244 BELL SYSTEM TECHNICAL JOURNAL 



E, the power of the sound source, is approximately 400 watts. For 

 other halls it may be seen that the power required for this source is 

 proportional to the volume of the hall and inversely proportional to 

 the reverberation time. A person would experience the sense of 

 feeling when closer than about 10 meters to such a source of 400 watts 

 power, even in free open space. Hence it would be unwise to have 

 seats closer than 10 or 15 meters from the stage when such powers are 

 to be used. 



These, then, are the general fundamental requirements for an ideal 

 transmission system. How near they can be realized with apparatus 

 that we now know how to build will be discussed in the papers included 

 in this symposium. 



A system approximately fulfilling these requirements was con- 

 structed and used to reproduce the music played by the Philadelphia 

 Orchestra. The first public demonstration was given in Constitution 

 Hall, Washington, D. C, on the evening of April 27, 1933, under the 

 auspices of the National Academy of Sciences. At that time, Dr. 

 Stokowski, Director of the Philadelphia Orchestra, manipulated the 

 electric controls from a box in the rear of Constitution Hall while the 

 orchestra, led by Associate Conductor Smallens, played in the Academy 

 of Music in Philadelphia. 



Three microphones of the type described in the paper by Wente and 

 Thuras (p. 259) were placed before the orchestra in Philadelphia, one on 

 each side and one in the center at about 20 feet in front of and 10 feet 

 above the first row of instruments in the orchestra. The electrical 

 vibrations generated in each of these microphones were amplied by 

 voltage amplifiers and then fed into a transmission line which was 

 extended to Washington by means of telephone cable. The con- 

 struction of these lines, the equipment used with them, and their 

 electrical properties, are described in the paper by Affel, Chesnut, and 

 Mills (p. 285). In Constitution Hall at Washington, D. C, these 

 transmission lines were connected to power amplifiers. The type of 

 power amplifiers and voltage amplifiers used are described in the paper 

 by Scriven (p. 278). The output of these amplifiers fed three sets of 

 loud speakers like those described in the paper by Wente and Thuras. 

 They were placed on the stage in Constitution Hall in positions cor- 

 responding to the microphones in the Academy of Music, Philadelphia. 



Judging from the expression of those who heard this concert, the 

 development of this system has opened many new possibilities for the 

 reproduction and transmission of music that will create even a greater 

 emotional appeal than that obtained when listening to the music 

 coming directly from the orchestra through the air. 



