IEC 60268-4:2018

Sound system equipment - Part 4: Microphones

IEC 60268-4:2018 specifies methods of measurement for the electrical impedance, sensitivity, directional response pattern, dynamic range and external influences of sound system microphones, and also details the characteristics to be specified by the manufacturer. It applies to sound system microphones for all applications for speech and music. It does not apply to measurement microphones, but it does apply to each audio channel of microphones having more than one channel, for example for stereo or similar use. It is also applicable to flush-mounted microphones and to the analogue characteristics of microphones with digital audio output. For the purposes of this International Standard, a microphone includes all such devices as transformers, pre-amplifiers, or other elements that form an integral part of the microphone, up to the output terminals specified by the manufacturer. The major characteristics of a microphone are considered in Clauses 6 to 21. Additional characteristics are considered in Annex A and Annex C. IEC 60268-4:2018 cancels and replaces the fifth edition published in 2014. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
a) subclause 19.4 on "pop" measurement replaces Annex C;
b) new Annex D for noise measurements in the digital domain.

Équipements pour systèmes électroacoustiques - Partie 4: Microphones

IEC 60268-4:2018 spécifie les méthodes de mesure portant sur l’impédance électrique, l’efficacité, le diagramme de directivité, la dynamique, et les perturbations extérieures des microphones pour les systèmes électroacoustiques, et décrit également les caractéristiques à spécifier par le fabricant.
Elle s’applique aux microphones pour les systèmes électroacoustiques appartenant aux applications relatives à la parole et à la musique. Elle ne s’applique pas aux microphones de mesure, mais elle s’applique à chaque voie audio des microphones multivoies, par exemple pour une utilisation stéréophonique ou similaire. Elle est également applicable aux microphones à montage encastré et aux caractéristiques analogiques des microphones à sortie audio numérique.
Pour les besoins de la présente Norme internationale, un microphone comprend tous les dispositifs, tels que les transformateurs, préamplificateurs ou autres éléments faisant partie intégrante du microphone, jusqu’aux bornes de sortie spécifiées par le fabricant.
Les principales caractéristiques d’un microphone sont examinées dans les Articles 6 à 21. Des caractéristiques supplémentaires sont étudiées dans l’Annexe A et l’Annexe C
IEC 60268-4:2018 annule et remplace la cinquième édition parue en 2014. Cette édition constitue une révision technique.
Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) le paragraphe 19.4 relatif à la mesure de l’effet "pop" remplace l’Annexe C;
b) nouvelle Annexe D pour les mesures du bruit dans le domaine numérique.

General Information

• Status: Published
• Publication Date: 11-Sep-2018
• ICS: 33.160.50 - Accessories
• Technical Committee: TA 20 - Analogue and digital audio
• Drafting Committee: MT 60268-4 - TC 100/TA 20/MT 60268-4
• Current Stage: PPUB - Publication issued
• Start Date: 12-Sep-2018
• Completion Date: 31-Aug-2018

Relations

• Revises: IEC 60268-4:2014 - Sound system equipment - Part 4: Microphones
• Effective Date: 05-Sep-2023

Overview

IEC 60268-4:2018 - Sound system equipment - Part 4: Microphones - is the international measurement standard that defines how to characterize and specify sound‑system microphones for speech and music. This 2018 (Edition 6.0) technical revision replaces the 2014 edition and specifies measurement methods and the set of microphone characteristics manufacturers should provide. It applies to each audio channel of multi‑channel microphones, flush‑mounted microphones and the analogue characteristics of microphones with digital outputs. Measurement microphones are explicitly excluded.

Key topics and technical requirements

The standard prescribes methods of measurement and the characteristics to be specified, covering major subjects (Clauses 6–21) and additional items in annexes:

• Electrical impedance - internal and rated impedance and methods of measurement
• Sensitivity - free‑field, diffuse‑field, near‑field/close‑talking and pressure sensitivities; rated sensitivity reporting
• Frequency response and effective frequency range - point‑by‑point and continuous sweep methods, calibration and graphical presentation
• Directional characteristics - polar/directional response patterns and directivity index measurements
• Dynamic range and non‑linearity - total harmonic distortion, harmonic orders and difference‑frequency distortion
• Limiting characteristics - rated maximum permissible peak sound pressure and overload conditions
• Noise - equivalent sound pressure due to inherent noise (Annex D adds noise measurements in the digital domain)
• External influences - mechanical vibration, wind noise, and transient “pop” effects (subclause 19.4 replaces previous Annex C)
• EMC and immunity - electromagnetic compatibility, immunity criteria and testing for preserving programme quality
• Environmental and physical conditions - ambient pressure, temperature, humidity ranges, connectors, reference point/axis, and pre‑conditioning requirements

The standard also defines measurement environments (free‑field, diffuse field, small cavity couplers), calibration approaches, and reporting/graphical presentation rules.

Practical applications and who uses it

IEC 60268-4:2018 is used by:

• Microphone manufacturers for R&D, product specification, datasheets and compliance testing
• Acoustic and audio test laboratories to produce reproducible, comparable measurements
• System integrators, broadcasters and live‑sound engineers to select microphones for speech or music applications
• Procurement and quality assurance teams to verify performance claims and acceptance testing

Practical uses include component selection, design verification, product comparison, certification and ensuring reliable operation under real‑world environmental and EMC conditions.

Related standards

• Part of the broader IEC 60268 series (sound system equipment); consult other parts of the series for loudspeaker, amplifiers and system‑level test methods.

Standards Content (Sample)

IEC 60268-4 ®
Edition 6.0 2018-09
INTERNATIONAL
STANDARD
Sound system equipment –
Part 4: Microphones
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IEC 60268-4 ®
Edition 6.0 2018-09
INTERNATIONAL
STANDARD
Sound system equipment –
Part 4: Microphones
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.160.50 ISBN 978-2-8322-5955-9

– 2 – IEC 60268-4:2018 © IEC 2018
CONTENTS
FOREWORD . 6
1 Scope . 8
2 Normative references. 8
3 Terms and definitions . 9
4 General conditions . 10
4.1 General . 10
4.2 Measurement conditions . 10
4.2.1 General . 10
4.2.2 Rated conditions . 11
5 Particular conditions . 12
5.1 Pre-conditioning . 12
5.2 Sound source . 12
5.3 Measurement of sound pressure . 12
5.4 Voltage measuring system . 12
5.5 Acoustical environment . 12
5.5.1 General . 12
5.5.2 Free-field conditions . 13
5.5.3 Diffuse field conditions . 14
5.5.4 Microphone coupled to a sound source by means of a small cavity
coupler . 15
5.6 Methods of measuring frequency response . 15
5.6.1 Point-by-point and continuous sweep frequency methods . 15
5.6.2 Calibration methods . 16
5.7 Overall accuracy . 16
5.8 Graphical presentation of results . 16
6 Type description (acoustical behaviour) . 17
6.1 Principle of the transducer . 17
6.2 Type of microphone . 17
6.3 Type of directional response characteristics . 17
6.4 Application . 17
7 Terminals and controls. 17
7.1 Marking . 17
7.2 Connectors and electrical interface values . 17
8 Reference point and axis . 18
8.1 Reference point . 18
8.2 Reference axis . 18
9 Rated power supply . 18
9.1 Characteristics to be specified . 18
9.2 Method of measurement . 18
10 Electrical impedance . 18
10.1 Internal impedance . 18
10.1.1 Characteristic to be specified . 18
10.1.2 Methods of measurement . 18
10.2 Rated impedance . 19
10.3 Rated minimum permitted load impedance . 19

