STANDARD VOLUME INDICATOR AND REFERENCE LEVEL 95 



If the student later enters the field of communication engineering, 

 he immediately encounters waves which are both very complex and 

 non-periodic. Examples of typical speech and music waves are shown 

 in the oscillograms of Fig. 1. When an attempt is made to measure 

 such waves in terms of average, r-m-s or peak values, it is found that 

 the results can no longer be expressed in simple numerical terms, as 

 these quantities are not constant but variable with time and, moreover, 

 are apparently affected by the characteristics of the measuring instru- 

 ment and the technique of measurement. However, the communica- 

 tions engineer is vitally concerned with the magnitude of waves of the 

 sort illustrated, as he must design and operate systems in which they 

 are amplified by vacuum tubes, transmitted over wire circuits, modu- 

 lated on carriers, and otherwise handled as required by the various 

 communication services. He needs a pra.ctical method of measuring 

 and expressing these magnitudes in simple numerical fashion. 



This need may be better appreciated by considering the communica- 

 tion systems employed for broadcasting. These are very complicated 

 networks spread over large geographical areas. A typical network 

 may include 15,000 miles of wire line and hundreds of amplifiers situ- 

 ated along the line and in the 50 to 100 connected broadcasting stations. 

 Every 15 minutes during the day the component parts of such a system 

 may be shifted and connected in different combinations in order to 

 provide for new points of origin of the programs, and for the addition 

 of new broadcasting stations and the removal of others from the 

 network. In whatever combination the parts of the system are put 

 together, it is necessary that the magnitude of the transmitted program 

 waves, at all times and at all parts of the system, remain within the 

 limits which the system can handle without impairment from over- 

 loading or noise. To accomplish this, some convenient method of 

 measuring the amplitude of program waves is needed. 



These considerations led to the conception of a fourth value, known 

 as "volume," whereby the magnitude of waves encountered in elec- 

 trical communications, such as telephone speech or program waves, 

 may be readily expressed. This value is a purely empirical thing, 

 evolved to meet a practical need. It is not definable by means of a 

 precise mathematical formula in terms of any of the familiar electrical 

 units of power, voltage or current. Volume is simply the reading of 

 an instrument known as a volume indicator, which has specified dy- 

 namic and other characteristics and which is calibrated and read in a 

 prescribed manner. Because of the rapidly changing character of the 

 program wave, the dynamic characteristics of the instrument are fully 

 as important as the value"of sine wave power used for calibration. The 



