Cross-frequency coherence of ITDs (“straightness”) minimizes the effects of binaural cue ambiguity in stereo perception

Rayleigh's original Duplex Theory posits that sound-source localization relies on different cues from low- and high-frequency sounds: low frequencies provide interaural time differences (ITDs), which often dominate localization, while high frequencies offer spatial information through interaural level differences (ILDs). An updated perspective on the Duplex Theory emerged from perceptual studies investigating binaural hearing by pitting ITDs against ILDs [Hartmann et al., J. Acoust. Soc. Am. 139, 968–985 (2016)]. Hartmann and colleagues examined free-field localization of sine tones at 250, 500, and 750 Hz, revealing two key findings: (1) slipped-cycle ITDs at higher frequencies (e.g., 750 Hz) can lead to significant left-right confusions, and (2) naturally occurring ILDs with the same sign as ITDs can resolve these confusions. Building on this work, we applied stereophonic techniques to investigate free-field localization of noise stimuli with conflicting ITDs and ILDs across different frequency ranges. Our findings indicate that ILDs can modulate the effective ITDs in both low-pass and high-pass noise stimuli with a cutoff frequency of 1500 Hz. However, a coherent ITD across frequencies (“straightness”) is a stronger grouping cue than same-sign ITD and ILDs in reducing left-right confusions.