542 BELL SYSTEM TECHNICAL JOURNAL 



can be determined by theoretical considerations.^ The performance 

 of the condenser transmitter is determined by making its diaphragm 

 a wall of a simple closed chamber in which the thermophone is placed. 

 By this means a known pressure wave of any frequency over the 

 range desired can be impressed upon the diaphragm of the transmitter. 

 The voltage output of the transmitter for a specified circuit condition 

 is then measured. From this measurement the ratio of the voltage 

 output to the acoustic pressure on the diaphragm is established for 

 that instrument and circuit condition. With the performance of the 

 transmitter thus established, the performance of the receiver element 

 of the reference system is measured by acoustically coupling the receiver 

 to the condenser transmitter, so that the receiver actuates the trans- 

 mitter, and then determining the relation of the pressure generated 

 by the receiver in the coupler to the voltage input to the receiver. 

 The performance of the line element is determined by well-known 

 means. The performance of the whole system can then be expressed 

 in terms of the pressure produced by the receiver with respect to the 

 pressure on the diaphragm of the transmitter. 



The performance of this system is practically free from distortion 

 for the energies which it is required to handle, and probably materially 

 excels in this respect that of any previous system. With this system, 

 volume relations between output and input sounds can be varied over 

 a wide range with practically no accompanying distortion. In com- 

 paring such a system, however, with commercial systems, it is ad- 

 vantageous also to be able to control distortion. This is particularly 

 the case when using the instruments of the master system for rating 

 the volume efficiency of commercial types of transmitters and receivers. 

 To facilitate this, arrangements are made for the introduction into 

 the amplifiers associated with the transmitter and receiver, of net- 

 works which may be designed to give a variation of efficiency with 

 frequency which corresponds to that obtained with commercial 

 apparatus. These networks and their distortion effect can, of course, 

 be definitely specified. The line element of this master system can 

 be replaced by a line or network giving any type of distortion desired. 

 Also, known amounts of extraneous currents to produce noise can be 

 introduced into this circuit without otherwise appreciably affecting its 

 performance. This is accomplished by connecting a relatively high 

 impedance source of voltage of the desired wave shape across a circuit 

 element of relatively low resistance. 



This system provides a performance which is definitely known, and 



^ The theory of the thermophone as a precision source of sound is outlined in 

 papers by H. D. Arnold and I. B. Crandall, Physical Review, July, 1917, and by E. 

 C. Wente in the Physical Revietv, April, 1922. 



