306 BELL SYSTEM TECHNICAL JOURNAL 



give an over-all flat response-frequency characteristic. If the system, 

 including the air path from the loud speakers to one position in the 

 auditorium, is made flat, it will not, in general, be flat for other posi- 

 tions or for other paths in the room. This variation in characteristic 

 is due partly to the variation in the ratio of direct to reverberant 

 sound, and partly to the fact that the sounds of higher frequency are 

 absorbed more rapidly by the air during transmission.^- ^ This latter 

 effect is of considerable importance; it depends upon the humidity and 

 temperature of the air, and may cause a loss of more than 10 db in 

 the high frequencies at the more distant positions in a large auditorium. 

 Some compromise in the amount of equalization employed therefore 

 is necessary. Probably the most straightforward procedure would be 

 to design the networks according to the response curves obtained with 

 the microphones near the loud speakers. This would insure that for 

 both the response measurements and the pick-up the microphone 

 characteristics would be the same, and any deviation from a uniform 

 response in the microphones would be corrected for in this way, along 

 with variations in the loud speaker output. This procedure was 

 modified somewhat for the case under discussion, however, because 

 by far the greater portion of the audience was at a distance from the 

 stage such that they received a relatively large ratio of reverberant 

 sound, and it was believed that a better effect would be achieved by 

 equalizing the system characteristic in accordance with response 

 measurements taken at some distance from the loud speakers. 



Control Equipment 



In addition to the equalizing circuits used to obtain a uniform re- 

 sponse characteristic, two sets of quality control networks which could be 

 switched in or out of the three channels simultaneously were employed. 

 One set modified the low frequencies as shown at A, B, and C of Fig. 5, 

 while the other gave high frequency characteristics as shown at D, E, 

 F, and G. These latter networks permitted the director to take 

 advantage of the fact that the electrical transmission and reproduction 

 of music permits the introduction of control of volume and quality 

 which can be superimposed on the orchestral variations. Quality of 

 sound can be divorced from loudness to a greater degree than is 

 possible in the actual playing of instruments, and the quality can be 

 varied while the loudness range is increased or decreased. Electrical 

 transmission therefore not only enlarges the audience of the orchestra, 

 but also enlarges the capacity of the orchestra for creating musical 

 effects. 



The quality control networks and their associated switches were 



