One of the earliest attempts to measure by machine the intelligibility of a speech transmission system, the Articulation Index was developed by Bell Telephone Laboratories in the 1940’s. AI is based on the idea that the response of a speech communication system can be divided into twenty frequency bands, each of which carries an independent contribution to the intelligibility of the system, and that the total contribution of all the bands is the sum of the contributions of the individual bands. (AI may also be measured using one-third octave or octave bands.) Signal-to-noise ratios are computed for each individual band, then weighted and combined to yield an intelligibility score. The AI varies in value from 0 (completely unintelligible) to 1 (perfect intelligibility). An AI of 0.3 or below is considered unsatisfactory, 0.3 to 0.5 satisfactory, 0.5 to 0.7 good, and greater than 0.7 very good to excellent. Return to

A measure for rating speech intelligibility, where an AI 0.05 is representative of very poor speech intelligibility, and an AI 0.80 represents good speech intelligibility. The Speech Privacy Index (PI) is derived from the AI.

Having to do with the intelligibility of speech. This is measured using listeners who try to identify randomly presented 'nonsense' words and phrases. The Articulation Index is the percentage of correct identifications. Used mainly in large venues.