Sound image movement of approaching and retreating sounds

We investigated how listeners perceive sound image movements of moving sound sources horizontally approaching and retreating from a listener’s head under three conditions: directly listening to approaching and retreating real sound sources, listening to binaurally recorded sounds with headphones, and listening to binaurally synthesized sounds with headphones. White noise was emanated from a small loudspeaker put on a slider. The moving directions of the loudspeaker were eight radial horizontal directions at 45° intervals, and the moving distance was 5 to 105 cm from the head. Personal earplug microphones were used for the binaural recording. Personal head-related transfer functions were used to synthesize the binaural signals. Sounds approaching and retreating from listeners’ heads in most radial horizontal directions produced straight approaching and retreating sound images. Those in the 45° and 315° radial horizontal directions, however, yielded anomalous sound image movement. The sound image moved straight forward, bent to the rear as the sound approached, and then followed the same path in reverse as the sound retreated for the real, binaurally recorded, and binaurally synthesized sounds. The analysis of HRTFs suggests that the growth of the inter-aural level difference very close to the head can be the cause of the anomalous sound image movement.