Analysis of the HRTF characteristics with respect to both azimut and elevation localisation cues would seem to suggest that, whilst inter-aural time difference and inter-aural level difference information give strong azimut localisation cues, we only have spectral variations in the vertical plane to generate localisation cues with respect to elevation. Of course positioning of a 2nd layer of loudspeakers above the horizontal reference plane will already introduce listener spectral differences relative to the horizontal plane reproduction, but the microphone array that feeds this 2nd layer must not generate information that will be in conflict with the localisation cues already generated in the horizontal plane of the 1st layer or main array. In the event of Time Difference and Level Difference information being generated between microphones in both layers, localisation characteristics must be considered as projected onto the main horizontal plane of localisation information.
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