Auditory Numerosity Judgement is Driven by Non-Spatial Factors Across Azimuth, Elevation, and Distance

Everyday listening often requires the auditory system to move beyond the perception of individual sound sources towards a more global representation of acoustic scenes. While auditory behaviour has been well characterised for one or a few sources, it remains unclear how listeners perceive scenes containing many simultaneous sounds. One way to address this question is through auditory numerosity judgement, the ability to estimate how many distinct sound sources are present in an acoustic scene. Because numerosity perception is known to saturate at higher source counts, it provides a useful window into how representations shift from discrete sources towards more scene-level percepts. Here, we measured numerosity judgements separately along the three dimensions of auditory space: azimuth, elevation, and distance, allowing each spatial axis to be examined in isolation. As these dimensions rely on markedly different spatial cues and differ strongly in spatial acuity, this design enabled the dissociation of spatial and non-spatial contributions to numerosity perception. Across dimensions, numerosity judgements showed no systematic relationship with individual spatial hearing performance. Speech intelligibility likewise exerted only a weak influence, suggesting a limited role for higher-level or linguistic processing. In contrast, perceived numerosity was significantly modulated by the spectrotemporal coverage of the acoustic mixture, a low-level property independent of spatial cues. Together, these findings suggest that auditory numerosity judgement is largely insensitive to spatial hearing precision and instead reflects more general constraints on how complex sound mixtures are represented at the scene level.