SIST EN IEC 60268-16:2021
(Main)Sound system equipment - Part 16: Objective rating of speech intelligibility by speech transmission index (IEC 60268-16:2020)
Sound system equipment - Part 16: Objective rating of speech intelligibility by speech transmission index (IEC 60268-16:2020)
This part of IEC 60268 defines the STI model, test signals, measurement and prediction
methods.
The objective of this document is to provide a comprehensive manual for all types of users of
the STI model in the fields of audio, communications and acoustics.
This document does not provide STI criteria for certification of transmission channels (e.g.
criteria for a voice-alarm system), but some typical application values are provided in Annex G.
Every measurement method has limitations, and the reader is referred to clauses relating to
limitations such as speech privacy, echo and systems using digital voice compression
(vocoders).
This document does not cover the case of fluctuating noise on the STI, although some general
comments on dealing with this complex issue are provided in 7.13 and 8.9.3.
Elektroakustische Geräte - Teil 16: Objektive Bewertung der Sprachverständlichkeit durch den Sprachübertragungsindex (IEC 60268-16:2020)
Equipements pour systèmes électroacoustiques - Partie 16: Evaluation objective de l'intelligibilité de la parole au moyen de l'indice de transmission de la parole (IEC 60268-16:2020)
L'IEC 60268-16:2020 définit le modèle, les signaux d'essai, la mesure et les méthodes de prévision STI.
L'objectif du présent document est de mettre un manuel complet à la disposition de tous les types d'utilisateurs du modèle STI dans les domaines de l'audio, des communications et de l'acoustique.
Le présent document ne donne pas de critères STI destinés à la certification des canaux de transmission (par exemple, critères pour un système d'alarme vocale), mais certaines valeurs d'application courante sont fournies dans l'Annexe G.
Chaque méthode de mesure présente ses limites, et le lecteur est prié de se référer aux articles correspondants, par exemple concernant la confidentialité des conversations, l'écho et les systèmes qui utilisent la compression vocale numérique (vocodeurs).
Le présent document ne couvre pas les cas du bruit fluctuant sur le STI, bien que certains commentaires généraux qui traitent de cette question complexe soient fournis en 7.13 et 8.9.3.
L'IEC 60268-16:2020 annule et remplace la quatrième édition parue en 2011. Cette édition constitue une révision technique.
Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) le spectre du signal d'essai de la parole masculine a été modifié pour appliquer des réductions significatives au niveau des bandes de 125 Hz et 250 Hz;
b) certaines formules ont été corrigées;
c) des informations supplémentaires relatives aux modes opératoires de prédiction et de mesure ont été incluses;
d) le spectre et les facteurs de pondération pour la parole féminine ont été supprimés;
e) des informations de vérification pour les dispositifs de mesure STI ont été ajoutées;
f) les relations entre le STI et plusieurs autres mesures de l'intelligibilité de la parole ont été mises à jour en Annexe E;
g) des informations supplémentaires sont données en Annexe M concernant les ajustements des résultats STI mesurés afin de simuler les effets d'autres niveaux de bruit ambiant et de parole.
Oprema za zvokovne sisteme - 16. del: Objektivno ocenjevanje govorne razumljivosti z uporabo indeksa prenosa govora (IEC 60268-16:2020)
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN IEC 60268-16:2021
01-julij-2021
Nadomešča:
SIST EN 60268-16:2011
Oprema za zvokovne sisteme - 16. del: Objektivno ocenjevanje govorne
razumljivosti z uporabo indeksa prenosa govora (IEC 60268-16:2020)
Sound system equipment - Part 16: Objective rating of speech intelligibility by speech
transmission index (IEC 60268-16:2020)
Elektroakustische Geräte - Teil 16: Objektive Bewertung der Sprachverständlichkeit
durch den Sprachübertragungsindex (IEC 60268-16:2020)
Equipements pour systèmes électroacoustiques - Partie 16: Evaluation objective de
l'intelligibilité de la parole au moyen de l'indice de transmission de la parole (IEC 60268-
16:2020)
Ta slovenski standard je istoveten z: EN IEC 60268-16:2020
ICS:
33.160.30 Avdio sistemi Audio systems
