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

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.

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

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.

General Information

Status
Published
Publication Date
24-Sep-2020
Current Stage
PPUB - Publication issued
Completion Date
25-Sep-2020
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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
IEC 60268-16:2020-09(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
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

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---------------------- Page: 3 ----------------------
– 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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– 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

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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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– 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
...

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