The effects of spatial separation in distance on the informational and energetic masking of a nearby speech signal

Although many studies have shown that intelligibility improves when a speech signal and an interfering sound source are spatially separated in azimuth, little is known about the effect that spatial separation in distance has on the perception of competing sound sources near the head. In this experiment, head-related transfer functions (HRTFs) were used to process stimuli in order to simulate a target talker and a masking sound located at different distances along the listener's interaural axis. One of the signals was always presented at a distance of 1 m, and the other signal was presented 1 m, 25 cm, or 12 cm from the center of the listener's head. The results show that distance separation has very different effects on speech segregation for different types of maskers. When speech-shaped noise was used as the masker, most of the intelligibility advantages of spatial separation could be accounted for by spectral differences in the target and masking signals at the ear with the higher signal-to-noise ratio (SNR). When a same-sex talker was used as the masker, the intelligibility advantages of spatial separation in distance were dominated by binaural effects that produced the same performance improvements as a 4-5-dB increase in the SNR of a diotic stimulus. These results suggest that distance-dependent changes in the interaural difference cues of nearby sources play a much larger role in the reduction of the informational masking produced by an interfering speech signal than in the reduction of the energetic masking produced by an interfering noise source.