SIST EN IEC 60268-16:2021 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN IEC 60268-16:2021
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SIST EN IEC 60268-16:2021
EUROPEAN STANDARD EN IEC 60268-16
NORME EUROPÉENNE
EUROPÄISCHE NORM
November 2020
ICS 33.160.01 Supersedes EN 60268-16:2011 and all of its
amendments and corrigenda (if any)
English Version
Sound system equipment - Part 16: Objective rating of speech
intelligibility by speech transmission index
(IEC 60268-16:2020)
Équipements pour systèmes électroacoustiques - Partie 16: Elektroakustische Geräte - Teil 16: Objektive Bewertung der
Évaluation objective de l'intelligibilité de la parole au moyen Sprachverständlichkeit durch den Sprachübertragungsindex
de l'indice de transmission de la parole (IEC 60268-16:2020)
(IEC 60268-16:2020)
This European Standard was approved by CENELEC on 2020-10-30. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60268-16:2020 E
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SIST EN IEC 60268-16:2021
EN IEC 60268-16:2020 (E)
European foreword
The text of document 100/3202/CDV, future edition 5 of IEC 60268-16, prepared by IEC/TC 100
"Audio, video and multimedia systems and equipment" was submitted to the IEC-CENELEC parallel
vote and approved by CENELEC as EN IEC 60268-16:2020.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2021-07-30
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2023-10-30
document have to be withdrawn
This document supersedes EN 60268-16:2011 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Endorsement notice
The text of the International Standard IEC 60268-16:2020 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards
indicated:
IEC 60318-1:2009 NOTE Harmonized as EN 60318-1:2009 (not modified)
IEC 61672-1 NOTE Harmonized as EN 61672-1
IEC 60118-4 NOTE Harmonized as EN 60118-4
ISO 9921:2003 NOTE Harmonized as EN ISO 9921:2003 (not modified)
ISO/TR 22411:2008 NOTE Harmonized as CEN ISO/TR 22411:2011 (not modified)
ISO 3382-1:2009 NOTE Harmonized as EN ISO 3382-1:2009 (not modified)
2
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SIST EN IEC 60268-16:2021
EN IEC 60268-16:2020 (E)
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 61260-1 2014 Electroacoustics - Octave-band and fractional- EN 61260-1 2014
octave-band filters - Part 1: Specifications
3
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SIST EN IEC 60268-16:2021
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SIST EN IEC 60268-16:2021
IEC 60268-16
®
Edition 5.0 2020-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Sound system equipment –
Part 16: Objective rating of speech intelligibility by speech transmission index
Équipements 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
INTERNATIONALE
ICS 33.160.01 ISBN 978-2-8322-8862-7
Warning! Make sure that you obtained this publication from an authorized distributor.
Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
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SIST EN IEC 60268-16:2021
– 2 – IEC 60268-16:2020 © IEC 2020
CONTENTS
FOREWORD . 7
INTRODUCTION . 9
1 Scope . 12
2 Normative references . 12
3 Terms and definitions . 12
4 Description of the STI model. 18
4.1 Overview. 18
4.2 Applicability of the STI model . 19
4.3 Theoretical details . 19
4.3.1 Envelope function and envelope spectrum . 19
4.3.2 Reduction of modulation . 20
4.3.3 Role of the octave-band noise carriers . 20
4.3.4 Theoretical overview . 20
4.4 Measurement of STI . 22
4.4.1 Direct and indirect methods . 22
4.4.2 Full STI . 23
4.4.3 STIPA . 23
4.4.4 Choice of method . 23
5 Direct method of measuring STI – User guidance . 25
5.1 Overview. 25
5.2 STIPA . 26
5.3 Application . 26
5.4 Limitations . 26
6 Indirect method of measuring STI (impulse response) – User guidance . 27
6.1 Overview. 27
6.2 Application . 27
6.3 Limitations (non-linear distortion) . 28
7 Limitations of the STI model . 29
7.1 General . 29
7.2 Frequency shifts . 30
7.3 Centre clipping . 30
7.4 Dropouts . 30
7.5 Jitter . 30
7.6 Digital voice compression systems . 30
7.7 Overestimation of STI under low background noise conditions . 31
7.8 Frequency response . 31
7.9 Echoes . 32
7.10 Fast amplitude compression and expansion . 34
7.11 Non-linear distortion. 35
7.12 Hearing impaired listeners . 35
7.13 Impulsive and fluctuating noise . 35
7.14 Conclusion . 35
