2.2.2 Speech Intelligibility
Speech intelligibility is a measure of the quality of the transmission of speech. Speech intelligibility is particularly important in public spaces where occupants need to be able to clearly hear and understand instructions, whether the instructions are coming from a person in the same room or via an electronic public address or voice alarm system. For instance, this could apply to classrooms, auditoriums, churches, conference rooms, concert halls, airports, trains stations and shopping centres.
Speech intelligibility is calculated according to a standard index using acoustical measurements of speech and noise. A number of factors influence speech intelligibility, including ambient noise level, reverberation time, the frequency response of a room, psychoacoustic masking effects, as well as the quality of any sound reproduction equipment being used to transmit sound in the space.
The Speech intelligibility index (SII) is represented on a numeric scale called the Common Intelligibility Scale (CIS). The value ranges from 0 to 1, or bad to excellent, and indicates the degree to which a space, aka transmission channel, degrades speech intelligibility.)
The Speech Intelligibility Index (SII)
| Category | Nominal STI value | Type of message information | Examples of typical usage | Comment |
|---|---|---|---|---|
| A+ | 0,76 | Recording studios | Excellent intelligibility but rarely achievable in most environments | |
| A | 0,74 | Complex messages, unfamiliar words | Theatres, Speech auditoria, parlaments, courts, Assistive Hearing Systems (AHS) | High speech intelligibility |
| B | 0,7 | Complex messages, unfamiliar words | Theatres, Speech auditoria, parlaments, courts, Assistive Hearing Systems (AHS) | High speech intelligibility |
| C | 0,66 | Complex messages, unfamiliar words | Theatres, Speech auditoria, teleconferencing, parlaments, courts | High speech intelligibility |
| D | 0,62 | Complex messages, familiar words | Lecture theatres, classrooms, concert halls | Good speech intelligibility |
| E | 0,58 | Complex messages, familiar context | Concert halls, modern churches | High quality PA systems |
| F | 0,54 | Complex messages, familiar context | PA systems in shopping malls, public building offices, VA systems, cathedrals | Good quality PA systems |
| G | 0,5 | Complex messages, familiar context | Shopping malls, public building offices, VA systems | Target value for VA systems |
| H | 0,46 | Simple messages, familiar words | VA and PA systems in difficult acoustic environments | Normal lower limit for VA systems |
| I | 0,42 | Simple messages, familiar context | VA and PA systems in very difficult spaces | |
| J | 0,38 | Not suitable for PA systems | ||
| U | <0,36 | Not suitable for PA systems |
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