11 Sensitivity . 19
11.1 General . 19
11.2 Sensitivities with respect to acoustical environment . 20
11.2.1 Free-field sensitivity . 20
11.2.2 Diffuse-field sensitivity . 21
11.2.3 Close-talking and near-field sensitivity . 21
11.2.4 Pressure sensitivity . 22
11.3 Rated sensitivity . 22
12 Response . 22
12.1 Frequency response . 22
12.1.1 Characteristic to be specified . 22
12.1.2 Method of measurement . 23
12.1.3 Graphical presentation of results . 23
12.2 Effective frequency range . 23
12.2.1 Characteristic to be specified . 23
12.2.2 Method of measurement . 23
13 Directional characteristics . 23
13.1 Directional pattern . 23
13.1.1 Characteristic to be specified . 23
13.1.2 Methods of measurement . 24
13.1.3 Graphical presentation of results . 25
13.2 Directivity index . 25
13.2.1 Characteristic to be specified . 25
13.2.2 Method of measurement . 25
14 Amplitude non-linearity . 25
14.1 General . 25
14.2 Total harmonic distortion . 25
14.2.1 Characteristic to be specified . 25
14.2.2 Method of measurement . 26
th
14.3 Harmonic distortion of the n order (n = 2, 3,.) . 26
14.3.1 Characteristic to be specified . 26
14.3.2 Method of measurement . 26
14.4 Difference frequency distortion of second order . 27
14.4.1 Characteristic to be specified . 27
14.4.2 Method of measurement . 27
15 Limiting characteristics . 28
15.1 Rated maximum permissible peak sound pressure . 28
15.2 Overload sound pressure . 28
15.2.1 Characteristic to be specified . 28
15.2.2 Method of measurement . 28
16 Balance . 28
16.1 Balance of the microphone output . 28
16.2 Balance under working conditions . 29
17 Equivalent sound pressure level due to inherent noise . 29
17.1 Characteristic to be specified . 29
17.2 Method of measurement . 29
18 Ambient conditions . 30
18.1 General . 30

– 4 – IEC 60268-4:2018 © IEC 2018
18.2 Pressure range . 30
18.3 Temperature range . 30
18.4 Relative humidity range . 30
19 External influences . 30
19.1 General . 30
19.1.1 Specification and methods of measurement . 30
19.1.2 Other external interferences . 31
19.2 Equivalent sound pressure due to mechanical vibration . 31
19.2.1 Characteristic to be specified . 31
19.2.2 Method of measurement . 31
19.3 Equivalent sound pressure due to wind . 31
19.3.1 Characteristic to be specified . 31
19.3.2 Method of measurement . 32
19.4 Transient equivalent sound pressure due to "pop" effect . 35
19.4.1 General . 35
19.4.2 Characteristic to be specified . 35
19.4.3 Method of measurement . 36
20 Electromagnetic compatibility (EMC) . 37
20.1 Regulatory requirements . 37
20.2 Requirements for preserving programme quality . 37
20.3 Performance criteria . 38
20.3.1 Criterion A . 38
20.3.2 Criterion B . 38
20.4 Testing for immunity to disturbances in the presence of acoustical noise . 39
20.5 Immunity to frequency-modulated radiated disturbances . 39
20.6 Immunity to magnetic fields . 39
20.7 Immunity to ripple on d.c. power supply . 39
20.8 Permanent magnetic field . 40
20.9 Evaluation and reporting of the test results . 40
21 Physical characteristics . 40
21.1 Dimensions . 40
21.2 Weight . 40
21.3 Cables and connectors . 40
22 Classification of the characteristics to be specified . 40
Annex A (normative) Additional characteristics . 43
A.1 Characteristic sensitivity for speech . 43
A.1.1 Characteristic to be specified . 43
A.1.2 Method of measurement . 43
A.2 Front-to-rear sensitivity index (0° – 180°) . 44
A.2.1 Characteristic to be specified . 44
A.2.2 Method of measurement . 44
A.3 Noise-cancelling index . 44
A.3.1 Characteristic to be specified . 44
A.3.2 Method of measurement . 44
A.4 Special characteristics for stereo microphones . 45
A.4.1 General . 45
A.4.2 Included angle of an XY (left-right) microphone . 45
A.4.3 Acceptance angle . 45

Annex B (informative) Sound insulation device . 46
Annex C (informative) Recommendations for professional digital microphones . 47
C.1 General . 47
C.2 Data sheets for digital microphones . 47
Annex D (informative) Recommended method for measuring noise levels according to
ITU-R BS.468-4 in the digital domain . 50
D.1 General . 50
D.2 Recommended method . 50
D.3 Matlab code . 51
Bibliography . 54

Figure 1 – Balance of the output . 28
Figure 2 – Balance under working conditions . 29
Figure 3 – Measurement set-up for wind influence . 32
Figure 4 – Wind generators, type 1 (Figure 4a) and type 2 (Figure 4b) . 34
Figure 5 – Electrical and mechanical setup for the measuring of the "pop" effect . 36
Figure B.1 – Sound insulation device . 46
Figure D.1 – ITU weighting filter for weighted and unweighted measurements . 50
Figure D.2 – Peak value rectifier scheme . 51

Table 1 – Reverberation time of the empty room . 15
Table 2 – Examples of EMC regulations and standards . 37
Table 3 – Basic EMC standards and their application to microphones . 38
Table 4 – Classification of characteristics . 41
Table A.1 – Speech power weighting factor at octave-band centre frequencies . 43
Table C.1 – Classification of the characteristics recommended to be specified . 47
Table C.2 – Additional digital characteristics to be specified . 49
Table D.1 – Time constants for the two PVRs . 51

– 6 – IEC 60268-4:2018 © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SOUND SYSTEM EQUIPMENT –
Part 4: Microphones
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
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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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
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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 60268-4 has been prepared by IEC technical committee 100:
Audio, video and multimedia systems and equipment.
This sixth edition cancels and replaces the fifth edition published in 2014. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Subclause 19.4 on "pop" measurement replaces Annex C;
b) new Annex D for noise measurements in the digital domain.
The text of this International Standard is based on the following documents:
CDV Report on voting
100/2992/CDV 100/3109/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 of the IEC 60268 series, 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.
A bilingual version of this publication may be issued at a later date.

– 8 – IEC 60268-4:2018 © IEC 2018
SOUND SYSTEM EQUIPMENT –
Part 4: Microphones
1 Scope
This part of IEC 60268 specifies methods of measurement for the electrical impedance,
sensitivity, directional response pattern, dynamic range and external influences of sound
system microphones, and also details the characteristics to be specified by the manufacturer.
It applies to sound system microphones for all applications for speech and music. It does not
apply to measurement microphones, but it does apply to each audio channel of microphones
having more than one channel, for example for stereo or similar use. It is also applicable to
flush-mounted microphones and to the analogue characteristics of microphones with digital
audio output.
For the purposes of this International Standard, a microphone includes all such devices as
transformers, pre-amplifiers, or other elements that form an integral part of the microphone,
up to the output terminals specified by the manufacturer.
The major characteristics of a microphone are considered in Clauses 6 to 21. Additional
characteristics are considered in Annex A and Annex C.
NOTE The characteristics specified in this document do not describe the subjective response of the microphone.
Further work is necessary to find new definitions and measurement procedures for a later introduction of objective
characteristics for at least some of the subjective descriptions used to describe microphone performance.
2 Normative references
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.
CISPR 35:2016, Electromagnetic compatibility of multimedia equipment – Immunity
requirements
IEC 60268-1:1985, Sound system equipment – Part 1: General
IEC 60268-1:1985/AMD1:1988
IEC 60268-1:1985/AMD2:1988
IEC 60268-2:1987, Sound system equipment – Part 2: Explanation of general terms and
calculation methods
IEC 60268-2:1987/AMD1:1991
IEC 60268-3:2013, Sound system equipment – Part 3: Amplifiers
IEC 60268-5:2003, Sound system equipment – Part 5: Loudspeakers
IEC 60268-5:2003/AMD1:2007
IEC 60268-11:1987, Sound system equipment – Part 11: Application of connectors for the
interconnection of sound system components
IEC 60268-11:1987/AMD1:1989
IEC 60268-11:1987/AMD2:1991
IEC 60268-12:1987, Sound system equipment – Part 12: Application of connectors for
broadcast and similar use
IEC 60268-12:1987/AMD1:1991
IEC 60268-12:1987/AMD2:1994
IEC 61000-4-2:2008, Electromagnetic compatibility (EMC) – Part 4-2: Testing and
measurement techniques – Electrostatic discharge immunity test
IEC 61000-4-3:2006, Electromagnetic compatibility (EMC) – Part 4-3: Testing and
measurement techniques – Radiated, radio-frequency, electromagnetic field immunity test
IEC 61000-4-3:2006/AMD1:2007
IEC 61000-4-3:2006/AMD2:2010
IEC 61000-4-4:2012, Electromagnetic compatibility (EMC) – Part 4-4: Testing and
measurement techniques – Electrical fast transient/burst immunity test
IEC 61000-4-6:2013, Electromagnetic compatibility (EMC) – Part 4-6: Testing and
measurement techniques – Immunity to conducted disturbances, induced by radio-frequency
fields
IEC 61000-4-8:2009, Electromagnetic compatibility (EMC) – Part 4-8: Testing and
measurement techniques – Power frequency magnetic field immunity test
IEC 61000-4-16:2015, Electromagnetic compatibility (EMC) – Part 4-16: Testing and
measurement techniques – Test for immunity to conducted, common mode disturbances in
the frequency range 0 Hz to 150 kHz
IEC 61000-4-17:1999, Electromagnetic compatibility (EMC) – Part 4-17: Testing and
measurement techniques – Ripple on d.c. input power port immunity test
61000-4-17:1999/AMD1:2001
61000-4-17:1999/AMD2:2008
IEC 61260-1:2014, Electroacoustics – Octave-band and fractional-octave-band filters –
Part 1: Specifications
IEC 61938:2013, Multimedia systems – Guide to the recommended characteristics of
analogue interfaces to achieve interoperability
ITU-T Recommendation P.51:1996, Artificial mouth
EN 55103-2:2009, Electromagnetic compatibility – Product family standard for audio, video,
audio-visual and entertainment lighting control apparatus for professional use – Part 2:
Immunity
EN 300 422-2 V1.3.1:2011, Electromagnetic compatibility and radio spectrum matters (ERM)
– Wireless microphones in the 25 MHz to 3 GHz frequency range – Part 2: Harmonized EN
covering the essential requirements of article 3.2 of the R&TTE Directive
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60268-1 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/

