IEC 60268-16:2011

Sound system equipment - Part 16: Objective rating of speech intelligibility by speech transmission index

IEC 60268-16:2011 specifies objective methods for rating the transmission quality of speech with respect to intelligibility. It provides a comprehensive manual for all types of users of the STI method in the fields of audio, communications and acoustics. Three methods are presented, which are closely related and are referred to as STI, STIPA, and STITEL. The first two methods are intended for rating speech transmission performance with or without sound systems. The STITEL method has more restricted uses. This fourth edition cancels and replaces the third edition, published in 2003, and constitutes a technical revision. It includes the following significant technical changes with respect to the previous edition:
- development of more comprehensive, complete and unambiguous standardization of the STI methodology;
- the term STIr is discontinued. A new function for the prediction of auditory masking effects is introduced;
- the concept of 'speech level' and the setting of the level of the test signal have been introduced;
- additional information has been included on prediction and measurement procedures. The French version of this standard has not been voted upon.

Equipements pour systèmes électroacoustiques - Partie 16: Evaluation objective de l'intelligibilité de la parole au moyen de l'indice de transmission de la parole

La CEI 60268-16:2011 spécifie des méthodes objectives d'évaluation de la qualité de transmission de la parole sous l'angle de l'intelligibilité. Elle met un manuel complet à la disposition de tous les types d'utilisateurs de la méthode STI dans les domaines de l'audio, des communications et de l'acoustique. Trois méthodes, étroitement liées, sont présentées sous le nom de STI, STIPA, et STITEL. Les deux premières méthodes sont destinées à l'évaluation des performances de la transmission de la parole avec ou sans systèmes électroacoustiques. La méthode STITEL a des utilisations plus restreintes. Cette quatrième édition annule et remplace la troisième édition parue en 2003, et constitue une révision technique. Elle inclut les modifications techniques importantes suivantes par rapport à l'édition précédente:
- Le développement d'une normalisation plus aboutie, plus complète et moins ambiguë de la méthodologie STI;
- le terme STIr n'est plus utilisé. Une nouvelle fonction pour la prédiction des effets du masquage auditif est introduite;
- le concept de "niveau de parole" et le réglage du niveau du signal d'essai ont été introduits;
- des informations supplémentaires relatives aux modes opératoires de prédiction et de mesures ont été incluses. La version française de cette norme n'a pas été soumise au vote.

General Information

• Status: Published
• Publication Date: 27-Jun-2011
• ICS: 01 - GENERALITIES. TERMINOLOGY. STANDARDIZATION. DOCUMENTATION
• ICS: 33.160.01 - Audio, video and audiovisual systems in general
• Technical Committee: TA 20 - Analogue and digital audio
• Current Stage: DELPUB - Deleted Publication
• Completion Date: 25-Sep-2020

Relations

• Revised: IEC 60268-16:2020 - Sound system equipment - Part 16: Objective rating of speech intelligibility by speech transmission index
• Effective Date: 05-Sep-2023
• Revises: IEC 60268-16:2003 - Sound system equipment - Part 16: Objective rating of speech intelligibility by speech transmission index
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 60268-16 ®
Edition 4.0 2011-06
INTERNATIONAL
STANDARD
Sound system equipment –
Part 16: Objective rating of speech intelligibility by speech transmission index

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form

or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester.
If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication,

please contact the address below or your local IEC member National Committee for further information.

IEC Central Office
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About the IEC
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International Standards for all electrical, electronic and related technologies.

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IEC 60268-16 ®
Edition 4.0 2011-06
INTERNATIONAL
STANDARD
Sound system equipment –
Part 16: Objective rating of speech intelligibility by speech transmission index

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
XB
ICS 33.160.01 ISBN 978-2-88912-522-7

– 2 – 60268-16  IEC:2011(E)
CONTENTS
FOREWORD . 5

INTRODUCTION . 7

1 Scope . 9

2 Normative references . 9

3 Terms and definitions . 9

4 Description of the STI method . 15

4.1 General . 15

4.1.1 Rationale for the STI method . 15
4.1.2 Applicability of the STI method . 15
4.2 Background of the STI method . 16
4.2.1 General . 16
4.2.2 Theoretical overview . 17
4.2.3 Measurement of STI . 18
4.3 Applicability of STI test methods . 19
4.4 Use of direct and indirect methods . 20
4.5 Limitations of the STI method . 21
4.5.1 General . 21
4.5.2 Frequency shifts . 21
4.5.3 Centre clipping . 22
4.5.4 Drop outs . 22
4.5.5 Jitter . 22
4.5.6 Vocoders . 22
4.5.7 Overestimation of STI under low background noise conditions . 22
4.5.8 Frequency response . 22
4.5.9 Echoes . 23
4.5.10 Fast amplitude compression and expansion . 23
4.5.11 Non-linear distortion . 24
4.5.12 Impulsive and fluctuating noise . 24
4.5.13 Hearing impaired listeners . 24
4.6 Conclusion . 24
5 Direct method of measuring STI . 24
5.1 Overview . 24

