Spatial unmasking of nearby speech sources in a simulated anechoic environment.

Spatial unmasking of speech has traditionally been studied with target and masker at the same, relatively large distance. The present study investigated spatial unmasking for configurations in which the simulated sources varied in azimuth and could be either near or far from the head. Target sentences and speech-shaped noise maskers were simulated over headphones using head-related transfer functions derived from a spherical-head model. Speech reception thresholds were measured adaptively, varying target level while keeping the masker level constant at the "better" ear. Results demonstrate that small positional changes can result in very large changes in speech intelligibility when sources are near the listener as a result of large changes in the overall level of the stimuli reaching the ears. In addition, the difference in the target-to-masker ratios at the two ears can be substantially larger for nearby sources than for relatively distant sources. Predictions from an existing model of binaural speech intelligibility are in good agreement with results from all conditions comparable to those that have been tested previously. However, small but important deviations between the measured and predicted results are observed for other spatial configurations, suggesting that current theories do not accurately account for speech intelligibility for some of the novel spatial configurations tested.