– 10 – IEC 60268-4:2018 © IEC 2018
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
far-field microphone
microphone for use mainly at a distance of 1 m or more from the source of sound
Note 1 to entry: The term "far-field microphone" describes a microphone application (see 6.4) and not a type of
microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
3.2
near-field microphone
microphone for use by an individual performer at a distance of approximately 30 cm
Note 1 to entry: The term "near-field microphone" describes a microphone application (see 6.4) and not a type of
microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
3.3
close-talking microphone
microphone for use at a distance of approximately 25 mm from the source of sound
Note 1 to entry: The term "close-talking microphone" describes a microphone application (see 6.4) and not a type
of microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
4 General conditions
4.1 General
Special reference is made to IEC 60268-1, concerning:
• units and system of measurement;
• frequencies of measurement;
• quantities to be specified and their accuracy (see also 5.7);
• marking (see also 7.1);
• ambient conditions;
• filters, networks and measuring instruments for noise specification and measurement;
• individual specifications and type specifications;
• graphical presentation of characteristics;
• scales for graphical presentation;
• personal safety and prevention of spread of fire;
• method of producing a uniform alternating magnetic field;
• search coils for measuring the magnetic field strength,
and to IEC 61938 concerning powering of microphones.
4.2 Measurement conditions
4.2.1 General
For convenience in specifying how microphones shall be set up for measurement, three sets
of conditions have been defined in this document, under the title of "rated conditions".
Microphones should be measured in conditions approximating those in which they are
intended to be used. Three sets of measurement conditions are specified in this document:
free-field, near-field and close-talking. The differences between these sets of conditions are in

the distance to the sound source and the sound pressure level of the measurement.
Measurements shall be reported using at least one of these sets of conditions. Additional data
may be included, provided that the measurement conditions are specified.
Three ratings are basic to the formulation of these concepts:
• rated power supply (see 9.1);
• rated impedance (see 10.2);
• rated sensitivity (see 11.3).
To obtain the correct conditions for measurement, the above-mentioned ratings shall be taken
from the specifications supplied by the manufacturer of the equipment.
The term "rated" applied to other characteristics relates to the specification or measurement
of the particular characteristic under rated conditions or under conditions unambiguously
connected to them. This applies, for example, to the following two characteristics:
• rated output voltage;
• rated equivalent sound pressure level due to inherent noise.
Methods of measurement are given in this document for electrical impedance, sensitivity,
directional pattern, dynamic range and external influences. Where alternative methods are
given, the chosen method shall be specified.
4.2.2 Rated conditions
The microphone is understood to be working under rated conditions when the following
conditions are fulfilled:
• the microphone is connected to the resistive load specified in 5.4, or as specified by the
manufacturer;
• if the microphone needs a power supply, this is the rated power supply;
• the microphone (except a close-talking or near-field microphone) is placed in a sound field
meeting the free-field conditions in 5.5.2, the waves having zero-degree incidence with
respect to the reference direction;
• far-field microphones shall be measured in free-field conditions;
• the undisturbed sound pressure (in the absence of the microphone) in the sound field at
the reference point of the microphone is sinusoidal and set at a level of 1 Pa (94 dB SPL);
• for close-talking microphones, the microphone is placed at a stated distance, no more
than 25 mm from the artificial mouth complying with ITU-T Recommendation P.51, and the
undisturbed sound pressure in the sound field at the reference point of microphone is
sinusoidal and set at a level of 3 Pa (104 dB SPL);
• for near-field microphones, the microphone is placed at 30 cm from the artificial mouth
complying with ITU-T Recommendation P.51, and the undisturbed sound pressure in the
sound field at the reference point of microphone is sinusoidal and set at a level of 1 Pa
(94 dB SPL);
• if a special microphone needs a different measurement level or procedure (e.g. Lavalier or
boundary layer microphones), it shall be stated in the technical data together with the
reason for this. Levels related to the normal reference level of 94 dB by multiples of 10 dB
are preferred;
• controls, if any, are set to the position recommended by the manufacturer;
• in the absence of a clear reason to the contrary, the measurement frequency is 1 000 Hz
(see IEC 60268-1);
• the ambient pressure, relative humidity and ambient temperature are within the limits
given in IEC 60268-1, and shall be stated.

– 12 – IEC 60268-4:2018 © IEC 2018
Measurements may be made at a sound pressure of 0,3 Pa if this is necessary due to
limitations of the performance of the loudspeaker or other measurement equipment, and only
if any change in performance between the level used and the reference level is known with
the necessary accuracy for the relevant characteristics.
5 Particular conditions
5.1 Pre-conditioning
A microphone with preamplifier shall be switched on for the period of time specified by the
manufacturer, before measurements are made, to allow the components to reach the
stationary temperature for rated conditions. If the manufacturer specifies no period, a period
of 10 s shall be allowed for stabilization. If the microphone contains a vacuum tube or other
heating device the time shall be 10 min.
5.2 Sound source
The sound source shall be capable of producing at the microphone position the sound
pressure level as defined for rated conditions. The amplitude non-linearity of the sound
source shall be held to such a value that the effect on the measured response does not
exceed 0,5 dB. If the conditions of measurement preclude the possibility of securing
sufficiently low distortion, a narrow-band filter may be used at the microphone output
terminals, which allows the response at the fundamental frequency to be measured.
For free-field calibration and calibration of near-field microphones, the sound source shall be
contained in an enclosure which radiates sound from one well-defined opening fulfilling the
sound field requirements, preferably by only one being radially symmetrical with respect to the
axis of the reference direction of the microphone.
5.3 Measurement of sound pressure
A calibrated reference pressure microphone shall be used to measure the sound pressure.
The reference microphone shall be calibrated with an accuracy of ±1 dB at 1 kHz or better.
5.4 Voltage measuring system
The voltage generated by the microphone, when in a sound field, shall be determined by
using a voltmeter with an input resistance negligible to the specified microphone load. This
load is five times the rated impedance of the microphone, unless otherwise stated by the
manufacturer. If external equipment, such as a power supply, applies an impedance in parallel
with the microphone, its impedance shall be taken into account.
NOTE Microphones having a rated impedance of 200 Ω often have an actual internal impedance in the order of
50 Ω, and many of them perform best with a minimum load impedance around 1 000 Ω.
5.5 Acoustical environment
5.5.1 General
The microphone can be measured in different acoustical environments:
a) in a free field or similar with negligible boundary effects, e.g. by using special computer-
generated sound source signals:
– spherical waves, or
– plane waves, or
– waves produced by a specific sound source (artificial mouth or artificial head);
b) in a diffuse field;
c) coupled to a sound source by means of a small cavity (coupler).