8 Measurement procedures, post-processing of data and applications . 36
8.1 General . 36
8.2 Acoustical input . 36
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IEC 60268-16:2020 © IEC 2020 – 3 –
8.3 Acoustical output . 38
8.4 Electrical input . 38
8.5 Electrical output . 38
8.6 Examples of input/output combinations . 38
8.6.1 Acoustical input – Acoustical output . 38
8.6.2 Electrical input – Electrical output (e.g. assessment of wired and
wireless) communication systems) . 38
8.6.3 Acoustical input – Electrical output (e.g. assessment of microphones) . 38
8.6.4 Electrical input – Acoustical output (e.g. assessment of PA systems) . 39
8.7 Spatial averaging of STI measurements . 39
8.8 Post-processing of measured MTF data . 40
8.9 Issues concerning noise . 40
8.9.1 General . 40
8.9.2 Measurement of background noise . 41
8.9.3 Fluctuating noise . 41
8.10 Analysis and interpretation of the results. 41
8.11 Binaural STI measurements . 42
9 Use of the STI as a design prediction tool . 42
9.1 Overview. 42
9.2 Statistical predictions . 43
9.3 Prediction from simulated impulse response . 43
Annex A (informative) The basis of the STI concept . 44
A.1 Introduction to this annex . 44
A.1.1 Purpose . 44
A.1.2 Modulation transfer function (MTF) . 44
A.1.3 STI model . 45
A.1.4 STI modulation frequencies . 46
A.2 Calculation of the STI . 46
A.2.1 General equation for STI . 46
A.2.2 Gender-specific octave band weighting and redundancy factors . 47
A.2.3 Adjustment of the MTF for ambient noise . 48
A.2.4 Adjustment of the MTF for auditory masking and threshold effects . 48
A.3 Calculation of the modulation transfer ratio values . 49
A.3.1 Direct method: Analysis of the STI test signal . 49
A.3.2 Indirect method: Determination of the modulation transfer
function (MTF) . 50
A.4 Auditory effects on the STI . 51
A.4.1 Overview . 51
A.4.2 Level-dependent auditory masking. 51
A.4.3 Absolute speech reception threshold . 54
A.5 Generation of the STI test signal (direct method) . 54
A.5.1 Pink noise source signal . 54
A.5.2 Generating octave band carrier signals . 54
A.5.3 Intensity modulation of the carrier signals . 55
A.5.4 Applying the speech spectrum to the STI test signal . 55
A.6 Spectrum of STI test signal . 55
A.6.1 Standardized speech spectrum . 55
A.6.2 Speech-shaped noise . 55
Annex B (normative) STIPA method . 57
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B.1 Overview. 57
B.2 Test signal . 57
Annex C (normative) Verification of STI measuring devices . 59
C.1 Specification of the measuring device . 59
C.2 Signals for testing STI implementations . 59
C.3 Testing the dynamic range in the modulation domain . 59
C.3.1 General . 59
C.3.2 Modulation depth testing for STIPA direct method . 59
C.3.3 Modulation depth testing for STI indirect method . 60
C.4 Testing of cross-talk between octave-band filters . 61
C.4.1 Flank attenuation slopes . 61
C.4.2 Octave band filter testing – STIPA direct method . 61
C.4.3 Performance verification files . 62
Annex D (informative) Use of STI measuring devices . 63
D.1 Overview. 63
D.2 STIPA characterises only the speech transmission channel . 63
D.3 Examples of test scenarios for STIPA tests . 64
D.4 Equipment and resources needed for a STIPA test . 67
D.4.1 Availability of the test signal . 67
D.4.2 A source of the STIPA test signal . 67
D.4.3 A STIPA analyser . 67
D.5 Steps in the overall procedure . 67
Annex E (informative) Qualification of the STI and relationships with other speech
intelligibility measures . 68
E.1 Relationship between the STI and word/sentence scores . 68
E.2 Relationship between STI and listening difficulty . 68
Annex F (informative) Nominal qualification bands for STI . 70
Annex G (informative) Examples of STI qualification bands and typical applications . 71
Annex H (informative) Non-native listeners . 72
Annex I (informative) Effect of age-related hearing loss and hearing impairment on
speech intelligibility . 73
Annex J (normative) Setting and adjustment of STI test signal level . 74
J.1 Overview. 74
J.2 The concept of 'speech level' and the method of measurement . 74
J.3 Real speech level . 74
J.4 Corrected speech level derived from real speech level . 75
J.5 Comparison of dynamic structures of speech and test signals . 75
Annex K (informative) Example test report sheet for STI measurements . 77
