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BELL SYSTEM TECHNICAL JOURNAL 



Also the following condition when Lk is larger than Lm must be satisfied, 

 namely, 6^ = 1 when AL = some value somewhat smaller than + T. 

 The value of T can be obtained from masking curves. An examination 

 of these data indicates that to a good approximation the value of T is 

 dependent upon the single variable fk — Ijm- A curve showing the 

 relation between T and this variable is shown in Fig. 12. It will be 

 seen that for most practical cases the value of T is 25. It cannot be 

 claimed that the curve of Fig. 12 is an accurate representation of the 

 masking data, but it is sufficiently accurate for the purpose of loudness 

 calculation since rather large changes in T will produce a very slight 

 change in the final calculated loudness level. 



60 



-100 100 300 500 700 



VALUES or Af-fm=f -affy, 

 Fig. 12 — Values of the masking T 



Data were taken in an effort to determine how this function de- 

 pended upon AL but it was not possible to obtain sufficient accuracy 

 in the experimental results. The difference between the resultant 

 loudness level when half the tones are down so as not to contribute to 

 loudness and when these are equal is not more than 4 or 5 db, which is 

 not much more than the observational errors in such results. 



A series of tests were made with tones similar to those used to obtain 

 the results shown in Figs. 8 and 9 except that every other component 

 was down in loudness level 5 db. Also a second series was made in 

 which every other component was down 10 db. Although these data 

 were not used in determining the function described above, it was 

 useful as a check on the final equations derived for calculating the 

 loudness of tones of this sort. 



The factor finally chosen for representing the dependence of hk upon 

 AL is 10^^/^. This factor is unity for AL = 0, fulfilling the first 



