270 BELL SYSTEM TECHNICAL JOURNAL 



bilities. A continuing close contact with field experience is employed 

 to modify the designs towards securing the proper balance to meet 

 these factors. 



In order to indicate more clearly the present trends in design, I 

 shall refer briefly to the earlier art. In the early development of 

 transmitters and receivers, the matter of getting efficiency was of 

 primary importance since this could be evaluated directly in terms of 

 the amount of copper required in the connecting line. The early 

 transmitters, which were of the same construction as the receiver, 

 depended on the generator action of a diaphragm and coil and de- 

 veloped sufficient power to be heard over only a few miles of heavy- 

 gauge wire. Some amplification was necessary before telephone 

 communication could begin to assume the proportions of a widespread 

 service. This amplification was obtained at a reasonable cost in the 

 carbon contact transmitter. Transmitters of this type are in the 

 order of 60 db more efficient as transducers of acoustic to electric 

 energy than the earlier type. 



Both the transmitter and the receiver operate by means of dia- 

 phragms which have natural periods of vibration. These resonances 

 and the resonances of the air spaces on each side of the diaphragm 

 were used to obtain as efficient a transfer of energy as possible. In 

 the early design, a great deal of attention was also given to locating 

 these resonances at the portion of the frequency range where they 

 would tend to increase the intelligibility of the reproduced sound. 

 As a result, both instruments were made very efficient in the region 

 of 1000 cycles, which lies within the range where the ear and the 

 sensation of loudness are most sensitive. 



It was recognized that these resonances caused undesirable distor- 

 tion, but under the conditions the resulting increase in efficiency more 

 than compensated for this disadvantage. As time went on, the 

 diaphragm resonances came to be looked upon as practically inherent 

 in commercial transmitters and receivers, because no way was known 

 of eliminating them without making a very material sacrifice in the 

 efficiency of the instrument. 



About twenty years ago, the development of the vacuum tube 

 amplifier and the high quality condenser transmitter made it possible 

 to demonstrate and measure quantitatively the advantages of reducing 

 distortion. These high-quality instruments, the improvement in 

 measuring technique and the development of improved methods of 

 designing vibratory systems offered the promise of providing instru- 

 ments in which the resonance effect could be reduced without unduly 

 affecting efficiency. 