Annex L (normative) Prediction of the STI using statistical methods . 79
Annex M (informative) Adjustments to STI data to simulate alternative ambient noise
spectra and different speech levels . 81
Annex N (informative) Other methods of determining speech intelligibility . 91
N.1 Overview. 91
N.2 Word tests . 91
N.3 Modified rhyme tests . 91
N.4 Speech intelligibility index (SII) . 92
N.5 PESQ . 92
Annex O (informative) Alternative direct methods for measuring Full STI . 93
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IEC 60268-16:2020 © IEC 2020 – 5 –
Annex P (normative) Information to be provided by manufacturers . 94
P.1 Purpose of this annex . 94
P.2 Form in which the information is to be provided . 94
P.3 Required information . 94
P.4 Declaration . 94
Annex Q (informative) Effect of uncertainties of selected parameters on STI
uncertainty . 95
Q.1 STI calculation framework . 95
Q.1.1 Overview . 95
Q.1.2 Statistical MTF. 95
Q.1.3 Corrections . 95
Q.1.4 Effective SNR . 96
Q.1.5 Modulation transfer index (MTI) . 96
Q.1.6 Speech transmission index (STI) . 96
Q.2 The effect of RT uncertainty on STI uncertainty . 97
Q.2.1 General . 97
Q.2.2 Modulation transfer function. 97
Q.2.3 Uncertainty in the STI . 97
Q.2.4 Conclusions: . 99
Q.3 The effect of S/N uncertainty on STI uncertainty . 99
Q.3.1 General . 99
Q.3.2 Ideal transfer function . 99
Q.3.3 Reverberation . 100
Q.3.4 Conclusions: . 101
Q.4 The effect of signal level uncertainty on STI uncertainty . 101
Q.4.1 Overview . 101
Q.4.2 Auditory masking . 101
Q.4.3 Conclusions . 103
Bibliography . 104
Figure 1 – Envelope function (panel A) of a 10 s speech signal for the 250 Hz octave
band and corresponding envelope spectrum (panel B) . 20
Figure 2 – Modulation transfer function – Input/output comparison . 21
Figure 3 – Effect of a single delayed arrival on the MTF (idealised conditions) . 33
Figure 4 – Idealised STI (Male speech Spectrum) versus delay and level of secondary
arrival . 34
Figure A.1 – Theoretical expression of the MTF . 44
Figure A.2 – Measurement system and frequencies for the STI method . 46
Figure A.3 – Auditory masking of octave band (k–1) on octave band (k) . 52
Figure A.4 – Relationship between STI and speech level for different reverberation
times. 53
Figure D.1 – Schematic representation of the definition of a speech transmission
channel . 64
Figure E.1 – Relationships between some speech intelligibility measures . 68
Figure E.2 – Relationship between STI, speech intelligibility scores and listening
difficulty ratings [43], [44] . 69
Figure F.1 – STI qualification bands . 70
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Figure Q.1 – Uncertainty in absolute value of STI vs reverberation time RT with
various degrees of uncertainty in RT . 99
Figure Q.2 – Uncertainty in absolute value of STI vs reverberation time RT with 1 dB
uncertainty in SNR at various SNRs . 101
Figure Q.3 – Uncertainty in absolute value of STI versus reverberation time RT with
various degrees of masking. .
...
SLOVENSKI STANDARD
oSIST prEN IEC 60268-16:2019
01-maj-2019
Oprema za zvokovne sisteme - 16. del: Objektivno ocenjevanje govorne
razumljivosti z uporabo indeksa prenosa govora
Sound system equipment - Part 16: Objective rating of speech intelligibility by speech
transmission index
Elektroakustische Geräte - Teil 16: Objektive Bewertung der Sprachverständlichkeit
durch den Sprachübertragungsindex
Equipements pour systèmes électroacoustiques - Partie 16: Evaluation objective de
l'intelligibilité de la parole au moyen de l'indice de transmission de la parole
Ta slovenski standard je istoveten z: prEN IEC 60268-16:2019
ICS:
33.160.30 Avdio sistemi Audio systems
oSIST prEN IEC 60268-16:2019 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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oSIST prEN IEC 60268-16:2019
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oSIST prEN IEC 60268-16:2019
100/3202/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 60268-16 ED5
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2019-03-08 2019-05-31
SUPERSEDES DOCUMENTS:
100/3012/CD,100/3171/CC
IEC TA 20 : ANALOGUE AND DIGITAL AUDIO
SECRETARIAT: SECRETARY:
Japan Mr Gen Ichimura
OF INTEREST TO THE FOLLOWING COMMITTEES: PROPOSED HORIZONTAL STANDARD:
Other TC/SCs are requested to indicate their interest, if
any, in this CDV to the secretary.