5.2 STIPA . 25
5.3 Application . 26
5.4 Limitations . 26
6 Indirect method of measuring STI using the impulse response . 26
6.1 Overview . 26
6.2 Application . 27
6.3 Limitations (non-linear distortion) . 28
7 Measurement procedures, post-processing of data and applications . 28
7.1 General . 28
7.2 Acoustical input . 28
7.3 Acoustical output . 30
7.4 Electrical input . 30
7.5 Electrical output . 30
7.6 Examples of input/output combinations. 30

60268-16  IEC:2011(E) – 3 –
7.6.1 Acoustical input – Acoustical output . 30

7.6.2 Electrical input – Electrical output (e.g. assessment of wired and

wireless) communication systems) . 30

7.6.3 Acoustical input – Electrical output (e.g. assessment of microphones) . 31

7.6.4 Electrical input – Acoustical output (e.g. assessment of PA systems) . 31

7.7 Post-processing of measured MTF data . 31

7.8 Issues concerning noise . 31

7.8.1 General . 31

7.8.2 Measurement of background noise . 32

7.8.3 Fluctuating noise . 32

7.9 Analysis and interpretation of the results . 32
7.10 Binaural STI measurements . 33
8 Use of STI as a design prediction tool . 33
8.1 Overview . 33
8.2 Statistical predictions . 33
8.3 Prediction from simulated impulse response . 34
Annex A (normative) Speech transmission index (STI) and revised STI methods . 35
Annex B (normative) STIPA method . 47
Annex C (normative) STITEL method . 48
Annex D (informative) RASTI method (obsolete) . 49
Annex E (informative) Qualification of the STI and relationships with other speech
intelligibility measures. 51
Annex F (informative) Nominal qualification bands for STI . 53
Annex G (informative)  Examples of STI qualification bands and typical applications . 54
Annex H (informative) Non-native listeners . 55
Annex I (informative) Effect of age-related hearing loss and hearing impairment on
speech intelligibility. 56
Annex J (normative) Calibration of STI test signal level . 57
Annex K (informative) Example test report sheet for STI measurements . 59
Annex L (normative) Prediction of STI using statistical methods . 61
Annex M (informative) Adjustments to measured STI and STIPA results for simulation
of occupancy noise and different speech levels . 63
Annex N (informative) Other methods of determining speech intelligibility . 67
Bibliography . 69

Figure 1 – Concept of the reduction in modulation due to a transmission channel . 16
Figure 2 – Modulation transfer function – Input/output comparison . 17
Figure A.1 – Envelope function (panel A) of a 10 s speech signal for the 250 Hz
octave band and corresponding envelope spectrum (panel B) . 35
Figure A.2 – Theoretical expression of the MTF . 36
Figure A.3 – Measurement system and frequencies for the STI method . 38
Figure A.4 – Auditory masking of octave band (k – 1) on octave band (k) . 39
Figure D.1 – Illustration of a practical RASTI test signal . 50
Figure E.1 – Relationships between some speech intelligibility measures . 51
Figure E.2 – Relationship between STI, speech intelligibility scores and listening
difficulty ratings [34], [35] . 52
Figure F.1 – STI qualification bands . 53

– 4 – 60268-16  IEC:2011(E)
Table 1 – Comparison of STI test methods for different types of distortion . 19

Table 2 – Applicability of test . 20

Table 3 – Choice of method . 21

Table A.1 – Auditory masking as a function of the octave band level. 40

Table A.2 – Absolute speech reception threshold level in octave bands . 41

Table A.3 – MTI octave band weighting factors . 41

Table A.4 – Octave band levels (dB) relative to the A-weighted speech level . 42

Table B.1 – Modulation frequencies for the STIPA method . 47
Table C.1 – Modulation frequencies for the STITEL method . 48
Table D.1
...

IEC 60268-16 ®
Edition 4.0 2011-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Sound system equipment –
Part 16: Objective rating of speech intelligibility by speech transmission index

Equipements pour systèmes électroacoustiques –
Partie 16: Evaluation objective de l'intelligibilité de la parole au moyen de l'indice
de transmission de la parole
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by
any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either IEC or
IEC's member National Committee in the country of the requester.
If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication,
please contact the address below or your local IEC member National Committee for further information.

