118 BELL SYSTEM TECHNICAL JOURNAL 



volume. If the loud speaker so amplifies this voice that it reaches 

 the audience with such volume that their instinct tells them that the 

 speaker should be shouting, the system appears to make his voice 

 sound quite heavy and somewhat unnatural. It has been found 

 necessary, therefore, to so regulate the amount of amplification that 

 the people at the furthest portion of the hall can hear comfortably 

 and the volume of sound shall not be permitted to become any louder 

 than necessary to meet this condition. On the other hand, if the 

 volume is insufficiently loud, certain of the weaker speech sounds are 

 entirely lost, and it becomes difficult to understand. 4 



Solution of the Problem 



With these considerations in mind it may be interesting to take 

 a brief survey of the whole problem and the method by which the 

 solution was reached. Two general methods of attack were con- 

 sidered. The first was to attempt to make each unit of the system 

 faithfully reproduce its input, while the second was to make any dis- 

 tortions of one part of the system, cancel those of another portion, 

 so that the complete system would operate satisfactorily. In either 

 case, it was desirable to keep each unit free from asymmetric dis- 

 tortion, as this type of distortion cannot be easily compensated for. 



While it would probably have been simpler to follow the second 

 line of attack, the greatly increased flexibility of a system in which 

 each part is correct in itself was of sufficient value to cause the attempt 

 to be made that way. When it is realized that these systems, to be 

 commercially successful, must be capable of operating for various 

 sized audiences, ranging possibly from one thousand to several hun- 

 dred thousand, that they must be used in connection with long dis- 

 tance telephone lines, as well as with either radio broadcasting or 

 receiving stations, the desire for flexibility can be understood. 5 



As a result of attempting the development in the manner already 

 described, there has resulted a system which involves four functional 

 units; a "pick-up" mechanism or transmitter unit, a preliminary 

 amplifier unit, commonly called the speech input equipment, a second 

 amplifier unit commonly called the power amplifier, and a receiver- 

 projector unit for transforming the amplified currents back into 

 sound, and properly distributing it throughout the space to be covered. 



4 Physical Examination of Hearing, R. L. Wegel, Proceedings of the National 

 Academy of Sciences, Volume 8, Number 7, July, 1922. 



8 LTse of Public Address Systems with Telephone Lines, W. H. Martin and A. B. 

 Clark. Presented before A. I. E. E., Feb. 14, 1923. 



