Direct control and evaluation of the perceived auditory source width (ASW) of phantom images produced by two or more loudspeakers is beneficial during the design and implementation of multi-channel audio reproduction systems. Numerous quantitative metrics have been proposed as indicators of ASW. Among the most common are the early lateral energy fraction (LFE) and the early interaural cross-correlation coefficient (IACCE). While these quantities have been shown to indicate ASW to various extents, they are derived primarily from observations of objective binaural measurements with little regard to the psychoacoustic behaviour of the human auditory system. This paper proposes a new metric to quantify and evaluate ASW, the Interaural Thresholded Level Distribution (ITLD). The metric is a psychophysically-motivated quantity derived from the properties and processes of the human auditory system specific to the inner ear. A preliminary evaluation of ITLD and IACCE as suitable metrics of ASW is presented, using binaural measurements taken in anechoic and reverberant conditions. While measurements indicate some general similarities between the ITLD and IACCE, the former is shown to correlate more strongly to theoretical results for on-axis listening positions, suggesting that ITLD is a more direct metric for evaluating ASW.
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