Sound Source and Loudspeaker Base Angle Dependency of Phantom Image Elevation Effect

Previous research showed that when identical noise signals are presented from two loudspeakers equidistant from the listener, the resulting phantom image is perceived as being elevated in the median plane. In this study, listening tests used eleven natural sources and four noise sources with different spectral and temporal characteristics reproduced with seven loudspeaker base angles between 0° and 360°. While the degree of perceived elevation depends on the base angle of the loudspeakers, the spectral and temporal characteristics of the sound source also play a significant role in determining perceived elevation. Results generally suggest that the effect is stronger for sources that have a transient nature and a flat frequency spectrum as compared to continuous and low-frequency sources. It is proposed that the perceived degree of elevation is determined by a relative cue related to the spectral energy distribution at high frequencies and also by an absolute cue associated with the acoustic crosstalk and torso reflections at low frequencies. A novel hypothesis about the role of acoustic crosstalk and torso reflection at low frequencies is explored. At frequencies below 3 kHz, the brain might use the first notch in the ear-input spectrum, which is produced by the combination of acoustic crosstalk and torso reflection, as a cue for localizing a phantom source at an elevated position in the median plane. These results may prove useful for 3D sound panning, recording, and mixing without elevated speakers.