5.5.2 Free-field conditions
5.5.2.1 General
A free-field sound wave is normally divergent in character. In certain circumstances it can
approximate an ideal plane wave. Free-field conditions can be obtained:
• in open air, ambient noise and wind permitting, or
• in an anechoic room, or
• in a duct.
A sound source of small dimensions with respect to the wavelength produces a spherical
wave in these environments. The spherical wave can be approximated to a plane wave in a
region of measurement located at a sufficient distance from the source. Spherical waves can
be used to measure pressure microphones but it is necessary to use almost perfect plane
waves in the low-frequency range for the measurement of pressure gradient microphones.
For microphones responding both to pressure and to pressure gradient, having a sufficiently
flat frequency response in a plane-wave free sound field (i.e. at a sufficient distance from the
source), the response as a function of frequency f of distance r from a centre of spherical
diverging waves and of angle of incidence θ of the waves at the microphone, can be given in
a complex form:
 
(1− B)+ B1+ cosθ
 
j kr
 
where
1 – B is the contribution of the pressure component;
B is the contribution of the pressure gradient component;
k = 2π/λ or 2πf/c (λ – wavelength, c – speed of sound);
B = 0 for the omnidirectional pressure type;
B = 0,5 for the cardioid type;
B = 1 for the bidirectional pressure gradient type.
At low frequencies, it becomes difficult to realize plane wave conditions in an anechoic room.
A plane wave at low frequencies, below the cut-off frequency of the anechoic room, can
therefore be better produced under other conditions.
Free-field conditions are considered to be sufficiently realized in the region around the
microphone if the following conditions are met:
• within a distance of 200 mm in front, behind, right, left, above and below the position of
the microphone the sound pressure level is measured at every measuring frequency by
means of a pressure transducer;
• the axis of the transducer shall point towards the reference point of the loudspeaker (see
IEC 60268-5);
• the corresponding sound pressure levels on axis positioned at different distances from the
loudspeaker shall not differ by more than 0,5 dB from the calculated levels in the ideal
sound field;
• the values at a nearly constant distance to the sound source, right, left, above and below
the microphone shall not differ by more than 1 dB from the level at the reference point of
...

IEC 60268-4 ®
Edition 6.0 2018-09
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Sound system equipment –
Part 4: Microphones
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IEC 60268-4 ®
Edition 6.0 2018-09
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Sound system equipment –
Part 4: Microphones
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.160.50 ISBN 978-2-8322-6047-0

– 2 – IEC 60268-4:2018 RLV © IEC 2018
CONTENTS
FOREWORD . 6
1 Scope . 8
2 Normative references. 8
3 Terms and definitions . 9
4 General conditions . 10
4.1 General . 10
4.2 Measurement conditions . 10
4.2.1 General . 10
4.2.2 Rated conditions . 11
5 Particular conditions . 12
5.1 Pre-conditioning . 12
5.2 Sound source . 12
5.3 Measurement of sound pressure . 12
5.4 Voltage measuring system . 12
5.5 Acoustical environment . 12
5.5.1 General . 12
5.5.2 Free-field conditions . 13
5.5.3 Diffuse field conditions . 15
5.5.4 Microphone coupled to a sound source by means of a small cavity
coupler . 15
5.6 Methods of measuring frequency response . 16
5.6.1 Point-by-point and continuous sweep frequency methods . 16
5.6.2 Calibration methods . 16
5.7 Overall accuracy . 17
5.8 Graphical presentation of results . 17
6 Type description (acoustical behaviour) . 17
6.1 Principle of the transducer . 17
6.2 Type of microphone . 17
6.3 Type of directional response characteristics . 17
6.4 Application . 17
7 Terminals and controls. 17
7.1 Marking . 17
7.2 Connectors and electrical interface values . 18
8 Reference point and axis . 18
8.1 Reference point . 18
8.2 Reference axis . 18
9 Rated power supply . 18
9.1 Characteristics to be specified . 18
9.2 Method of measurement . 18
10 Electrical impedance . 19
10.1 Internal impedance . 19
10.1.1 Characteristic to be specified . 19
10.1.2 Methods of measurement . 19
10.2 Rated impedance . 19
10.3 Rated minimum permitted load impedance . 20

11 Sensitivity . 20
11.1 General . 20
11.2 Sensitivities with respect to acoustical environment . 20
11.2.1 Free-field sensitivity . 20
11.2.2 Diffuse-field sensitivity . 21
11.2.3 Close-talking or and near-field sensitivity . 22
11.2.4 Pressure sensitivity . 22
11.3 Rated sensitivity . 22
12 Response . 23
12.1 Frequency response . 23
12.1.1 Characteristic to be specified . 23
12.1.2 Method of measurement . 23
12.1.3 Graphical presentation of results . 23
12.2 Effective frequency range . 23
12.2.1 Characteristic to be specified . 23
12.2.2 Method of measurement . 23
13 Directional characteristics . 24
13.1 Directional pattern . 24
13.1.1 Characteristic to be specified . 24
13.1.2 Methods of measurement . 24
13.1.3 Graphical presentation of results . 25
13.2 Directivity index . 25
13.2.1 Characteristic to be specified . 25
13.2.2 Method of measurement . 25
14 Amplitude non-linearity . 26
14.1 General . 26
14.2 Total harmonic distortion . 26
14.2.1 Characteristic to be specified . 26
14.2.2 Method of measurement . 26
th
14.3 Harmonic distortion of the n order (n = 2, 3,.) . 27
14.3.1 Characteristic to be specified . 27
14.3.2 Method of measurement . 27
14.4 Difference frequency distortion of second order . 27
14.4.1 Characteristic to be specified . 27
14.4.2 Method of measurement . 27
15 Limiting characteristics . 28
15.1 Rated maximum permissible peak sound pressure . 28
15.2 Overload sound pressure . 28
15.2.1 Characteristic to be specified . 28
15.2.2 Method of measurement . 28
16 Balance . 29
16.1 Balance of the microphone output . 29
16.2 Balance under working conditions . 29
17 Equivalent sound pressure level due to inherent noise . 30
17.1 Characteristic to be specified . 30
17.2 Method of measurement . 30
18 Ambient conditions . 30
18.1 General . 30

– 4 – IEC 60268-4:2018 RLV © IEC 2018
18.2 Pressure range . 31
18.3 Temperature range . 31
18.4 Relative humidity range . 31
19 External influences . 31
19.1 General . 31
19.1.1 Specification and methods of measurement . 31
19.1.2 Other external interferences . 31
19.2 Equivalent sound pressure due to mechanical vibration . 31
19.2.1 Characteristic to be specified . 31
19.2.2 Method of measurement . 32
19.3 Equivalent sound pressure due to wind . 32
19.3.1 Characteristic to be specified . 32
19.3.2 Method of measurement . 32
19.4 Transient equivalent sound pressure due to "pop" effect . 36
19.4.1 General . 36
19.4.2 Characteristic to be specified . 36
19.4.3 Method of measurement . 38
20 Electromagnetic compatibility (EMC) . 39
20.1 Regulatory requirements . 39
20.2 Requirements for preserving programme quality . 39
20.3 Performance criteria . 40
20.3.1 Criterion A . 40
20.3.2 Criterion B . 40
20.4 Testing for immunity to disturbances in the presence of acoustical noise . 41
20.5 Immunity to frequency-modulated radiated disturbances . 41
20.6 Immunity to magnetic fields . 41
20.7 Immunity to ripple on DC power supply . 41
20.8 Permanent magnetic field . 42
20.9 Evaluation and reporting of the test results . 42
21 Physical characteristics . 42
21.1 Dimensions . 42
21.2 Weight . 42
21.3 Cables and connectors . 42
22 Classification of the characteristics to be specified . 42
Annex A (normative) Additional characteristics . 45
A.1 Characteristic sensitivity for speech . 45
A.1.1 Characteristic to be specified . 45
A.1.2 Method of measurement . 45
A.2 Front-to-rear sensitivity index (0° to 180°) . 46
A.2.1 Characteristic to be specified . 46
A.2.2 Method of measurement . 46
A.3 Noise-cancelling index . 46
A.3.1 Characteristic to be specified . 46
A.3.2 Method of measurement . 46
A.4 Special characteristics for stereo microphones . 47
A.4.1 General . 47
A.4.2 Included angle of an XY (left-right) microphone . 47
A.4.3 Acceptance angle . 47

