STANDARD VOLUME INDICATOR AND REFERENCE LEVEL 117 



this, the opinions of technicians, accustomed to reading volume indi- 

 cators as a part of their regularly assigned duties, were sought, as well 

 as those of the engineers. The instruments studied included a con- 

 siderable range of speeds of response and of damping. From this work, 

 the following conclusions were reached : 



a. For ease of reading and minimum of eye fatigue, the movement 

 should not be too fast. As a result of observations under service con- 

 ditions and other tests the requirement was adopted that the sudden 

 application of a 1000-cycle sine-wave of such amplitude as to give a 

 steady deflection at the scale point where the instrument is to be read, 

 shall cause the pointer to read 99 per cent of the final deflection in 0.3 

 second. 



b. The movement shall be slightly less than critically damped, so 

 that the pointer will overswing not less than 1 per cent nor more than 

 1.5 per cent when the above sine-wave is suddenly applied. 



This last point deserves further discussion. It was noted that on 

 speech or program waves, instruments which were critically damped 

 or slightly overdamped had a more "jittery" action than instruments 

 slightly underdamped, and the strain of reading them was greater. 

 The reason for this will be understood by reference to the theoretical 

 curves shown in Fig. 10. These curves represent, for three different 

 degrees of damping, the deflection versus time following the sudden 

 application of a steady sine-wave. Curve A is for a movement under- 

 damped by the amount specified above. Curve B is for a critically 

 damped movement, while curve C is for a movement which is over- 

 damped by the same factor that A is underdamped. It is assumed 

 that the periods of the three movements are so adjusted that all reach 

 a deflection of 99 per cent in the same time and that the sensitivities of 

 each are the same. 



It will be noted that the velocity of the pointer in curve A is more 

 nearly uniform than in the other curves, and that the maximum veloc- 

 ity in A is only about half that in C. Because of the lower and more 

 uniform velocity, there will be much less eye strain in watching pointer 

 A as it dances about in response to program waves than either of the 

 others. Moreover, the same curves inverted will equally well repre- 

 sent the motion of the pointers when the applied wave is suddenly 

 stopped. It is evident, by inspection of the region shown near zero, 

 that pointers B and C will start downward very rapidly whereas 

 pointer A will pause for a moment and then start downward more 

 slowly. This is of importance since it is the maximum excursions of 

 the pointer which must be observed in reading volume levels. The 



