Spectral manipulation techniques offer a means of generating virtual sound-source elevation using horizontal loudspeakers. In comParison to cross-talk cancellation systems, these techniques can be more flexible and operate with even a single loudspeaker. However, the azimuthal stability of such approaches remains uncharacterised. This study evaluates the effectiveness of magnitude-based difference-spectrum filtering across lateral source positions, including intermediate positions rendered via amplitude panning, in loudspeaker-based reproduction. Direction-dependent filters derived from a mean HRTF magnitude response were applied over a horizontal-plane loudspeaker array, with physically elevated loudspeakers at matched azimuths serving as perceptual references. Perceived virtual elevation was quantified using the illusion ratio, a novel metric expressing virtual elevation shift as a proportion of the physical elevation shift at each azimuth. Virtual elevation reached approximately 50% of the physical elevation shift at central azimuths, decreasing significantly with lateral displacement, consistent with the reduced effectiveness of monaural spectral cues at lateral positions. A greater virtual elevation effect was observed for ipsilateral rather than contralateral source positions relative to the filter ear. Stimulus class did not significantly alter the azimuth-dependent structure of the effect. These results demonstrate that magnitude-based spectral elevation synthesis produces a measurable and robust elevation effect, most pronounced for central sources.
