been pointed 

 out, the past custom of describing the volume measurements as so 

 many decibels above or below reference level has been ambiguous 

 because of differences in instruments and standards of calibration. 

 It was thought, therefore, that there would be less confusion in adopt- 

 ing the new standards if a new name were coined for expressing the 

 measurements. The term selected is "vu," the number of vu being 

 numerically the same as the number of db above or below the new 

 reference volume level. It is hoped that in the future this new term 

 will be restricted to its intended use so that, whenever a volume level reading 

 is encountered expressed as so many vu, it will be understood that the 

 reading was made with an instrument having the characteristics of the new 

 volume indicator and is expressed with respect to the new reference level. 



« A. Van Dyck, " Preferred Numbers," Proc. I. R. E., Vol. 24, pp. 159-179 (1936). 



