CHANGES IX PROGRAM BAXD WIDTH 3 



conditions were unknown to the observers, being designated to them only 

 by the letters in the signal. At the conclusion of the test the observers were 

 asked to mark on a ballot which letter appeared to coincide with the wider 

 band (not which they preferred). A series of tests consisted of comparisons 

 between substantially all of the possible band widths among those available. 

 There were also included in some of the series as a check, one or two tests 

 in which the band width was the same for both positions of the switch. 

 Ten complete series of tests were carried out, two on each of five different 

 programs. 



The programs consisted of a dance orchestra, two large symphony orches- 

 tras, speech from a male speaker repeating a test sentence, and a radio 

 dramatic sketch. The programs, except for the spoken test sentences, 

 were obtained by special arrangement over direct wire lines from the studio 

 or theater in which the performance took place. The entire system from 

 microphones to and including the loud-speaker had a substantially flat trans- 

 mission characteristic from 40 to 15,000 cycles, with no filters in the circuit. 

 The loud-speaker was of the two-unit type and was one of a number built 

 for the demonstration of auditor}^ perspective in 1933. The tests were 

 conducted in the program laboratory of the Bell Telephone Laboratories 

 where the acoustic noise level was about -\-30 decibels. The noise con- 

 tributed by the electrical parts of the system was considerably below the 

 acoustic noise. The loudness of the programs was adjusted to about unity 

 reproduction, that is, to the volume that would be heard by listeners in a 

 favorable position at the original performance. 



The observers were engineers having a considerable experience in tests of 

 program quality. They were doubtless therefore considerably more 

 critical than the average radio listener. The number of observers varied 

 somewhat during the tests but averaged about sixteen. The ages of the 

 observers were in the 30's and 40's so that neither very young nor very old 

 ears were represented. 



The immediate outcome of the tests was some 2,000 ballots which were 

 meaningless until analyzed. Before the analysis could be made, however, 

 it was necessary to decide how to express the results. 



There are no familiar units to express fidelity or program quality. It was 

 decided therefore to employ the very useful concept of the limen and the 

 liminal unit. These terms have occasionally been applied to other subjec- 

 tive data and may be roughly defined as the least change in a quantity 

 which is detectable. In the present case, if the band widths being com- 

 pared differ greatly, there will be a nearly unanimous agreement among the 

 observers as to which is the wider. If they differ only slightly, however, 

 many of the observers will vote wrongly for the narrower band and on suc- 

 cessive repetitions of the test many will reverse themselves. An average of 



