BASIC REQUIREMENTS 241 



such a change are below the threshold of audibility it is obvious that 

 their elimination will not be detected by the average normal ear. 

 Consequently, for highgrade reproduction of sounds it is obvious that, 

 except in very special cases, the range of frequencies that the system 

 must transmit is determined by the range of hearing rather than by 

 the kind of sound that is being reproduced. 



Tests have indicated that, for those having normal hearing, pure 

 tones ranging in frequency from 20 to 20,000 cycles per second can be 

 heard. In order to sense the sounds at either of these extreme limits, 

 they must have very high intensity. In music these frequencies 

 usually are at such low intensities that the elimination of frequencies 

 below 40 c.p.s. and those above 15,000 c.p.s. produces no detectable 

 difference in the reproduction of symphonic music. These same tests 

 also indicated that the further elimination of frequencies beyond either 

 of these limits did begin to produce noticeable effects, particularly on 

 a certain class of sounds produced in the orchestra. For example, the 

 elimination of all frequencies above 13,000 c.p.s. produced a detectable 

 change in the reproduced sound of the snare drum, cymbals, and 

 castanets. Also, the elimination of frequencies below 40 c.p.s. 

 produced detectable differences in reproduced music of the base viol, 

 the bass tuba, and particularly of the organ. 



Within this range of frequencies the system (the combination of the 

 microphone, transmission line, and loud speaker) should reproduce the 

 various frequencies with the same efficiency. Such a general statement 

 sounds correct, but a careful analysis of it would reveal that when any 

 one tried to build such a system or tried to meet such a requirement 

 he would have great difficulty in understanding what it meant. 



For example, for reproducing all the frequencies within this band, 

 a certain system may be said to have a uniform efficiency when it 

 operates between two rooms under the condition that the pressure 

 variation at a certain distance away from the sound projector is the 

 same as the pressure variation at a certain position in front of the 

 microphone. It is obvious, however, that in other positions in the two 

 rooms this relation would not in general hold. Also, if the system 

 were transferred into another pair of rooms the situation would be 

 entirely changed. These difficulties and the way they were met are 

 discussed in the papers of this symposium that deal with loud speakers 

 and microphones (p. 259) and with methods of applying the reproducing 

 system to the concert hall (p. 301). It will be obvious from these 

 papers that the criterion for determining the ideal frequency charac- 

 teristics to be given to the system is arbitrary within certain limits. 

 However, solving the problem according to criteria adopted produced 

 a system that gave very satisfactory results. 



