Enhancing Externalization of Frontal Virtual Sound Sources in Binaural Synthesis via Level and Reflection Adjustment of Non-individualized BRIRs

This study addresses the common issue of low externalization in binaural synthesis when listening to virtual sound sources through wearable devices. The improvement of externalization of frontal sound sources (0°–30° azimuth), which are perceived as being closer to the listener, was investigated. A consistent pattern across different rooms was identified using a single-speaker non-individualized binaural room impulse response (NI-BRIR). The binaural level decreases regularly with increasing azimuth up to 45°(front right) but levels off beyond this point. Based on the strong correlation among binaural level, azimuth, and externalization, we developed a two-phase approach. In Phase I, scaling factors were applied to the NI-BRIRs to equalize the binaural level across different frontal azimuths. Our results showed this adjustment significantly improved externalization, with a large effect size for azimuths from 0°–30°. Building on this adjustment, we further enhanced the early reflections of the NI-BRIR for 0° and 15° to simulate the temporal signal conditions of angles with better externalization (Phase II), aiming to modify the ratio between direct sound and main reflections. This modification had a positive and additive effect. The effective enhancement of externalization of frontal virtual sound sources was demonstrated by a consistent set of scaling factors and strategic adjustments to early reflections. These findings suggest a practical corrective method for single-speaker NI-BRIRs, contributing to more realistic binaural synthesis in applications such as wearable audio technology and hearing support systems.