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)
IEC 60268-16:2020 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.
IEC 60268-16:2020 cancels and replaces the fourth edition published in 2011. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous edition:
a) the spectrum of the male speech test signal has been changed, with significant reductions in the 125 Hz and 250 Hz bands being implemented;
b) some corrections to formulae have been made;
c) additional information has been included on prediction and measurement procedures;
d) spectrum and weighting factors for female speech have been removed;
e) verification information for STI measurement devices added;
f) the relationships between STI and number of other speech intelligibility measures have been updated in Annex E;
g) greater information is given in Annex M about adjustments to the measured STI results to simulate effects of alternative ambient noise and speech levels.
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
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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
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)
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
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
– 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
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
– 4 – IEC 60268-16:2020 © IEC 2020
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
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
– 6 – IEC 60268-16:2020 © IEC 2020
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. . 103
Table 1 – How to use this document . 10
Table 2 – Comparison of direct and indirect methods . 22
Table 3 – Suitability of STI test methods for different types of distortion . 24
Table 4 – Test-method suitability . 24
Table 5 – Measurement applications . 25
Table A.1 – MTI octave band weighting factors . 48
Table A.2 – Auditory masking as a function of the octave band level . 53
Table A.3 – Absolute speech reception threshold level in octave bands . 54
Table A.4 – Octave band levels (dB) relative to the A-weighted speech level . 55
Table A.5 – Filter parameters and s-plane polynomials that produce speech-shaped
pink noise. . 56
Table B.1 – Modulation frequencies for the STIPA method . 57
Table C.1 – Specification of an STI measuring device . 59
Table D.1 – Scenario 1, PA with "live" announcer . 65
Table D.2 – Scenario 2, PA with pre-recorded announcements . 65
Table D.3 – Scenario 3, "live" meetings and conversations . 66
Table D.4 – Scenario 4, lecture . 66
Table E.1 – Categories for listening difficulty . 69
Table G.1 – Examples between STI qualification bands and typical applications . 71
Table H.1 – Adjusted intelligibility qualification tables for non-native listeners . 72
Table I.1 – Adjusted intelligibility qualification tables for normal listeners and people
over 60 years old with hearing loss . 73
Table J.1 – Typical speech and test signal dynamics . 75
Table J.2 – Comparison of speech and the test signal . 76
Table K.1 – Example test report sheet . 77
Table K.2 – Measurement data record sheet . 78
Table M.1 – Flow chart of post-processing adjustment steps . 82
Table M.2 – Example calculation. 87
IEC 60268-16:2020 © IEC 2020 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SOUND SYSTEM EQUIPMENT –
Part 16: Objective rating of speech intelligibility
by speech transmission index
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60268-16 has been prepared by IEC technical committee 100: Audio,
video and multimedia equipment and systems.
This fifth edition cancels and replaces the fourth edition published in 2011. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) the spectrum of the male speech test signal has been changed, with significant reductions
in the 125 Hz and 250 Hz bands being implemented;
b) some corrections to formulae have been made;
c) additional information has been included on prediction and measurement procedures;
d) spectrum and weighting factors for female speech have been removed;
e) verification information for STI measurement devices added;
f) the relationships between STI and number of other speech intelligibility measures have been
updated in Annex E;
– 8 – IEC 60268-16:2020 © IEC 2020
g) greater information is given in Annex M about adjustments to the measured STI results to
simulate effects of alternative ambient noise and speech levels.
NOTE See Introduction for a historical summary listing the various changes from the first to the fifth edition (current
edition).
The text of this International Standard is based on the following documents:
CDV Report on voting
100/3202/CDV 100/3422/RVC
Full information on the voting for the approval of this International Standard can be found in the
report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60268 series, published under the general title Sound system
equipment, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.
IEC 60268-16:2020 © IEC 2020 – 9 –
INTRODUCTION
Speech is considered to be the major method of communication between humans. In many
situations, the speech signal is degraded by the signal path or the transmission channel
between talker and listener, resulting in a reduction of the intelligibility of the speech at the
listener’s location.
