The Articulation Index is a single-number metric that predicts the intelligibility of speech in the presence of background noise. Originally defined in ANSI S3.5, it quantifies the proportion of speech cues that are audible above the masking noise across critical frequency bands.
Speech intelligibility depends on the signal-to-noise ratio (SNR) across the frequency bands that carry speech information. The AI is computed as a weighted sum of the effective SNR contributions across these bands:
where wi is the importance weighting for band i (reflecting how much that band contributes to speech understanding), and Ai is the effective audibility factor for that band.
The audibility factor for each band is derived from the SNR in that band, clipped to the range [0, 1]:
An SNR of −12 dB or below yields Ai = 0 (fully masked). An SNR of +30 dB or above yields Ai = 1 (fully audible). The 42 dB dynamic range corresponds to the useful range of speech-to-noise ratios for intelligibility.
The importance weights wi reflect the concentration of speech information across frequency. The bands centered around 1–3 kHz receive the highest weights because consonant discrimination depends primarily on this range.
| Band Center (Hz) | Weight (wi) | Speech Content |
|---|---|---|
| 250 | 0.0024 | Vowel fundamentals, voicing cues |
| 500 | 0.0048 | First formant of most vowels |
| 1000 | 0.0074 | Second formant, nasal consonants |
| 2000 | 0.0109 | Fricatives, plosive bursts |
| 4000 | 0.0078 | Sibilants (/s/, /z/), high-frequency consonants |
| 8000 | 0.0032 | Upper harmonics, breathiness |
| AI Value | Intelligibility | Description |
|---|---|---|
| 0.7 – 1.0 | Excellent | Virtually all speech cues are audible. Normal conversation is fully intelligible. |
| 0.5 – 0.7 | Good | Most speech is understood. Some effort required in difficult listening conditions. |
| 0.3 – 0.5 | Fair | Significant portions of speech are masked. Listeners frequently ask for repetition. |
| 0.1 – 0.3 | Poor | Only familiar phrases are recognized. Continuous communication is impractical. |
| 0.0 – 0.1 | Unintelligible | Speech is essentially inaudible in the noise. |
| Parameter | Options | Description |
|---|---|---|
| Input Channel | Any signal channel | Channel carrying the combined speech-plus-noise or noise-only signal. |
| Noise Spectrum | Measured / Reference | Source of the background noise spectrum for SNR computation. |
| Speech Level | Normal (60 dB), Raised (65 dB), Loud (75 dB) | Assumed speech effort level at 1 m distance. |
| Calculation Standard | ANSI S3.5, Modified | Selection of weighting and band definition. |
| Frequency Range | 200 Hz – 8 kHz | Analysis bandwidth for band decomposition. |
In automotive NVH testing, the Articulation Index is tracked across the engine speed range to identify RPM conditions where cabin noise most severely impacts speech communication. The AI vs RPM curve reveals critical speeds where masking is worst, helping engineers target specific operating conditions for noise reduction.
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