Binaural perception of the modulation depth of AM signals

The purpose of this study was to determine the binaurally perceived modulation depth (m) of the low rate amplitude modulated (AM) signals, under conditions of their dichotic presentation, i.e., when the AM signals presented to the left and the right ear had different modulation depths. The modulation depth was determined as a point of subjective equality between the sensations of the modulation depth of AM signals presented to the left and right ear, using a one-up, one-down adaptive procedure. Measurements were made for the carrier frequencies (fc) of 250, 1000, and 4000 Hz, and the modulation frequency (fm) of 4 Hz. Experimental data showed that, for sufficiently small interaural difference in modulation depth (Δm), the perceived modulation approximated the mean of the modulation depths presented to the left and the right ear. However, for moderate and large Δm, the binaurally perceived modulation was lower than the mean of ml and mr and the steepness of the function m = f(Δm) gradually decreased with an increase of Δm. Results of the calculation of the binaurally perceived modulation depth, obtained on the assumption of binaural loudness summation, were found to be consistent within the limit of standard deviation, with the experimental data for relatively wide range of Δm.