In audiology, the Articulation Index (AI) is a tool used to predict the amount of speech that is audible to a patient with a specific hearing loss.
The calculation is also used in industrial settings for the design of safety devices, such as flight helmets, where audio signals are required to be clearly heard.
The AI is often used as a counseling tool since it presents an individual's hearing loss in terms of percentage of speech that is audible during a typical one-on-one conversation.
In aerospace settings the Speech Intelligibility Index (SII), published in 1986 by the American National Standards Institute, is a major revision of the AI standard and defines computational methods "that produce results highly correlated with the intelligibility of speech under a variety of adverse listening conditions, such as noise masking, filtering and reverberation".
[1] Amlani, A., Punch, J., and Ching, T. (2002), "Methods and Applications of the Audibility Index in Hearing Aid Selection and Fitting" in Trends in Amplification, 6(3), pp.