Scale
STI is a numeric representation measure of communication channel characteristics whose value varies from 0 = bad to 1 = excellent. On this scale, an STI of at least .5 is desirable for most applications.
Barnett (1995, 1999) proposed to use a reference scale, the Common Intelligibility Scale (CIS), based on a mathematical relation with STI (CIS = 1 + log (STI)).
STI predicts the likelihood of syllables, words and sentences being comprehended. As an example, for native speakers, this likelihood is given by:
| STI Value | Quality according to IEC 60268-16 | Intelligibility of Syllables in % | Intelligibility of Words in % | Intelligibility of Sentences in % |
|---|---|---|---|---|
| 0 - 0.3 | bad | 0 - 34 | 0 - 67 | 0 - 89 |
| 0.3 - 0.45 | poor | 34 - 48 | 67 - 78 | 89 - 92 |
| 0.45 - 0.6 | fair | 48 - 67 | 78 - 87 | 92 - 95 |
| 0.6 - 0.75 | good | 67 - 90 | 87 - 94 | 95 - 96 |
| 0.75 - 1 | excellent | 90 - 96 | 94 - 96 | 96 - 100 |
If non-native speakers, people with speech disorders or hard-of-hearing people are involved, other probabilities hold.
It is interesting but not astonishing that STI prediction is independent of the language spoken - not astonishing, as the ability of the channel to transport patterns of physical speech is measured.
Another method is defined for computing a physical measure that is highly correlated with the intelligibility of speech as evaluated by speech perception tests given a group of talkers and listeners. This measure is called the Speech Intelligibility Index, or SII.
Read more about this topic: Speech Transmission Index
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