To quantify the deterioration of the speech intelligibility induced by the transmission channel, a
fast and objective measuring method was developed; the Speech Transmission Index (STI).
The STI method applies a specific test signal to the transmission channel and by analysing the
received test signal; the speech transmission quality of the channel is derived and expressed
in a value between 0 and 1, as the Speech Transmission Index (STI). Using the obtained STI-
value, the potential speech intelligibility can be determined.
Although there are limitations to the STI method, the use of STI has proved useful in many
situations and has gained international acceptance.
The STI method has been the subject of ongoing development and refinement since its
introduction in the 1970s. Major improvements of the STI have been consolidated by
incorporating them in successive revisions of IEC 60268-16.
To avoid misinterpretation of STI results, it is important that all users of the STI understand the
basic principles behind the operation of the STI, the application domain and the limitations. This
document provides substantial information to assist users.
Potential applications of the STI
The STI can be used to measure the potential intelligibility of a wide range of electronic systems
and acoustic environments. Typical applications include:
• measurement of public address and sound reinforcement systems;
• measurement and certification of emergency sound and communication systems;
• measurement of communication channels and systems such as intercoms and wireless
communication;
• measurement of potential speech intelligibility and communication in rooms and auditoria;
• evaluation of direct speech communication (situations without electronic amplification) in
rooms or acoustic spaces, including vehicles;
• evaluation of the potential intelligibility of assistive hearing systems.
NOTE The STI method was not designed for the measurement and evaluation of speech privacy or speech masking
systems and, therefore, has not been validated for these situations. It is not recommended to use the STI below 0,3,
but if this is to be undertaken, specialist expertise and techniques beyond the scope of this standard are required.
Potential users of STI
The range of users of STI measurements is diverse. Among the users who might apply this
method are:
• certifiers of voice alarm and other types of emergency systems;
• certifiers of sound reinforcement and audio systems;
• audio and telecommunication equipment manufacturers;
• audio and communication engineers;
• acoustic and electroacoustic consultants;
• sound system installers;
• researchers into STI methods and developers of instruments to measure the STI.
– 10 – IEC 60268-16:2020 © IEC 2020
Table 1 summarises which sections of the document may apply to different users and
applications.
Table 1 – How to use this document
Purpose Topic Clauses
All users Introduction to the STI method
Routine check of voice-alarm or sound system
Direct method of measuring STI 4
with STIPA
Description of the STI method 5
Direct method of measuring STI 4 and 5
Indirect method of measuring STI using the
4 and 6
impulse response
Measurement procedures, and applications 8
In-depth check of or to certify sound system
Post-processing of measured MTF data 8.8
with STIPA and/or impulse response methods
Limitations of the measurement methods 5.4, 6.3
Optional: Theory and equations governing STI Annex A and
methods Annex B
Optional: Relationship between subjective and
Annex F
objective measures of intelligibility
Optional: Measurement uncertainties Annex Q
Measure telecommunication equipment Direct method only 8.6.2
Annex A and
Theory and equations governing STI methods
Annex B
Manufacturer of STIPA device Verification of STI measurement device
Annex C
performance
Information to be provided Annex D
Theory and equations governing STI methods Annex A
Manufacturer of acoustical analyser and
Calibration of STI instruments Annex C
simulation software
Information to be provided Annex P
Annex A and
Research into intelligibility Theory and equations governing STI methods
Annex B
Using simulation software Prediction methods Annex M
Post processing measurement results Annex M
Post processing of STI and STIPA Optional – As per in-depth measurements of
measurement STI listed above
Optional -Worked calculation example Annex M
As per in-depth measurements of STI listed
above
Evaluation of the potential intelligibility of
Assistive Listening Systems
Special process for Assistive Listening
8.6.3
Systems
Revision history
The history of revisions is as follows:
• Revision 1: 1988. In the first version of the STI standard, a gender-independent test signal
spectrum was used.
• Revision 2: 1998. Gender-specific test signals were introduced, for male and female talkers,
each gender relating to a specific set of weighting factors. In addition, weightings were
introduced for redundancy factors. The term STI was introduced to signify the use of these
r
redundancy factors.
IEC 60268-16:2020 © IEC 202
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