Spatial unmasking of circular moving sound sources in the free field

This study investigates the unmasking of a horizontally moving tonal source in noise under free-field presentation. Thresholds of static and circularly moving 500 Hz sine tones presented in octave-band noise from the front were measured with four normal hearing listeners with an adaptive forced choice method. Six velocities were tested from 10°/s to 200°/s. Stimuli were of fixed duration, started in the front and moved to an azimuthal end position, determined by the overall probe tone duration in relation to their speed. The moving sound sources were generated with 17th order Ambisonics and played via 36 horizontally arranged loudspeakers in an anechoic chamber. Preliminary results suggest that for the six velocities tested, spatial unmasking increased with increasing velocity up to 10 dB for 200°/s. However, unmasking remained substantially lower than for static sources, at the respective most lateral position reached in the movement trajectory at the end of the overall probe tone duration. This is consistent with observations in the literature on binaural sluggishness. Even for a small velocity of 50°/s, and therefore relatively slow changes in interaural phase, spatial unmasking decreased by 4 dB. This velocity is comparable to that of a walking person in 2 meters distance. It seems that in the free field, also slow changes in the interaural cues can lead to a decrease in binaural unmasking. This finding is important for extending binaural models to more realistic, dynamic listening situations in the free field.