Annex B (informative) Sound insulation device . 48
Annex (informative) Simplified procedure for “pop” measurements .
General .
Measurement set-up .
Measurement procedure .
Approximate inclusion of different frequency responses .
Annex C (informative) Recommendations for professional digital microphones . 52
C.1 General . 52
C.2 Data sheets for digital microphones . 52
Annex D (informative) Recommended method for measuring noise levels according to
ITU-R BS.468-4 in the digital domain . 55
D.1 General . 55
D.2 Recommended method . 55
D.3 Matlab code . 56
Bibliography . 59

Figure – Measurement set-up .
Figure – Test fixture for the sound field sensitivity .
Figure 1 – Balance of the output . 29
Figure 2 – Balance under working conditions . 29
Figure 3 – Measurement set-up for wind influence . 33
Figure 4 – Wind generators, type 1 (Figure 4a) and type 2 (Figure 4b) . 35
Figure 5 – Electrical and mechanical setup for the measuring of the "pop" effect . 37
Figure B.1 – Sound insulation device . 48
Figure D.1 – ITU weighting filter for weighted and unweighted measurements . 55
Figure D.2 – Peak value rectifier scheme . 56

Table – Reference signal and characteristics .
Table 1 – Reverberation time of the empty room . 15
Table 2 – Examples of EMC regulations and standards . 39
Table 3 – Basic EMC standards and their application to microphones . 40
Table 4 – Classification of characteristics . 43
Table A.1 – Speech power weighting factor at octave-band centre frequencies . 45
Table C.1 – Classification of the characteristics recommended to be specified . 52
Table C.2 – Additional digital characteristics to be specified . 54
Table D.1 – Time constants for the two PVRs . 56

– 6 – IEC 60268-4:2018 RLV © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SOUND SYSTEM EQUIPMENT –
Part 4: Microphones
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
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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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
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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.
This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition. A vertical bar appears in the margin wherever a change
has been made. Additions are in green text, deletions are in strikethrough red text.

International Standard IEC 60268-4 has been prepared by IEC technical committee 100:
Audio, video and multimedia systems and equipment.
This sixth edition cancels and replaces the fifth edition published in 2014. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Subclause 19.4 on "pop" measurement replaces Annex C;
b) new Annex D for noise measurements in the digital domain.
The text of this International Standard is based on the following documents:
CDV Report on voting
100/2992/CDV 100/3109/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 of the IEC 60268 series, 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 publication using a colour printer.

– 8 – IEC 60268-4:2018 RLV © IEC 2018
A bilingual version of this publication may be issued at a later
date.SOUND SYSTEM EQUIPMENT –
Part 4: Microphones
1 Scope
This part of IEC 60268 specifies methods of measurement for the electrical impedance,
sensitivity, directional response pattern, dynamic range and external influences of sound
system microphones, and also details the characteristics to be specified by the manufacturer.
It applies to sound system microphones for all applications for speech and music. It does not
apply to measurement microphones, but it does apply to each audio channel of microphones
having more than one channel, for example for stereo or similar use. It is also applicable to
flush-mounted microphones and to the analogue characteristics of microphones with digital
audio output.
For the purposes of this International Standard, a microphone includes all such devices as
transformers, pre-amplifiers, or other elements that form an integral part of the microphone,
up to the output terminals specified by the manufacturer.
The major characteristics of a microphone are considered in Clauses 6 to 21. Additional
characteristics are considered in Annex A, Annex C and Annex DC.
NOTE The characteristics specified in this document do not completely describe the subjective response of the
microphone. Further work is necessary to find new definitions and measurement procedures for a later replacement
by introduction of objective characteristics of for at least some of the subjective descriptions used to describe
microphone performance.
2 Normative references
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.
CISPR 35:2016, Electromagnetic compatibility of multimedia equipment – Immunity
requirements
IEC 60268-1:1985, Sound system equipment – Part 1: General
IEC 60268-1:1985/AMD1:1988
IEC 60268-1:1985/AMD2:1988
IEC 60268-2:1987, Sound system equipment – Part 2: Explanation of general terms and
calculation methods
IEC 60268-2:1987/AMD1:1991
IEC 60268-3:2013, Sound system equipment – Part 3: Amplifiers
IEC 60268-5:2003, Sound system equipment – Part 5: Loudspeakers
IEC 60268-5:2003/AMD1:2007
IEC 60268-11:1987, Sound system equipment – Part 11: Application of connectors for the
interconnection of sound system components

IEC 60268-11:1987/AMD1:1989
IEC 60268-11:1987/AMD2:1991
IEC 60268-12:1987, Sound system equipment – Part 12: Application of connectors for
broadcast and similar use
IEC 60268-12:1987/AMD1:1991
IEC 60268-12:1987/AMD2:1994
IEC 61000-4-2:2008, Electromagnetic compatibility (EMC) – Part 4-2: Testing and
measurement techniques – Electrostatic discharge immunity test
IEC 61000-4-3:2006, Electromagnetic compatibility (EMC) – Part 4-3: Testing and
measurement techniques – Radiated, radio-frequency, electromagnetic field immunity test
IEC 61000-4-3:2006/AMD1:2007
IEC 61000-4-3:2006/AMD2:2010
IEC 61000-4-4:2012, Electromagnetic compatibility (EMC) – Part 4-4: Testing and
measurement techniques – Electrical fast transient/burst immunity test
IEC 61000-4-6:2008 2013, Electromagnetic compatibility (EMC) – Part 4-6: Testing and
measurement techniques – Immunity to conducted disturbances, induced by radio-frequency
fields
IEC 61000-4-8:2009, Electromagnetic compatibility (EMC) – Part 4-8: Testing and
measurement techniques – Power frequency magnetic field immunity test
IEC 61000-4-16:2015, Electromagnetic compatibility (EMC) – Part 4-16: Testing and
measurement techniques – Test for immunity to conducted, common mode disturbances in
the frequency range 0 Hz to 150 kHz
IEC 61000-4-17:1999, Electromagnetic compatibility (EMC) – Part 4-17: Testing and
measurement techniques – Ripple on d.c. input power port immunity test
61000-4-17:1999/AMD1:2001
61000-4-17:1999/AMD2:2008
IEC 61260-1:2014, Electroacoustics – Octave-band and fractional-octave-band filters –
Part 1: Specifications
IEC 61938:2013, Multimedia systems – Guide to the recommended characteristics of
analogue interfaces to achieve interoperability
ITU-T Recommendation P.51:1996, Artificial mouth
EN 55103-2:2009, Electromagnetic compatibility – Product family standard for audio, video,
audio-visual and entertainment lighting control apparatus for professional use – Part 2:
Immunity
EN 300 422-2 V1.3.1:2011, Electromagnetic compatibility and radio spectrum matters (ERM)
– Wireless microphones in the 25 MHz to 3 GHz frequency range – Part 2: Harmonized EN
covering the essential requirements of article 3.2 of the R&TTE Directive
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60268-1 and the
following apply.
– 10 – IEC 60268-4:2018 RLV © IEC 2018
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
far-field microphone
microphone for use mainly at a distance of 1 m or more than 1 m from the source of sound
Note 1 to entry: The term "far-field microphone" describes a microphone application (see 6.4) and not a type of
microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
3.2
near-field microphone
microphone for use by an individual performer at a distance of approximately 30 cm
Note 1 to entry: The term "near-field microphone" describes a microphone application (see 6.4) and not a type of
microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
3.3
close-talking microphone
microphone for use at a distance of approximately 25 mm from the source of sound
Note 1 to entry: The term "close-talking microphone" describes a microphone application (see 6.4) and not a type
of microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
4 General conditions
4.1 General
Special reference is made to IEC 60268-1, concerning:
• units and system of measurement;
• frequencies of measurement;
• quantities to be specified and their accuracy (see also 5.7);
• marking (see also 7.1);
• ambient conditions;
• filters, networks and measuring instruments for noise specification and measurement;
• individual specifications and type specifications;
• graphical presentation of characteristics;
• scales for graphical presentation;
• personal safety and prevention of spread of fire;
• method of producing a uniform alternating magnetic field;
• search coils for measuring the magnetic field strength,
and to IEC 61938 concerning powering of microphones.
4.2 Measurement conditions
4.2.1 General
For convenience in specifying how microphones shall be set up for measurement, three sets
of conditions have been defined in this document, under the title of "rated conditions".

