Articulation Index

The Articulation Index (AI) is a measure of speech intelligibility expressed as a value from 0.00 (no intelligibility) to 1.00 (perfect intelligibility). Originally defined by French and Steinberg (1947) and standardized in ANSI S3.5, AI calculates the proportion of the speech spectrum that is audible above the background noise at the listener’s position. The calculation divides the speech frequency range into bands and weights each by its contribution to intelligibility. An AI below 0.05 indicates confidential privacy (speech unintelligible), 0.05–0.20 indicates normal privacy, and above 0.50 indicates poor privacy. AI has been largely superseded by the Speech Intelligibility Index (SII, ANSI S3.5-1997) for general use and by STI (IEC 60268-16) for room acoustic applications, but it remains the basis for Speech Privacy Class (SPC) calculations per ASTM E1130. AI is directly related to Privacy Index: PI = 1 − AI.