Chapter 2.0 Essential measurments
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
Notes
- These values should be regarded as minimum target values.
- Perceieved intelligibility relating to each category will also depend on the frequency response at each listening position.
- The SII values refer to measured values in sample listening positions or as required by specific apllication standards.
