Auditory localization in a free field using discrimination procedures

A rotating boom positioned a loudspeaker at ear height at a distance of 4 ft from the center of the head of S, who was seated in a darkened large anechoic chamber (684 ft); 750-msec bursts of pink noise, separated by 3.25 sec, at 54 db SPL, were presented. During the interstimulus interval the speaker was moved cw or ccw in the presence of a masking noise at 73 db SPL. Practiced young adults (2F, 1 M) judged azimuth "same" or "different"; minimum discriminable angle (MDA) was taken as P(c) = .70, corrected for false alarms (judgments "s" or "d" with no movement). Standard azimuths relative to S were at the 8 cardinal points. MDAs were best at 0 degree (c. 4.5 degrees) but were still relatively good (c. 8.5 degrees) at 180 degrees. In general, MDAs were poorer in the rear quadrants. One S excelled at MDA and exhibited the lowest false alarm rate. The effect of the masker, and random selection of standard and azimuth and of comparison stimuli at every trial, help explain the rather large MDAs compared with the minimum audible angles in the literature. The rather large determination at 90 degrees azimuth as determined by Mills (1958) was not found in these data.