Microphones should be measured in conditions approximating those in which they are
intended to be used. Three sets of measurement conditions are specified in this document:
free-field, near-field and close-talking. The differences between these sets of conditions are in
the distance to the sound source and the sound pressure level of the measurement.
Measurements shall be reported using at least one of these sets of conditions. Additional data
may be included, provided that the measurement conditions are specified.
Three ratings are basic to the formulation of these concepts:
• rated power supply (see 9.1);
• rated impedance (see 10.2);
• rated sensitivity (see 11.3).
To obtain the correct conditions for measurement, the above-mentioned ratings shall be taken
from the specifications supplied by the manufacturer of the equipment.
The term "rated" applied to other characteristics relates to the specification or measurement
of the particular characteristic under rated conditions or under conditions unambiguously
connected to them. This applies, for example, to the following two characteristics:
• rated output voltage;
• rated equivalent sound pressure level due to inherent noise.
Methods of measurement are given in this document for electrical impedance, sensitivity,
directional pattern, dynamic range and external influences. Where alternative methods are
given, the chosen method shall be specified.
4.2.2 Rated conditions
The microphone is understood to be working under rated conditions when the following
conditions are fulfilled:
• the microphone is connected to the resistive load specified in 5.4, or as specified by the
manufacturer;
• if the microphone needs a power supply, this is the rated power supply;
• the microphone (except a close-talking or near-field microphone) is placed in a sound field
meeting the free-field conditions in 5.5.2, the waves having zero-degree incidence with
respect to the reference direction;
• far-field microphones shall be measured in free-field conditions;
• the undisturbed sound pressure (in the absence of the microphone) in the sound field at
the reference point of the microphone is sinusoidal and set at a level of 1 Pa (94 dB SPL);
• for close-talking microphones, the microphone is placed at a stated distance, no more
than 25 mm from the artificial mouth complying with ITU-T Recommendation P.51, and the
undisturbed sound pressure in the sound field at the reference point of microphone is
sinusoidal and set at a level of 3 Pa (104 dB SPL);
• for near-field microphones, the microphone is placed at 30 cm from the artificial mouth
complying with ITU-T Recommendation P.51, and the undisturbed sound pressure in the
sound field at the reference point of microphone is sinusoidal and set at a level of 1 Pa
(94 dB SPL);
• if a special microphone needs a different measurement level or procedure (e.g. Lavalier or
boundary layer microphones), it shall be stated in the technical data together with the
reason for this. Levels related to the normal reference level of 94 dB by multiples of 10 dB
are preferred;
• controls, if any, are set to the position recommended by the manufacturer;

– 12 – IEC 60268-4:2018 RLV © IEC 2018
• in the absence of a clear reason to the contrary, the measurement frequency is 1 000 Hz
(see IEC 60268-1);
• the ambient pressure, relative humidity and ambient temperature are within the limits
given in IEC 60268-1, and shall be stated.
Measurements may be made at a sound pressure of 0,3 Pa if this is necessary due to
limitations of the performance of the loudspeaker or other measurement equipment, and only
if any change in performance between the level used and the reference level is known with
the necessary accuracy for the relevant characteristics.
5 Particular conditions
5.1 Pre-conditioning
A microphone with preamplifier shall be switched on for the period of time specified by the
manufacturer, before measurements are made, to allow the components to reach the
stationary temperature for rated conditions. If the manufacturer specifies no period, a period
of 10 s shall be allowed for stabilization. If the microphone contains a vacuum tube or other
heating device the time shall be 10 min.
5.2 Sound source
The sound source shall be capable of producing at the microphone position the sound
pressure level as defined for rated conditions. The amplitude non-linearity of the sound
source shall be held to such a value that the effect on the measured response does not
exceed 0,5 dB. If the conditions of measurement preclude the possibility of securing
sufficiently low distortion, a narrow-band filter may be used at the microphone output
terminals, which allows the response at the fundamental frequency to be measured.
For free-field calibration and calibration of near-field microphones, the sound source shall be
contained in an enclosure which radiates sound from one well-defined opening only, and such
an opening shall be fulfilling the sound field requirements, preferably by only one being
radially symmetrical with respect to the axis of the reference direction of the microphone.
5.3 Measurement of sound pressure
A calibrated reference pressure microphone shall be used to measure the sound pressure.
The reference microphone shall be calibrated with an accuracy of ±1 dB at 1 kHz or better.
5.4 Voltage measuring system
The voltage generated by the microphone, when in a sound field, shall be determined by
using a voltmeter with an input resistance of negligible to the specified microphone load. This
load is five times the rated impedance of the microphone, unless otherwise stated by the
manufacturer. If external equipment, such as a power supply, applies an impedance in parallel
with the microphone, its impedance shall be taken into account.
NOTE Microphones having a rated impedance of 200 Ω often have an actual internal impedance in the order of
50 Ω, and many of them perform best with a minimum load impedance around 1 000 Ω.
5.5 Acoustical environment
5.5.1 General
The microphone can be measured in different acoustical environments:
a) in a free field or similar with negligible boundary effects, e.g. by using special computer-
generated sound source signals:
– spherical waves, or
– plane waves, or
– waves produced by a specific sound source (artificial mouth or artificial head);
b) in a diffuse field;
c) coupled to a sound source by means of a small cavity (coupler).
5.5.2 Free-field conditions
5.5.2.1 General
A free-field sound wave is normally divergent in character. In certain circumstances it can
approximate an ideal plane wave. Free-field conditions can be obtained:
• in open air, ambient noise and wind permitting, or
• in an anechoic room, or
• in a duct.
A sound source of small dimensions with respect to the wavelength produces a spherical
wave in these environments. The spherical wave can be approximated to a plane wave in a
region of measurement located at a sufficient distance from the source. Spherical waves can
be used to measure pressure microphones but it is necessary to use almost perfect plane
waves in the low-frequency range for the measurement of pressure gradient microphones.
For microphones responding both to pressure and to pressure gradient, having a sufficiently
flat frequency response in a plane-wave free soun
...

