540 BELL SYSTEM TECHNICAL JOURNAL 



of perfection, and with which provision may be made for modifying 

 the speech in definite and reproducible ways, afifords a convenient 

 means for studying the capabiHties of telephone circuits of different 

 physical performances. These investigations, however, are outside the 

 purpose of this paper, which is to describe the new reference system 

 and its application in making volume ratings. 



General Requirements 



The outstanding conception of the new reference system is that its 

 performance should be suitable to serve as a reference base line for 

 indicating the performance of all telephone circuits and that the 

 transmitter and receiver elements of the new system should provide 

 similar base lines for the performance of electroacoustic converters. 

 To meet these needs properly the performance of the system and its 

 parts should be capable of being measured and definitely specified in 

 terms of physical quantities. In this connection, there arises a matter 

 which has been the subject of much discussion, namely, as to whether 

 or not the specified performances of the reference transmitter and 

 receiver should be those which are realized when used as telephone in- 

 struments. In regard to the transmitter, the difficulty comes in speci- 

 fying the input when it is placed in front of the mouth of the talker. 

 This is due to the non-uniformity of the sound field from the speaker's 

 mouth, the nature of the waves of speech sounds and the reflections of 

 these waves from the transmitter. In regard to the receiver, the diffi- 

 culty is due to determining the output when the receiver is held to the 

 ear. To obviate these, it has been decided to specify the performance 

 of the transmitter in terms of the electrical output for a given pres- 

 sure on the diaphragm of the transmitter and of the receiver in terms 

 of pressure set up in a simple closed chamber for a given electrical 

 input to the receiver. These conditions are definite and reproducible. 



The most important requirement, then, for the reference system 

 is that the physical performance of the system and of its component 

 parts should be capable of being measured and definitely specified in 

 terms of physical quantities. If this requirement is realized, a system 

 providing the specified performance can be set up wherever desired. 

 This has been the main criterion in the design of the master reference 

 system for telephone transmission which is described here. 



The second main requirement is that specifiable and predeterminable 

 changes can be made with respect to the performance which is selected 

 as the reference. These changes must be capable of varying the 

 relation between the loudness of the reproduced sounds with respect 

 to the initial sounds, the distortion of the wave shape of these repro- 



