Public Address Systems 1 



By I. W. GREEN and J. P. MAXFIELD 



Synopsis: A public address system comprises electrical equipment to 

 greatly amplify a speaker's voice so it will reach a much larger assemblage 

 than he could speak to unaided. Beginning with the presidential conven- 

 tions of the two major parties in 1920 and the inaugural address of President 

 Harding in March 1921, when a special address system installed by the 

 telephone engineers enabled him to address an audience estimated at 125,- 

 000, there followed in rapid succession, many public events demonstrat- 

 ing the value of such systems. One of the most notable of these occurred 

 on Armistice Day 1921, when the speeches, prayers and music at Arlington, 

 Virginia, were heard, not only by 100,000 persons gathered there at the 

 National Cemetery, but by some 35,000 in New York City and 20,000 in 

 San Francisco. On this occasion the three public address systems, one 

 for each of these cities, were joined by long distance telephone circuits. 



The fundamental requirements of a satisfactory public address system 

 are naturalness of reproduction and wide range of output volume. The 

 meeting of these two requirements for music proves more difficult than for 

 speech. 



The public address system here described is most readily considered in 

 three sections — "pick-up" apparatus which is placed in the neighborhood 

 of the speaker and converts his words into undulatory electric currents; a 

 vacuum tube amplifier for amplifying these currents; and a "receiver- 

 projector" for reconverting the current into sound waves and distributing 

 the sound over all of the audience. In the present system each of these 

 three parts of the equipment has been designed with the intention of 

 making it as nearly distortionless as possible, so that the various parts 

 might be adaptable for audiences ranging in size from possibly one 

 thousand to several hundred thousand, and might also be used in connec- 

 tion with the long distance telephone lines and with either radio broad- 

 casting or receiving stations. One of the larger public address systems is 

 easily capable of magnifying a speaker's voice as many as 10,000 times. 



The pick-up device whether of the carbon microphone variety or a con- 

 denser transmitter need not be placed close to the speaker's lips but will 

 operate satisfactorily when four or five feet away. The loud-speaking 

 receiver mechanism is so designed that it will carry a power of several watts 

 with small distortion. Under normal conditions, 40 watts distributed 

 among a number of receiver-projectors arranged in a circle is ample to 

 reach an audience of 700,000 persons. — Editor. 



f I A HIS paper aims to present the problems encountered in the 

 -*- development of electrical systems for amplifying the voices of 

 public speakers and music; and to describe the equipment as brought 

 to a commercial state and now in use in the United States and various 

 other countries. 



The two main requirements of a successful public address or loud 

 speaking system are, first, that it shall reproduce the sounds, such as 

 speech or music, faithfully; and second, that this faithful reproduction 

 shall be loud enough and sufficiently well distributed for all of the 

 audience to hear it comfortably. Most of the development work 



•Presented at the Midwinter Convention of the A. I. E. E., New York, N. Y., 

 February 14-17, 1923. Published in the Journal of the A. I. E. E. for April, 1923. 



113 