FUNCTIONS CONCERNED:
EMC ENVIRONMENT QUALITY ASSURANCE SAFETY
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of
which they are aware and to provide supporting documentation.
TITLE:
Sound system equipment - Part 16: Objective rating of speech intelligibility by speech
transmission index
PROPOSED STABILITY DATE: 2022
NOTE FROM TC/SC OFFICERS:
Copyright © 2019 International Electrotechnical Commission, IEC. All rights reserved. It is permitted to download this
electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without
permission in writing from IEC.
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oSIST prEN IEC 60268-16:2019
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1 CONTENTS
2
3 FOREWORD . 7
4 INTRODUCTION . 9
5 1 Scope . 12
6 2 Normative references . 12
7 3 Terms and definitions . 12
8 4 Description of the STI model . 17
9 4.1 Overview . 17
10 4.1.1 Applicability of the STI model . 18
11 4.2 Theoretical details . 18
12 4.2.1 Envelope function and envelope spectrum . 18
13 4.2.2 Reduction of modulation . 19
14 4.2.3 Role of the octave-band noise carriers . 19
15 4.2.4 Theoretical overview . 20
16 4.3 Measurement of STI. 22
17 4.3.1 Direct and Indirect Methods . 22
18 4.3.2 FULL STI . 22
19 4.3.3 STIPA . 23
20 4.3.4 Choice of method . 23
21 5 Direct method of measuring STI – User guidance . 25
22 5.1 Overview . 25
23 5.2 STIPA . 25
24 5.3 Application . 26
25 5.4 Limitations . 26
26 6 Indirect method of measuring STI (impulse response) – User guidance . 26
27 6.1 Overview . 26
28 6.2 Application . 27
29 6.3 Limitations (non-linear distortion) . 28
30 7 Limitations of the STI model . 28
31 7.1 General . 28
32 7.2 Frequency shifts . 29
33 7.3 Centre clipping . 29
34 7.4 Dropouts . 30
35 7.5 Jitter . 30
36 7.6 Digital voice compression systems . 30
37 7.7 Overestimation of STI under low background noise conditions . 30
38 7.8 Frequency response . 30
39 7.9 Echoes . 31
40 7.10 Fast amplitude compression and expansion . 33
41 7.11 Non-linear distortion . 34
42 7.12 Hearing impaired listeners . 34
43 7.13 Impulsive and fluctuating noise . 34
44 7.14 Conclusion . 35
45 8 Measurement procedures, post-processing of data and applications . 35
46 8.1 General . 35
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47 8.2 Acoustical input . 35
48 8.3 Acoustical output . 37
49 8.4 Electrical input . 37
50 8.5 Electrical output . 37
51 8.6 Examples of input/output combinations . 37
52 8.6.1 Acoustical input – Acoustical output . 37
53 8.6.2 Electrical input – Electrical output (e.g. assessment of wired and
54 wireless) communication systems) . 38
55 8.6.3 Acoustical input – Electrical output (e.g. assessment of microphones) . 38
56 8.6.4 Electrical input – Acoustical output (e.g. assessment of PA systems) . 38
57 8.7 Spatial averaging of STI measurements . 38
58 8.8 Post-processing of measured MTF data . 39
59 8.9 Issues concerning noise. 40
60 8.9.1 General . 40
61 8.9.2 Measurement of background noise . 40
62 8.9.3 Fluctuating noise . 40
63 8.10 Analysis and interpretation of the results . 41
64 8.11 Binaural STI measurements . 41
65 9 Use of STI as a design prediction tool . 42
66 9.1 Overview . 42
67 9.2 Statistical predictions . 42
68 9.3 Prediction from simulated impulse response . 42
69 Annex A (informative) The basis of the STI concept . 44
70 A.1 Introduction to this Annex . 44
71 A.1.1 Purpose . 44
72 A.1.2 Modulation transfer function (MTF) . 44
73 A.1.3 STI model . 45
74 A.1.4 STI modulation frequencies . 46
75 A.2 Calculation of the STI . 46
76 A.2.1 General Equation for STI . 46
77 A.2.2 Gender-specific octave band weighting and redundancy factors . 47
78 A.2.3 Adjustment of the MTF for ambient noise . 48
79 A.2.4 Adjustment of the MTF for auditory masking and threshold effects . 48
80 A.3 Calculation of the modulation transfer ratio values . 49
81 A.3.1 Direct method: Analysis of the STI test signal . 49
82 A.3.2 Indirect method: Determination of the modulation transfer function (MTF)
83 50
84 A.4 Auditory effects on the STI . 51
85 A.4.1 Overview . 51
