IACC (Inter-Aural Cross-Correlation)

Inter-Aural Cross-Correlation Coefficient (IACC) measures the similarity between the sound signals arriving at the left and right ears of a listener or dummy head, quantifying the spatial impression and envelopment in a room. Defined in ISO 3382-1:2009 §4.7, IACC is calculated from binaural impulse responses by cross-correlating the left and right ear signals. IACC values range from 0 (completely dissimilar signals, maximum spaciousness) to 1 (identical signals, no spatial impression). Low IACC values correlate with subjective feelings of being "immersed" in sound, which is desirable in concert halls. Early IACC (IACCE, 0–80 ms) relates to apparent source width, while late IACC (IACCL, 80 ms+) relates to listener envelopment. Preferred IACCE values for concert halls are below 0.50 at mid-frequencies. IACC is influenced by room width, lateral reflection patterns, and diffusion. Measurement requires a dummy head or binaural microphone system.