554 BELL SYSTEM TECHNICAL JOURNAL 



by the design and introduction in the master reference system of a 

 proper network. The advantage of such a procedure will be evident 

 to those familiar with the difficulties of making voice volume balances 

 between instruments of widely dififerent frequency characteristics. 

 The volume efficiency of any instrument can also be reproduced by 

 adjustment of the controls in the amplifiers of the reference trans- 

 mitter or receiver as the case may be. The results of voice volume 

 balances made with the distorted reference system can be referred to 

 the undistorted master reference system by applying a correction for 

 the volume effect of the distortion network in the master reference 

 system, this effect being determined independently as a rating of the 

 distortion network. 



Experience has shown that amplifiers properly designed and con- 

 structed remain essentially constant with time. Such changes in 

 gain as may occur because of replacement of vacuum tubes or other 

 apparatus may be readily compensated for by adjustments of po- 

 tentiometers provided for this purpose. Condenser transmitters are 

 affected to some extent by variations in temperature and barometric 

 pressure. The magnitude of variation at any frequency between 50 

 and 10,000 cycles per second from these causes may be as much as 

 2.5 db, although under normal operating conditions such as those 

 experienced in buildings in this climate, the variation is usually not 

 more than 1 db. Since changes of this character are gradual in nature, 

 their magnitude can be readily determined by a thermophone cali- 

 bration. Corrections for any change in sensitivity of the instrument 

 may then be made by adjustment of the controls in the amplifier 

 associated with the condenser transmitter to maintain the proper gain 

 in the reference transmitter. Similarly, any variations in the response 

 of the moving coil receiver may be compensated for by adjustment of 

 the gain controls in the amplifier associated with this receiver. The 

 magnitude of the variations, at any frequency between 50 and 10,000 

 cycles per second, in the moving coil receiver may be as much as 

 3 db although usually variations of less than 1.5 db are observed. 

 The calibration of the condenser transmitters, and indirectly of the 

 moving coil receivers, is dependent upon the gold leaf thermophone, 

 whose pressure characteristic is computed from physical measurements. 

 Results obtained with thermophones can be held within about 0.5 db 

 of the average obtained by using a group. 



