Dynamic spectral cues do not affect human sound localization during small head movements

Abstract

Natural listening involves a constant deployment of small head movement. Spatial listening is facilitated by head movements, especially when resolving front-back confusions, an otherwise common issue during sound localization under head-still conditions. The present study investigated which acoustic cues are utilized by human listeners to localize sounds using small head movements (below ±10° around the center). Seven normal-hearing subjects participated in a sound localization experiment in a virtual reality environment. Four acoustic cue stimulus conditions were presented (full spectrum, flattened spectrum, frozen spectrum, free-field) under three movement conditions (no movement, head rotations over the yaw axis and over the pitch axis). Localization performance was assessed using three metrics: lateral and polar precision error and front-back confusion rate. Analysis through mixed-effects models showed that even small yaw rotations provide a remarkable decrease in front-back confusion rate, whereas pitch rotations did not show much of an effect. Furthermore, MSS cues improved localization performance even in the presence of dITD cues. However, performance was similar between stimuli with and without dMSS cues. This indicates that human listeners utilize the MSS cues before the head moves, but do not rely on dMSS cues to localize sounds when utilizing small head movements.

Keywords: active localization; binaural audio; dynamic cues; front-back confusion; head rotation; spectral cues.

Figure 1

Spatial directions of the sound sources used in the experiment, plotted as two hemispheres cut through the frontal plane. In total 41 directions were used.

Figure 2

HRTFs of the subject NH257 as an example. (Left column) Energy time curves of the impulse responses (in dB) calculated for the HRTFs of a left ear along the azimuth angle. (Right column) Magnitude spectra of HRTFs (in dB) along the median plane as a function of the polar angle. (Top row) HRTFs from the condition “full." (Center row) HRTFs from the condition “flat,” here the MSS was significantly flattened, but the time of arrival, thus, the ITD and dITD cues remained unchanged. (Bottom row) HRTFs from the condition “frozen" to (0°,0°), i.e., for all spatial positions, the MSS was frozen to that of the spatial position in the front and at eye level, but the time of arrival, thus, the ITD and dITD cues, were identical to those from the actual spatial positions.

Figure 3

Localization performance grouped by the available acoustic cues. (A) Lateral precision error (in degrees). (B) Polar precision error (in degrees). (C) Front-back confusion rate (in %). Lower values indicate better performance. Each boxplot shows the statistics of all subjects (median, first and third quartiles, minima and maxima, outliers).

Figure 4

Localization performance grouped by the type of head rotation. (A) Lateral precision error (in degrees). (B) Polar precision error (in degrees). (C) Front-back confusion rate (in %). Lower values indicate better performance. Each boxplot shows the statistics of all subjects (median, first and third quartiles, minima and maxima, outliers).