IEC 60268-16:2003

INTERNATIONAL IEC
STANDARD
60268-16
Third edition
2003-05
Sound system equipment –
Part 16:
Objective rating of speech intelligibility
by speech transmission index
Equipements pour systèmes électroacoustiques –
Partie 16:
Evaluation objective de l'intelligibilité de la parole
au moyen de l'indice de transmission de la parole
Reference number
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INTERNATIONAL IEC
STANDARD
60268-16
Third edition
2003-05
Sound system equipment –
Part 16:
Objective rating of speech intelligibility
by speech transmission index
Equipements pour systèmes électroacoustiques –
Partie 16:
Evaluation objective de l'intelligibilité de la parole
au moyen de l'indice de transmission de la parole
 IEC 2003  Copyright - all rights reserved
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Международная Электротехническая Комиссия
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– 2 – 60268-16  IEC:2003(E)
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references. 6
3 Definitions and abbreviations. 6
4 Description of the methods . 7
4.1 General . 7
4.2 The STI method. 8
4.3 The STITEL method. 9
4.4 The STIPA method .10
4.5 The RASTI method .10
4.6 Methods of measurement.13
5 Methods of determining intelligibility .15
5.1 Word tests .15
5.2 Modified rhyme tests.15
5.3 Speech Intelligibility Index .15
5.4 Articulation loss of consonants .15
Annex A (normative) Speech transmission index (STI) and revised (STI ) methods .16
r
A.1 Background .16
A.2 The STI method .19
A.3 The test signals .23
Annex B (informative) The STITEL method.24
Annex C (informative) The STIPA method .25
Annex D (informative) The RASTI method .26
Annex E (informative) Qualification of the STI and relation with some subjective
intelligibility measures.27
Bibliography.28
Figure 1 – Modulation transfer function: input/output comparison . 7
Figure 2 – Relationship between the theoretical STI by the RASTI method and the STI
measured by a proprietary equipment with a measurement time of 12 s approximately .11
Figure 3 – Conditions under which RASTI results do not differ by more than 0,05 .12
Figure A.1 – Envelope function (panel A) of a 10 s speech signal for the 250 Hz
octave band and corresponding envelope spectrum (panel B) .16
Figure A.2 – Theoretical expression of the MTF .18
Figure A.3 – The measurement system and frequencies for the STI method.19
Figure A.4 – Auditory masking strength of octave band (k – 1) on that above (k).20
Figure A.5 – The relationship between effective signal-to-noise ratio and transmission
index for a shift of 15 dB and a range of 30 dB.22
Figure D.1 – Illustration of a practical RASTI test signal.26
Figure E.1 – Qualification of the STI and relation with some subjective intelligibility
measures .27

60268-16  IEC:2003(E) – 3 –
Table A.1 – Octave level specific slope of masking and corresponding auditory
masking factor (amf) .21
Table A.2 – STI octave band specific male and female weighting factors .23
r
Table A.3 – Octave band levels (dB) relative to the A-weighted long-term speech level.23
Table B.1 – STITEL: modulation frequencies for the seven octave bands.24
Table C.1 – STIPA: modulation frequencies for the seven octave bands .25
Table C.2 – STI octave band specific male and female weighting factors adopted
r
to STIPA .25

– 4 – 60268-16  IEC:2003(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SOUND SYSTEM EQUIPMENT –
Part 16: Objective rating of speech intelligibility
by speech transmission index
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The 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 systems and equipment.
This third edition cancels and replaces the second edition, published in 1998. This third
edition constitutes a technical revision.
The text of this standard is based on the following documents:
FDIS Report on voting
100/650/FDIS 100/677/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

60268-16  IEC:2003(E) – 5 –
The committee has decided that the contents of this publication will remain unchanged until
2005. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
A bilingual edition of this standard may be issued at a later date

– 6 – 60268-16  IEC:2003(E)
SOUND SYSTEM EQUIPMENT –
Part 16: Objective rating of speech intelligibility
by speech transmission index
1 Scope
This part of IEC 60268 defines objective methods for rating the transmission quality of speech
with respect to intelligibility. The four methods, which are closely related, are referred to as
the “STI,” the “STITEL”, the “STIPA” and the “RASTI” methods (see Clause 3). The methods
are intended for rating speech transmission with or without sound systems.
A survey of other methods of determining or predicting speech intelligibility is also included,
together with a method of correlating the results of different methods of determination.
2 Normative references
The following referenced documents are indispensable for the application 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.
ISO 4870:1991, Acoustics – The construction and calibration of speech intelligibility tests
ITU-T Recommendation P.51:1996, Artificial mouth
3 Definitions and abbreviations
For the purpose of this document, the following definitions apply.
3.1
speech transmission index (STI)
physical quantity representing the transmission quality of speech with respect to intelligibility
3.2
speech transmission index for telecommunication systems (STITEL)
index obtained by a condensed version of the STI method but still responsive to distortions
found in communication systems
3.3
speech transmission index for public address systems (STIPA)
index obtained by a condensed version of the STI method but still responsive to distortions
found in room acoustics including public address systems
3.4
room acoustics speech transmission index (RASTI)
index obtained by a condensed version of the STI method, to be used for screening
purposes and focused on direct communication between persons without making use of a
communication system. RASTI accounts for noise interference and distortions in the time
domain (echoes, reverberation)

60268-16  IEC:2003(E) – 7 –
4 Description of the methods
4.1 General
The methods can be used to compare speech transmission quality at various positions and for
various conditions within the same listening space, in particular for assessing the effect of
changes in the acoustic properties. This includes effects from the presence of an audience or
1)
of changes in any sound system [1] .The methods are also able to predict the absolute rating
of the speech transmission qualit
...