86 A.4.2 Level-dependent auditory masking . 51
87 A.4.3 Absolute speech reception threshold . 53
88 A.5 Generation of the STI test signal (direct method) . 54
89 A.5.1 Pink noise source signal . 54
90 A.5.2 Generating octave band carrier signals . 54
91 A.5.3 Intensity modulation of the carrier signals . 54
92 A.5.4 Applying the speech spectrum to the STI test signal . 55
93 A.6 Spectrum of STI test signal . 55
94 A.6.1 Speech shaped noise . 55
95 Annex B (normative) STIPA method . 57
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96 B.1 Overview . 57
97 B.2 Test signal . 57
98 Annex C (normative) . 59
99 C.1 Specification of the measuring device . 59
100 C.2 Signals for testing STI implementations . 59
101 C.3 Testing the dynamic range in the modulation domain . 59
102 C.3.1 Modulation depth testing for STIPA direct method . 59
103 C.3.2 Modulation depth testing for STI indirect method . 60
104 C.4 Testing of cross-talk between octave-band filters . 61
105 C.4.1 Flank attenuation slopes . 61
106 C.4.2 Octave band filter testing - STIPA direct method . 61
107 C.4.3 Performance verfication files . 62
108 Annex D (informative) Use of STI Measuring Devices . 63
109 D.1 Overview . 63
110 D.2 STIPA characterises only the speech transmission channel . 63
111 D.3 Examples of test scenarios for STIPA tests . 64
112 D.4 Equipment and resources needed for a STIPA test . 66
113 D.4.1 Availability of the test signal . 66
114 D.4.2 A source of the STIPA test signal . 67
115 D.5 Steps in the overall procedure . 67
116 Annex E (informative) Qualification of the STI and relationships with other speech
117 intelligibility measures . 68
118 E.1 Relationship between STI and word/sentence scores . 68
119 E.2 Relationship between STI and listening difficulty . 68
120 Annex F (informative) Nominal qualification bands for STI . 70
121 Annex G (informative) Examples of STI qualification bands and typical applications . 71
122 Annex H (informative) Non-native listeners . 72
123 Annex I (informative) Effect of age-related hearing loss and hearing impairment on
124 speech intelligibility . 73
125 Annex J (normative) Setting & adjustment of STI test signal level . 74
126 J.1 Overview . 74
127 J.2 The concept of 'speech level' and the method of measurement . 74
128 J.3 Real speech level . 74
129 J.4 Corrected speech level derived from real speech level . 75
130 J.5 Comparison of dynamic structures of speech and test signals . 75
131 Annex K (informative) Example test report sheet for STI measurements . 76
132 Annex L (normative) Prediction of STI using statistical methods . 78
133 Annex M (informative) Adjustments to STI data to simulate alternative ambient noise
134 spectra and different speech levels . 80
135 Annex N (informative) Other methods of determining speech intelligibility . 88
136 N.1 Overview . 88
137 N.2 Word tests . 88
138 N.3 Modified rhyme tests . 88
139 N.4 Speech Intelligibility Index (SII) . 88
140 N.5 PESQ . 89
141 Annex O (informative) Alternative direct methods for measuring Full STI . 90
142 Annex P (normative) Information to be provided by manufacturers . 91
143 P.1 Purpose of this Annex . 91
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144 P.2 Form in which the information is to be provided . 91
145 P.3 Required information . 91
146 P.4 Declaration . 91
147 Annex Q (informative) Effect of uncertainties of selected parameters on STI
148 uncertainty . 92
149 Q.1 STI calculation framework . 92
150 Q.1.1 Overview . 92
151 Q.1.2 Statistical MTF . 92
152 Q.1.3 Corrections . 92
153 Q.1.4 Effective SNR . 93
154 Q.1.5 Modulation Transfer Index MTI . 93
155 Q.1.6 Speech Transmission Index STI . 93
156 Q.2 The effect of RT uncertainty on STI uncertainty . 93
157 Q.2.1 Modulation Transfer Function . 93
158 Q.2.2 Uncertainty in STI . 93
159 Q.2.3 Conclusions: . 95
160 Q.3 The effect of S/N uncertainty on STI uncertainty . 95
161 Q.3.1 Ideal transfer function . 95
162 Q.3.2 Reverberation . 96
163 Q.3.3 Conclusions: . 97
164 Q.4 The effect of signal level uncertainty on STI uncertainty. 97
165 Q.4.1 Overview . 97
