SEVENTY-FIVE YEARS OF THE TELEPHONE 229 



This concept brought to bear on the development and design of electro- 

 acoustic devices all the wealth of electrical transmission theory and meas- 

 urement techniques, and especially the Campbell filter idea of designing a 

 system to transmit efficiently a band of frequencies. With this analytical 

 method, the means of controlling resonance could be explored quantitatively 

 and systematically. Also thereby, means could be studied of compensating 

 for limitations in the behavior of one part of the network by corrective 

 measures elsewhere. 



This electrical analog equivalent network concept not only facilitated the 

 analysis of the overall performance of electro-acoustic devices but also made 

 possible the study of the contribution of each element and of changes in 

 each characteristic of each element to that performance. This could be done, 

 mathematically or by measurement, on the simulating electrical network. 

 Such studies promoted the understanding of the functioning of such devices 

 and indicated what needed to be done to improve their performance. This 

 method of analysis made it readily possible to determine the effect of modi- 

 fications in the material properties and dimensions of the mechanical and 

 magnetic parts, and of damping and dissipation in the acoustic elements. 

 This pointed the way to meeting the response characteristics which were 

 shown to be desirable by the subjective measurements on the intelligibility 

 of reproduced sounds. 



With this technique, advances can be made intelUgently in the kinds of 

 materials*^ used for the diaphragms of instruments and for the other mag- 

 netic parts of receivers; and the designer has been put in the position not 

 solely of considering the materials that are offered to him by the metal- 

 lurgist and other material engineers, but also of giving to them the speci- 

 fications of desired properties. Incidentally, this specific tailoring of the 

 characteristics and dimensions of the material to the performance require- 

 ments of the part in which it is used is an important factor in the miniaturi- 

 zation of apparatus and in minimizing in its design the old "factor of safety" 

 (or "factor of ignorance" as it might be termed with present technology). 



Design for Performance 



With this evolution, the technology of telephone transmitters and re- 

 ceivers has made great progress since the beginning of the era of measure- 

 ment. The situation will be indicated by considering the development and 

 design of these instruments from several aspects and by noting certain 

 salient accomplishments. 



By "design for performance" is meant the process of determining the per- 

 formance characteristics to be striven for, then developing systematically 

 the means for meeting them and embodying these means in a suitable op- 

 erating design. In selecting the performance objectives, due regard must of 