IEC 60268-4 ®
Edition 6.0 2018-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Sound system equipment –
Part 4: Microphones
Équipements pour systèmes électroacoustiques –
Partie 4: Microphones
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IEC 60268-4 ®
Edition 6.0 2018-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Sound system equipment –
Part 4: Microphones
Équipements pour systèmes électroacoustiques –

Partie 4: Microphones
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.160.50 ISBN 978-2-8322-6616-8

– 2 – IEC 60268-4:2018 © IEC 2018
CONTENTS
FOREWORD . 6
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 General conditions. 10
4.1 General . 10
4.2 Measurement conditions . 10
4.2.1 General . 10
4.2.2 Rated conditions . 11
5 Particular conditions . 12
5.1 Pre-conditioning . 12
5.2 Sound source . 12
5.3 Measurement of sound pressure . 12
5.4 Voltage measuring system . 12
5.5 Acoustical environment . 12
5.5.1 General . 12
5.5.2 Free-field conditions . 13
5.5.3 Diffuse field conditions . 14
5.5.4 Microphone coupled to a sound source by means of a small cavity
coupler . 15
5.6 Methods of measuring frequency response . 15
5.6.1 Point-by-point and continuous sweep frequency methods . 15
5.6.2 Calibration methods . 16
5.7 Overall accuracy . 16
5.8 Graphical presentation of results. 16
6 Type description (acoustical behaviour) . 17
6.1 Principle of the transducer . 17
6.2 Type of microphone . 17
6.3 Type of directional response characteristics . 17
6.4 Application . 17
7 Terminals and controls . 17
7.1 Marking . 17
7.2 Connectors and electrical interface values . 17
8 Reference point and axis . 18
8.1 Reference point . 18
8.2 Reference axis . 18
9 Rated power supply . 18
9.1 Characteristics to be specified . 18
9.2 Method of measurement . 18
10 Electrical impedance . 18
10.1 Internal impedance . 18
10.1.1 Characteristic to be specified . 18
10.1.2 Methods of measurement . 18
10.2 Rated impedance . 19
10.3 Rated minimum permitted load impedance . 19

11 Sensitivity . 19
11.1 General . 19
11.2 Sensitivities with respect to acoustical environment . 20
11.2.1 Free-field sensitivity . 20
11.2.2 Diffuse-field sensitivity . 21
11.2.3 Close-talking and near-field sensitivity . 21
11.2.4 Pressure sensitivity . 22
11.3 Rated sensitivity . 22
12 Response . 22
12.1 Frequency response . 22
12.1.1 Characteristic to be specified . 22
12.1.2 Method of measurement . 23
12.1.3 Graphical presentation of results . 23
12.2 Effective frequency range . 23
12.2.1 Characteristic to be specified . 23
12.2.2 Method of measurement . 23
13 Directional characteristics . 23
13.1 Directional pattern . 23
13.1.1 Characteristic to be specified . 23
13.1.2 Methods of measurement . 24
13.1.3 Graphical presentation of results . 25
13.2 Directivity index . 25
13.2.1 Characteristic to be specified . 25
13.2.2 Method of measurement . 25
14 Amplitude non-linearity . 25
14.1 General . 25
14.2 Total harmonic distortion . 25
14.2.1 Characteristic to be specified . 25
14.2.2 Method of measurement . 26
th
14.3 Harmonic distortion of the n order (n = 2, 3,.) . 26
14.3.1 Characteristic to be specified . 26
14.3.2 Method of measurement . 26
14.4 Difference frequency distortion of second order . 27
14.4.1 Characteristic to be specified . 27
14.4.2 Method of measurement . 27
15 Limiting characteristics . 28
15.1 Rated maximum permissible peak sound pressure . 28
15.2 Overload sound pressure . 28
15.2.1 Characteristic to be specified . 28
15.2.2 Method of measurement . 28
16 Balance . 28
16.1 Balance of the microphone output . 28
16.2 Balance under working conditions . 29
17 Equivalent sound pressure level due to inherent noise . 29
17.1 Characteristic to be specified . 29
17.2 Method of measurement . 29
18 Ambient conditions . 30
18.1 General . 30

– 4 – IEC 60268-4:2018 © IEC 2018
18.2 Pressure range . 30
18.3 Temperature range . 30
18.4 Relative humidity range . 30
19 External influences . 30
19.1 General . 30
19.1.1 Specification and methods of measurement . 30
19.1.2 Other external interferences . 31
19.2 Equivalent sound pressure due to mechanical vibration . 31
19.2.1 Characteristic to be specified . 31
19.2.2 Method of measurement . 31
19.3 Equivalent sound pressure due to wind . 31
19.3.1 Characteristic to be specified . 31
19.3.2 Method of measurement . 32
19.4 Transient equivalent sound pressure due to "pop" effect . 35
19.4.1 General . 35
19.4.2 Characteristic to be specified . 35
19.4.3 Method of measurement . 36
20 Electromagnetic compatibility (EMC) . 37
20.1 Regulatory requirements . 37
20.2 Requirements for preserving programme quality . 37
20.3 Performance criteria . 38
20.3.1 Criterion A . 38
20.3.2 Criterion B . 38
20.4 Testing for immunity to disturbances in the presence of acoustical noise . 39
20.5 Immunity to frequency-modulated radiated disturbances . 39
20.6 Immunity to magnetic fields . 39
20.7 Immunity to ripple on d.c. power supply . 39
20.8 Permanent magnetic field . 40
20.9 Evaluation and reporting of the test results . 40
21 Physical characteristics . 40
21.1 Dimensions . 40
21.2 Weight . 40
21.3 Cables and connectors . 40
22 Classification of the characteristics to be specified . 40
Annex A (normative) Additional characteristics . 43
A.1 Characteristic sensitivity for speech . 43
A.1.1 Characteristic to be specified . 43
A.1.2 Method of measurement . 43
A.2 Front-to-rear sensitivity index (0° – 180°) . 44
A.2.1 Characteristic to be specified . 44
A.2.2 Method of measurement . 44
A.3 Noise-cancelling index . 44
A.3.1 Characteristic to be specified . 44
A.3.2 Method of measurement . 44
A.4 Special characteristics for stereo microphones . 45
A.4.1 General . 45
A.4.2 Included angle of an XY (left-right) microphone . 45
A.4.3 Acceptance angle . 45

Annex B (informative) Sound insulation device . 46
Annex C (informative) Recommendations for professional digital microphones . 47
C.1 General . 47
C.2 Data sheets for digital microphones . 47
Annex D (informative) Recommended method for measuring noise levels according to
ITU-R BS.468-4 in the digital domain . 50
D.1 General . 50
D.2 Recommended method . 50
D.3 Matlab code . 51
Bibliography . 54

Figure 1 – Balance of the output . 28
Figure 2 – Balance under working conditions . 29
Figure 3 – Measurement set-up for wind influence . 32
Figure 4 – Wind generators, type 1 (Figure 4a) and type 2 (Figure 4b) . 34
Figure 5 – Electrical and mechanical setup for the measuring of the "pop" effect . 36
Figure B.1 – Sound insulation device . 46
Figure D.1 – ITU weighting filter for weighted and unweighted measurements . 50
Figure D.2 – Peak value rectifier scheme . 51

Table 1 – Reverberation time of the empty room . 15
Table 2 – Examples of EMC regulations and standards . 37
Table 3 – Basic EMC standards and their application to microphones . 38
Table 4 – Classification of characteristics . 41
Table A.1 – Speech power weighting factor at octave-band centre frequencies . 43
Table C.1 – Classification of the characteristics recommended to be specified . 47
Table C.2 – Additional digital characteristics to be specified . 49
Table D.1 – Time constants for the two PVRs . 51

– 6 – IEC 60268-4:2018 © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SOUND SYSTEM EQUIPMENT –
Part 4: Microphones
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 60268-4 has been prepared by IEC technical committee 100:
Audio, video and multimedia systems and equipment.
This bilingual version (2019-02) corresponds to the monolingual English version, published in
2018-08.
This sixth edition cancels and replaces the fifth edition published in 2014. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Subclause 19.4 on "pop" measurement replaces Annex C;
b) new Annex D for noise measurements in the digital domain.
The text of this International Standard is based on the following documents:

