Interaural coherence and the lateralization of noise by interaural level differences

Abstract

Coherent noise, with an interaural cross correlation of unity, and incoherent noise, where signals to left and right ears are independent, sound very different. The former produces a compact lateralized image. The latter produces a diffuse image that fills the head. Nevertheless, listening experiments show that coherent and incoherent noise images seem to be equally affected by small interaural level differences (ILD). The ILD threshold for lateralizing incoherent noise is less than half a dB greater than the threshold for lateralizing coherent noise. In this sense, the human binaural system appears to behave like an ideal level meter, insensitive to the wave form and envelope fine structures that determine coherence. The small discrepancy (less than 0.5 dB) can be understood from a standard model of loudness perception—incorporating critical band filtering, half-wave rectification, amplitude compression (0.6 power law), and temporal integration (300 ms). The same model can also account for the small observed difference in lateralization ILD thresholds for coherent noise (N0) and antiphasic noise (Nπ). It can further account for the observed noise bandwidth dependence of lateralization ILD thresholds. [Work supported by the NIDCD.]