Horizontal and vertical MAAs for a wide range of sound source locations

Minimum audible angles (MAA) in both the horizontal and the vertical directional were measured for sound source locations distributed over virtually the complete sphere around the listener. Stimuli were noise bursts with either a fixed (flat) spectrum, or a random spectrum with 1/3-octave levels drawn from a uniform distribution with a 20-dB range. It was found that, in general, horizontal MAAs are smallest for sound sources in or near the median plane and vertical MAAs are smallest for sources around 90° azimuth. The results for the random-spectrum stimuli show some unexpected asymmetries. Though listeners were almost unable to localize sources in the median plane with elevations in excess of 30° (vertical MAAs were then 60° or more), they attained moderate MAAs for lower elevations. The MAA for frontal presentation was even similar to the result obtained with fixed-spectrum noise. Results furthermore show that horizontal MAAs for sources at 90° azimuth are much larger for positive elevations than for negative elevations. These findings indicate that localization acuity is optimal for sound sources around or below the horizontal plane.