Masking-Level Differences Determined with and without Interaural Disparities in Masker Intensity

Psychometric functions were obtained for three listeners for the interaural conditions Nm-Sm and N0-Sπ using seven different noise spectrum levels. In addition, data were collected on the N0-Sπ condition with the spectrum level in one ear smaller than that in the other ear. In these N0′-Sπ′ conditions, the signal-to-noise ratio was kept equal in the two ears. The masking-level differences (MLD's) for the N0-Sπ conditions declined gradually as the level of the noise was reduced, and for small spectrum levels, they were essentially the same as the MLD obtained when the masker was turned off. The MLD's declined more rapidly for the N0′-Sπ′ conditions, but a small MLD was obtained even when the spectrum level in one ear was 60 dB smaller than that in the other ear. For the Nm-Sm conditions, masking increased linearly with increases in spectrum level, but for N0-Sπ, the relation was not linear—a further indication that the basis for detection is different in MLD and non-MLD conditions. The results are discussed in terms of “internal noise,” the effect of which is to alter the effective interaural correlation as the masker level is reduced. Signal: 400 cps, 250 msec; masker: continuous, wide-band noise; method: two-interval forced choice.