Effect of Masker Spectrum Level on Masking‐Level Differences at Low Signal Frequencies

Most, if not all, models of binaural hearing predict a dependence of the magnitude of the masking‐level difference (MLD) on signal frequency. Specifically, the size of the MLD is expected to increase as the signal frequency is decreased. The results of many experiments in which signal frequency was varied, however, do not support the above prediction. The magnitude of the MLD appears to decrease at frequencies below 300 cps. The present experiment was designed to re‐examine the decrease in the size of the MLD at low frequencies and to test a qualitative explanation of this phenomenon based on the concept of internal noise. To do so, the size of the MLD in the N0Sπ masking condition and the N0S0 masking condition was estimated relative to the NMSM masking condition at signal frequencies of 150 and 300 cps at several masker spectrum levels. The results indicated that the magnitude of the MLD at both frequencies is dependent upon spectrum level of the masker. The results further indicate that the dependence of the size of the MLD on spectrum level varies as a function of the signal frequency. The size of the MLD at 150 cps is much more sensitive to changes in masker spectrum level than is the MLD at 300 cps. Both results agree with predictions based on the internal noise hypothesis.