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 investigat ed spatial unmasking for configurations in which the simulated sources vari ed in azimuth and could be either near or far from the head. Target sentenc es and speech-shaped noise maskers were simulated over headphones using hea d-related transfer functions derived from a spherical-head model. Speech re ception thresholds were measured adaptively, varying target level while kee ping the masker level constant at the "better" car. Results demonstrate tha t small positional changes can result in very large changes in speech intel ligibility 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 dif ference in the target-to-masker ratios at the two cars can be substantially larger for nearby sources than for relatively distant sources. Predictions from an existing model of binaural speech intelligibility are in good agre ement with results from all conditions comparable to those that have been t ested previously. However, small but important deviations between the measu red and predicted results are observed for other spatial configurations, su ggesting that current theories do not accurately account for speech intelli gibility for Some of the novel spatial configurations tested. (C) 2001 Acou stical Society of America.