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About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigenda or an amendment might have been published.
 Catalogue of IEC publications: www.iec.ch/searchpub
The IEC on-line Catalogue enables you to search by a variety of criteria (reference number, text, technical committee,…).
It also gives information on projects, withdrawn and replaced publications.
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The world's leading online dictionary of electronic and electrical terms containing more than 20 000 terms and definitions
in English and French, with equivalent terms in additional languages. Also known as the International Electrotechnical
Vocabulary online.
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IEC 60268-16 ®
Edition 4.0 2011-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Sound system equipment –
Part 16: Objective rating of speech intelligibility by speech transmission index

Equipements pour systèmes électroacoustiques –
Partie 16: Evaluation objective de l'intelligibilité de la parole au moyen de l'indice
de transmission de la parole
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XB
ICS 33.160.01 ISBN 978-2-88912-672-9

– 2 – 60268-16  IEC:2011
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 9
4 Description of the STI method . 15
4.1 General . 15
4.1.1 Rationale for the STI method . 15
4.1.2 Applicability of the STI method . 15
4.2 Background of the STI method . 16
4.2.1 General . 16
4.2.2 Theoretical overview . 17
4.2.3 Measurement of STI . 18
4.3 Applicability of STI test methods . 19
4.4 Use of direct and indirect methods . 20
4.5 Limitations of the STI method . 21
4.5.1 General . 21
4.5.2 Frequency shifts . 21
4.5.3 Centre clipping . 22
4.5.4 Drop outs . 22
4.5.5 Jitter . 22
4.5.6 Vocoders . 22
4.5.7 Overestimation of STI under low background noise conditions . 22
4.5.8 Frequency response . 22
4.5.9 Echoes . 23
4.5.10 Fast amplitude compression and expansion . 23
4.5.11 Non-linear distortion . 24
4.5.12 Impulsive and fluctuating noise . 24
4.5.13 Hearing impaired listeners . 24
4.6 Conclusion . 24
5 Direct method of measuring STI . 24
5.1 Overview . 24
5.2 STIPA . 25
5.3 Application . 26
5.4 Limitations . 26
6 Indirect method of measuring STI using the impulse response . 26
6.1 Overview . 26
6.2 Application . 27
6.3 Limitations (non-linear distortion) . 28
7 Measurement procedures, post-processing of data and applications . 28
7.1 General . 28
7.2 Acoustical input . 28
7.3 Acoustical output . 30
7.4 Electrical input . 30
7.5 Electrical output . 30
7.6 Examples of input/output combinations. 30

60268-16  IEC:2011 – 3 –
7.6.1 Acoustical input – Acoustical output . 30
7.6.2 Electrical input – Electrical output (e.g. assessment of wired and
wireless) communication systems) . 30
7.6.3 Acoustical input – Electrical output (e.g. assessment of microphones) . 31
7.6.4 Electrical input – Acoustical output (e.g. assessment of PA systems) . 31
7.7 Post-processing of measured MTF data . 31
7.8 Issues concerning noise . 31
7.8.1 General . 31
7.8.2 Measurement of background noise . 32
7.8.3 Fluctuating noise . 32
7.9 Analysis and interpretation of the results . 32
7.10 Binaural STI measurements . 33
8 Use of STI as a design prediction tool . 33
8.1 Overview . 33
8.2 Statistical predictions . 33
8.3 Prediction from simulated impulse response . 34
Annex A (normative) Speech transmission index (STI) and revised STI methods . 35
Annex B (normative) STIPA method . 48
Annex C (normative) STITEL method . 49
Annex D (informative) RASTI method (obsolete) . 50
Annex E (informative) Qualification of the STI and relationships with other speech
intelligibility measures. 52
Annex F (informative) Nominal qualification bands for STI . 54
Annex G (informative)  Examples of STI qualification bands and typical applications . 55
Annex H (informative) Non-native listeners . 56
Annex I (informative) Effect of age-related hearing loss and hearing impairment on
speech intelligibility. 57
Annex J (normative) Calibration of STI test signal level . 58
Annex K (informative) Example test report sheet for STI measurements . 60
Annex L (normative) Prediction of STI using statistical methods . 62
Annex M (informative) Adjustments to measured STI and STIPA results for simulation
of occupancy noise and different speech levels . 64
Annex N (informative) Other methods of determining speech intelligibility . 68
Bibliog
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