128 BELL SYSTEM TECHNICAL JOURNAL 



program which causes a standard volume indicator, when calibrated and 

 used in the accepted way, to read vu. 



It is especially cautioned that reference volume as applied to program 

 material should not be confused with the single frequency power used 

 to calibrate the zero volume setting of the volume indicator. If a 

 volume indicator is calibrated so as to read zero on a sine-wave power 

 of, say, one milliwatt in a stated impedance, a speech or program wave 

 in the same impedance whose intensity is such as to give a reading of 

 zero will have instantaneous peaks of power which are several times 

 one milliwatt and an average power which is only a small fraction of 

 a milliwatt. It is therefore erroneous to say that reference volume in 

 this case is one milliwatt. Only in the case of sine-wave measurements 

 does a reading of vu correspond to one milliwatt. 



It should be emphasized that, although it is convenient to measure 

 the performance of amplifiers and systems by means of single frequen- 

 cies, there is no exact universal relationship between the single fre- 

 quency load carrying capacity indicated by such measurements and 

 the load carrying capacity for a speech and program waves expressed 

 in terms of volume level. This relationship depends upon a number 

 of factors such as the rapidity of cutoff at the overload point, the fre- 

 quency band width being transmitted, the quality of service to be 

 rendered, etc. 



It has already been brought out that in the past there have been a 

 multiplicity of reference volumes differing from each other not only 

 because of the various single frequency calibrations which have been 

 employed, but also because of the dissimilar dynamic characteristics of 

 the different instruments used to measure volume levels. It is also 

 apparent that the introduction of a new volume indicator whose 

 characteristics are not identical with any of its predecessors inherently 

 means the introduction of a new reference volume no matter how it is 

 calibrated. Therefore, there did not seem to be any compelling reason 

 to make the calibration of the new instrument agree with any of the 

 calibrations used in the past. Moreover, to many there seemed to be 

 some advantage in setting the new reference level at a sufficiently 

 different order of magnitude from those which had been in most com- 

 mon use, so that there will be little chance of confusing the new stand- 

 ards with any of those that went before. 



After much thought and discussion, it was agreed that the new 

 reference volume should correspond to the reading of the new volume 

 indicator when calibrated with one milliwatt in 600 ohms across which 

 the volume indicator is bridged. Other calibrating values considered 

 were 10"^*^ watt, 6 milliwatts and 10 milliwatts, in 600 ohms or in 



