120 BELL SYSTEM TECHNICAL JOURNAL 



The procedure was based on determining the exponent "^" in the 

 equation 



i = ke^, 



which is equivalent to the actual performance of the instrument for 

 normal deflections. (In the equation "i" is the instantaneous current 

 in the instrument coil and "e" is the instantaneous potential applied 

 to the volume indicator.) Two methods were employed. One con- 

 sisted of determining the ratio of the magnitudes of the sine-wave 

 a.-c. and the d.-c. potential which when applied to the volume indicator 

 give the same deflection. The second method consisted of determining 

 the ratio of the single frequency potential to the potential of each of 

 two equal amplitude, non-harmonically related frequencies which 

 when simultaneously applied give the same deflection. 



Without going into the mathematics involved, several of the new 

 volume indicators were found to have average exponents of about 1.2, 

 so that they had characteristics that were between a linear (^ = 1) and 

 a square law or "r-m-s" {p = 2) characteristic. Applying the second 

 method to a Western Electric IG Volume Indicator, which is considered 

 to be an "r-m-s" instrument, the exponent was found to be 1.89. 



Instrument Scale 



Among the more important features to be considered in the develop- 

 ment of a volume indicator is the design of its scale. In broadcast 

 studios, volume indicators are under observation almost continuously 

 by the control operators, and the ease and accuracy of reading, and 

 the degree of eye strain are of major importance. 



Prior to the adoption of the new standard volume indicator there 

 was a wide variety of volume-level indicator scales in use by the 

 electrical communications industry. This, coupled with the use of a 

 number of different kinds of instruments, reference levels, etc., resulted 

 in considerable confusion when volume measurements were involved. 



Volume level indicators, as already explained, are used (a) as an aid 

 in compressing the wide dynamic range of an original performance to 

 that of the associated transmission medium and (Jb) for locating the 

 upper part of the dynamic range just within the overload point of 

 an equipment during its normal operation. For the first of these uses, 

 a scale having a wide decibel range is preferable. For the latter pur- 

 pose, a scale length of 10 db is usually adequate. Since a given in- 

 strument may be used for both applications, neither too large nor too 

 small a range is desirable in a volume level indicator for the above 

 purposes. A usable scale length covering 20 db appears to be a satis- 

 factory compromise. 



