240 BELL SYSTEM TECHNICAL JOURNAL 



two halls this time-sequence should be the same. Obviously, this 

 never can be true at every position unless the halls are the same size 

 and shape; corresponding positions would not otherwise exist. Let 

 us consider the case where the two halls are the same size and shape 

 and also have the same acoustical properties. Let us designate the 

 first hall in which the music originates by 0, and the second one in 

 which the music is reproduced by R. What requirements are necessary 

 to obtain perfect reproduction from into R such that any listener 

 in any part of R will receive the same sound effects as if he were in the 

 corresponding position in O? 



Suppose there were interposed between the orchestra and the audi- 

 ence a flexible curtain of such a nature that it did not interfere with a 

 free passage of the sound, and which at the same time had scattered 

 uniformly over it microphones which would pick up the sound waves 

 and produce a faithful electrical copy of them. Assume each micro- 

 phone to be connected with a perfect transmission line which termi- 

 nates in a projector occupying a corresponding position on a similar 

 curtain in hall R. By a perfect transmission line is meant one that 

 delivers to the projector electrical energy equal both in form and 

 magnitude to that which it receives from the microphone. If these 

 sound projectors faithfully transform the electrical vibrations into 

 sound vibrations, the audience in hall R should obtain the same effect 

 as those listening to the original music in hall 0. 



Theoretically, there should be an infinite number of such ideal sets 

 of microphones and sound projectors, and each one should be infin- 

 itesimally small. Practically, however, when the audience is at a 

 considerable distance from the orchestra, as usually is the case, only 

 a few of these sets are needed to give good auditory perspective; that 

 is, to give depth and a sense of extensiveness to the source of the 

 music. The arrangement of some of these simple systems, together 

 with their effect upon listeners in various parts of the hall, is described 

 in the paper by Steinberg and Snow (page 245). 



In any practical system it is important to know how near these ideal 

 requirements one must approach before the listener will be aware that 

 there has been any degradation from the ideal. For example, it is 

 well known that whenever a sound is suddenly stopped or started, the 

 frequency band required to transmit the change faithfully is infinitely 

 wide. Theoretically, then, in order to fulfill these ideal requirements for 

 transmitting such sounds, all three elements in the transmission system 

 should transmit all possible frequencies without change. Practically, 

 because of the limitations of hearing, this is not necessary. If the 

 intensities of some of the component frequencies required to represent 