166 Q.4.2 Auditory masking . 97
167 Q.4.3 Conclusions: . 98
168 Bibliography . 100
169
170 Figure 1– Envelope function (panel A) of a 10 s speech signal for the 250 Hz octave
171 band and corresponding envelope spectrum (panel B) . 19
172 Figure 2 – Modulation transfer function – Input/output comparison . 21
173 Figure 3 – Effect of a single delayed arrival on the MTF (idealised conditions) . 32
174 Figure 4 – Idealised STI (Male speech Spectrum) versus delay and level of secondary
175 arrival . 33
176 Figure A.1 – Theoretical expression of the MTF . 44
177 Figure A.2 – Measurement system and frequencies for the STI method . 46
178 Figure A.4 – Auditory masking of octave band (k–1) on octave band (k) . 52
179 Figure D.1. Schematic representation of the definition of a speech transmission
180 channel. . 64
181 Figure E.1 Relationships between some speech intelligibility measures . 68
182 Figure E.2 – Relationship between STI, speech intelligibility scores and listening
183 difficulty ratings [34], [35] . 69
184 Figure F.1 – STI qualification bands . 70
185 Figure Q.1 - Uncertainty in absolute value of STI vs reverberation time RT with various
186 degrees of uncertainty in RT. . 95
187 Figure Q.2 - Uncertainty in absolute value of STI vs reverberation time RT with 1 dB
188 uncertainty in SNR at various SNRs. . 97
189 Figure Q.3 - Uncertainty in absolute value of STI versus reverberation time RT with
190 various degrees of masking. . 98
191
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192 Table 1 How to use this standard . 10
193 Table 2 – Comparison of direct and indirect methods . 22
194 Table 3 – Suitability of STI test methods for different types of distortion . 23
195 Table 4 – Test-method suitability . 24
196 Table 5 – Measurement applications . 24
197 Table A.1 – MTI octave band weighting factors . 48
198 Table A.1 – Auditory masking as a function of the octave band level. 53
199 Table A.2 – Absolute speech reception threshold level in octave bands . 54
200 Table A.4 – Octave band levels (dB) relative to the A-weighted speech level . 55
201 Table A.5 – Filter parameters and s-plane polynomials that produce speech-shaped
202 pink noise. . 56
203 Table B.1 – Modulation frequencies for the STIPA method . 57
204 Table C 1 Specification of an STI measuring device . 59
205 Table D.1 Scenario 1. PA with “live” announcer . 64
206 Table D.2 Scenario 2. PA with pre-recorded announcements . 65
207 Table D.3 Scenario 3. “Live” meetings and conversations . 65
208 Table D.4 Scenario 4. Lecture . 66
209 Table E.1 – Categories for listening difficulty . 69
210 Table G.1. – Examples between STI qualification bands and typical applications . 71
211 Table H.1. – Adjusted intelligibility qualification tables for non-native listeners . 72
212 Table I.1. – Adjusted intelligibility qualification tables for normal listeners and people
213 over 60 years old with hearing loss . 73
214 Table J.2 Comparison of speech and the test signal . 75
215 Table K.1 Example test report sheet . 76
216 Table K.2 Measurement data record sheet . 77
217 Table M.1. - Flow chart of post-processing adjustment steps . 80
218 Table M.2 Example calculation . 84
219
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220 INTERNATIONAL ELECTROTECHNICAL COMMISSION
221 ____________
222
223 SOUND SYSTEM EQUIPMENT –
224
225 Part 16: Objective rating of speech intelligibility
226 by speech transmission index
227
228 FOREWORD
229 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
230 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
231 international co-operation on all questions concerning standardization in the electrical and electronic fields. To
232 this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
233 Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
234 Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
235 in the subject dealt with may participate in this preparatory work. International, governmental and non-
236 governmental organizations liaising with the IEC
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