Diotic and dichotic detection using multiplied-noise maskers.

Detection thresholds were measured with a multiplied-noise masker that was in phase in both ears and a sinusoidal signal which was either in phase or out of phase (NoSo and NoS pi conditions). The masker was generated by multiplying a low-pass noise with a sinusoidal carrier. The signal was a sinusoid with the same frequency as the carrier and a constant phase offset, theta, with respect to the carrier. By adjusting the phase offset, the stimulus properties were varied in such a way that only interaural time delays (theta = pi/2) or interaural intensity differences (theta = 0) were present within the NoS pi stimulus. Thresholds were measured at a center frequency of 4 kHz as a function of bandwidth for theta = pi/2 and for theta = 0. In a second experiment thresholds were measured for a bandwidth of 25 Hz as a function of the center frequency. The results show that narrow-band BMLDs at 4 kHz can amount to 30 dB for the theta = 0 condition. For this condition, narrow-band BMLDs are also reasonably constant across frequency, in contrast to results obtained with standard Gaussian-noise maskers. For theta = pi/2, BMLDs are restricted to the frequency region below 2 kHz provided that the masker is narrow band, but BMLDs of up to 15 dB are found at 4 kHz if the masker is 50 Hz or wider. The frequency dependence of the binaural thresholds seems to be best explained by assuming that the stimulus waveforms are compressed before binaural interaction.