Speech Articulation Index Calculator

The Articulation Index, defined in the ANSI S3.5 framework, is a number from 0 to 1 that predicts the proportion of speech cues audible above noise. AI near 0 means speech is unintelligible; AI near 1 means essentially all speech information is available. It is the predecessor of the modern Speech Intelligibility Index and is widely used in hearing science and room acoustics.

A common simplified estimate maps the effective speech-to-noise ratio onto a 30 dB dynamic range: AI = (SNR + 12) / 30, clamped between 0 and 1. The peaks of speech sit about 12 dB above the long-term average and the useful dynamic range of speech cues spans about 30 dB. So an average SNR of +18 dB gives AI = 1, and an SNR of -12 dB gives AI = 0.

AI correlates with word and sentence recognition: roughly, AI of 0.3 supports limited communication, AI of 0.5 supports fair intelligibility, and AI above 0.7 supports excellent intelligibility for everyday speech. The exact mapping depends on the speech material and the listener.

Speech energy is not constant; its instantaneous peaks rise about 12 dB above the long-term average level, and the range from the faint speech valleys to the loud peaks spans roughly 30 dB. The AI model credits intelligibility in proportion to how much of that 30 dB band sits above the noise floor, which is why the formula adds 12 and divides by 30.

No. STI is a different, modulation-based metric that also accounts for reverberation and distortion, measured on a 0 to 1 scale. The Articulation Index here is a simpler steady-noise model based on the speech-to-noise ratio across the speech dynamic range. Both aim to predict intelligibility but use different methods.