260 BELL SYSTEM TECHNICAL JOURNAL 



such as to give to a lover of symphonic music esthetic satisfaction at 

 least as great as that which would be given by the orchestra itself 

 playing in the same hall. This is more than a problem of instrument 

 design, but this paper will be restricted to a discussion of the require- 

 ments that must be met by the loud speakers and microphones, and 

 to a description of the principles of design of the instruments used in 

 the transmission of the music of the Philadelphia Orchestra from 

 Philadelphia to Constitution Hall in Washington. Some of the 

 requirements are found in the results of measurements that have been 

 made on the volume and frequency ranges of the music produced by 

 the orchestra. 



General Considerations 



The acoustic powers delivered by the several instruments of a 

 symphony orchestra, as well as by the orchestra as a whole, have been 

 investigated by Sivian, Dunn, and White. Figure 1 was drawn on 

 the basis of the values published by them.^ The ordinates of the 

 horizontal lines give the values of the peak powers within the octaves 

 indicated by the positions of the lines. For a more exact interpretation 

 of these values the reader is referred to the original paper, but the 

 chart here given will serve to indicate the power that a loud speaker 

 must be capable of delivering in the various frequency regions, if the 

 reproduced music is to be as loud as that given by the orchestra itself. 

 However, it was the plan in the Philadelphia-Washington experiment 

 to reproduce the orchestra, when desired, at a level 8 or 10 db higher, 

 so that with three channels each loud speaking system had to be able 

 to deliver two or three times the powers indicated in Fig. 1. Sivian, 

 Dunn, and White also found that for the whole frequency band the 

 peak powers in some cases reached values as high as 65 watts. In 

 order to go 8 db above this value, each channel would have to be capa- 

 ble of delivering in the neighborhood of 135 watts. 



The chart (Fig. 1) shows that the orchestra delivers sound of com- 

 parable intensity throughout practically the whole audible range. 

 Although it is conceivable that the ear would not be capable of 

 detecting a change in quality if some of the higher or lower frequencies 

 were suppressed, measurements published by W. B. Snow ^ show that 

 for any change in quality in any of the instruments to be undetectable 

 the frequency band should extend from about 40 to about 13,000 c.p.s. 

 The necessary frequency ranges that must be transmitted to obviate 

 noticeable change in quality for the different orchestral instruments 

 are indicated in the chart of Fig. 2, which is taken from the paper by 

 Snow. 



