The Book of Acoustics

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

  1. These values should be regarded as minimum target values.
  2. Perceieved intelligibility relating to each category will also depend on the frequency response at each listening position.
  3. The SII values refer to measured values in sample listening positions or as required by specific apllication standards.
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The Book of Acoustics

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