Intelligibility and localization of speech from virtual directions.

We studied the intelligibility of speech and the ability of listeners to localize speech when synthetic direction information was added to the signal. Masked thresholds for single synthesized words were measured for simulated, horizontal angular separations of speech in the presence of a masking noise. Speech recognition was measured using the Modified Rhyme Test; the masker was broad-band white noise. A single set of head-related transfer functions conditioned the speech and noise waveforms presented over headphones, and listeners were free to move their heads relative to the apparent directions of these signals. The masker was fixed at 0 deg, and when speech was presented from other azimuths, its intelligibility typically increased by 4 to 5 dB. Individuals' masked thresholds were variable yet repeatable, and the plot of threshold by azimuth seemed unique to each subject. In a separate test, the same listeners were asked to estimate the azimuth of items from the rhyme test. Localization was accurate and performance was similar to previous work, except that confusions of source locations in the front and rear hemispheres were evenly distributed across azimuth.