The Frequency and Loudspeaker-Azimuth Dependencies of Vertical Interchannel Decorrelation on the Vertical Spread of an Auditory Image

In horizontal stereophony, it is known that interchannel correlation relates to the horizontal spread of a phantom auditory image. However, little is known about the perceptual effect of interchannel correlation on vertical image spread (VIS) between two vertically-arranged loudspeakers. The present study investigates this through two subjective experiments: (i) a multiple comparison of relative VIS for stimuli with varying degrees of correlation; and (ii) the absolute measurement of upper and lower VIS boundaries for extreme stimuli conditions. Octave-band (center frequencies: 63 Hz to 16 kHz) and broadband pink noise signals have been decorrelated using two techniques: All-pass filtering and complementary comb-filtering. These stimuli were presented from vertically-spaced loudspeaker pairs at three azimuth angles (0^◦, ±30^◦, and ±110^◦), with each angle assessed discretely. Both the relative and absolute test results show no significant effect of vertical correlation on VIS for the 63 Hz, 125 Hz, and 250 Hz bands. For the 500 Hz band and above, there is a general tendency for VIS to increase as correlation decreases, which is observed for both decorrelation methods. This association is strongest at 0^◦ azimuth for the 500 Hz and 1 kHz bands; at ±30^◦ for 8 kHz and Broadband; and at ±110^◦ for 2 kHz, 4 kHz, and 16 kHz. The 8 kHz band at ±30^◦ has the strongest association of all conditions—post-hoc objective analysis indicates a potential relationship between HRTF localization cues (pinna filtering) and VIS perception within this frequency region. Furthermore, the absolute test results suggest that changes of VIS from interchannel decorrelation are fairly slight, with only the Broadband and 16 kHz bands showing a significant increase. The deviations of boundary scores also suggest a difficulty grading absolute VIS and/or potential disagreements among listeners.