CDV Report on voting
100/2992/CDV 100/3109/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.
The French version of this standard has not been voted upon.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60268 series, 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.
– 8 – IEC 60268-4:2018 © IEC 2018
SOUND SYSTEM EQUIPMENT –
Part 4: Microphones
1 Scope
This part of IEC 60268 specifies methods of measurement for the electrical impedance,
sensitivity, directional response pattern, dynamic range and external influences of sound
system microphones, and also details the characteristics to be specified by the manufacturer.
It applies to sound system microphones for all applications for speech and music. It does not
apply to measurement microphones, but it does apply to each audio channel of microphones
having more than one channel, for example for stereo or similar use. It is also applicable to
flush-mounted microphones and to the analogue characteristics of microphones with digital
audio output.
For the purposes of this International Standard, a microphone includes all such devices as
transformers, pre-amplifiers, or other elements that form an integral part of the microphone,
up to the output terminals specified by the manufacturer.
The major characteristics of a microphone are considered in Clauses 6 to 21. Additional
characteristics are considered in Annex A and Annex C.
NOTE The characteristics specified in this document do not describe the subjective response of the microphone.
Further work is necessary to find new definitions and measurement procedures for a later introduction of objective
characteristics for at least some of the subjective descriptions used to describe microphone performance.
2 Normative references
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.
CISPR 35:2016, Electromagnetic compatibility of multimedia equipment – Immunity
requirements
IEC 60268-1:1985, Sound system equipment – Part 1: General
IEC 60268-1:1985/AMD1:1988
IEC 60268-1:1985/AMD2:1988
IEC 60268-2:1987, Sound system equipment – Part 2: Explanation of general terms and
calculation methods
IEC 60268-2:1987/AMD1:1991
IEC 60268-3:2013, Sound system equipment – Part 3: Amplifiers
IEC 60268-5:2003, Sound system equipment – Part 5: Loudspeakers
IEC 60268-5:2003/AMD1:2007
IEC 60268-11:1987, Sound system equipment – Part 11: Application of connectors for the
interconnection of sound system components
IEC 60268-11:1987/AMD1:1989
IEC 60268-11:1987/AMD2:1991
IEC 60268-12:1987, Sound system equipment – Part 12: Application of connectors for
broadcast and similar use
IEC 60268-12:1987/AMD1:1991
IEC 60268-12:1987/AMD2:1994
IEC 61000-4-2:2008, Electromagnetic compatibility (EMC) – Part 4-2: Testing and
measurement techniques – Electrostatic discharge immunity test
IEC 61000-4-3:2006, Electromagnetic compatibility (EMC) – Part 4-3: Testing and
measurement techniques – Radiated, radio-frequency, electromagnetic field immunity test
IEC 61000-4-3:2006/AMD1:2007
IEC 61000-4-3:2006/AMD2:2010
IEC 61000-4-4:2012, Electromagnetic compatibility (EMC) – Part 4-4: Testing and
measurement techniques – Electrical fast transient/burst immunity test
IEC 61000-4-6:2013, Electromagnetic compatibility (EMC) – Part 4-6: Testing and
measurement techniques – Immunity to conducted disturbances, induced by radio-frequency
fields
IEC 61000-4-8:2009, Electromagnetic compatibility (EMC) – Part 4-8: Testing and
measurement techniques – Power frequency magnetic field immunity test
IEC 61000-4-16:2015, Electromagnetic compatibility (EMC) – Part 4-16: Testing and
measurement techniques – Test for immunity to conducted, common mode disturbances in
the frequency range 0 Hz to 150 kHz
IEC 61000-4-17:1999, Electromagnetic compatibility (EMC) – Part 4-17: Testing and
measurement techniques – Ripple on d.c. input power port immunity test
61000-4-17:1999/AMD1:2001
61000-4-17:1999/AMD2:2008
IEC 61260-1:2014, Electroacoustics – Octave-band and fractional-octave-band filters –
Part 1: Specifications
IEC 61938:2013, Multimedia systems – Guide to the recommended characteristics of
analogue interfaces to achieve interoperability
ITU-T Recommendation P.51:1996, Artificial mouth
EN 55103-2:2009, Electromagnetic compatibility – Product family standard for audio, video,
audio-visual and entertainment lighting control apparatus for professional use – Part 2:
Immunity
EN 300 422-2 V1.3.1:2011, Electromagnetic compatibility and radio spectrum matters (ERM)
– Wireless microphones in the 25 MHz to 3 GHz frequency range – Part 2: Harmonized EN
covering the essential requirements of article 3.2 of the R&TTE Directive
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60268-1 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/

– 10 – IEC 60268-4:2018 © IEC 2018
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
far-field microphone
microphone for use mainly at a distance of 1 m or more from the source of sound
Note 1 to entry: The term "far-field microphone" describes a microphone application (see 6.4) and not a type of
microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
3.2
near-field microphone
microphone for use by an individual performer at a distance of approximately 30 cm
Note 1 to entry: The term "near-field microphone" describes a microphone application (see 6.4) and not a type of
microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
3.3
close-talking microphone
microphone for use at a distance of approximately 25 mm from the source of sound
Note 1 to entry: The term "close-talking microphone" describes a microphone application (see 6.4) and not a type
of microphone. A specific microphone may have one or more applications, but the term is helpful in specifying the
method of measurement.
4 General conditions
4.1 General
Special reference is made to IEC 60268-1, concerning:
• units and system of measurement;
• frequencies of measurement;
• quantities to be specified and their accuracy (see also 5.7);
• marking (see also 7.1);
• ambient conditions;
• filters, networks and measuring instruments for noise specification and measurement;
• individual specifications and type specifications;
• graphical presentation of characteristics;
• scales for graphical presentation;
• personal safety and prevention of spread of fire;
• method of producing a uniform alternating magnetic field;
• search coils for measuring the magnetic field strength,
and to IEC 61938 concerning powering of microphones.
4.2 Measurement conditions
4.2.1 General
For convenience in specifying how microphones shall be set up for measurement, three sets
of conditions have been defined in this document, under the title of "rated conditions".
Microphones should be measured in conditions approximating those in which they are
intended to be used. Three sets of measurement conditions are specified in this document:
free-field, near-field and close-talking. The differences between these sets of conditions are in

the distance to the sound source and the sound pressure level of the measurement.
Measurements shall be reported using at least one of these sets of conditions. Additional data
may be included, provided that the measurement conditions are specified.
Three ratings are basic to the formulation of these concepts:
• rated power supply (see 9.1);
• rated impedance (see 10.2);
• rated sensitivity (see 11.3).
To obtain the correct conditions for measurement, the above-mentioned ratings shall be taken
from the specifications supplied by the manufacturer of the equipment.
The term "rated" applied to other characteristics relates to the specification or measurement
of the particular characteristic under rated conditions or under conditions unambiguously
connected to them. This applies, for example, to the following two characteristics:
• rated output voltage;
• rated equivalent sound pressure level due to inherent noise.
Methods of measurement are given in this document for electrical impedance, sensitivity,
directional pattern, dynamic range and external influences. Where alternative methods are
given, the chosen method shall be specified.
4.2.2 Rated conditions
The microphone is understood to be working under rated conditions when the following
conditions are fulfilled:
• the microphone is connected to the resistive load specified in 5.4, or as specified by the
manufacturer;
• if the microphone needs a power supply, this is the rated power supply;
• the microphone (except a close-talking or near-field microphone) is placed in a sound field
meeting the free-field conditions in 5.5.2, the waves having zero-degree incidence with
respect to the reference direction;
• far-field microphones shall be measured in free-field conditions;
• the undisturbed sound pressure (in the absence of the microphone) in the sound field at
the reference point of the microphone is sinusoidal and set at a level of 1 Pa (94 dB SPL);
• for close-talking microphones, the microphone is placed at a stated distance, no more
than 25 mm from the artificial mouth complying with ITU-T Recommendation P.51, and the
undisturbed sound pressure in the sound field at the reference point of microphone is
sinusoidal and set at a level of 3 Pa (104 dB SPL);
• for near-field microphones, the microphone is placed at 30 cm from the artificial mouth
complying with ITU-T Recommendation P.51, and the undisturbed sound pressure in the
sound field at the reference point of microphone is sinusoidal and set at a level of 1 Pa
(94 dB SPL);
• if a special microphone needs a different measurement level or procedure (e.g. Lavalier or
boundary layer microphones), it shall be stated in the technical data together with the
reason for this. Levels related to the normal reference level of 94 dB by multiples of 10 dB
are preferred;
• controls, if any, are set to the position recommended by the manufacturer;
• in the absence of a clear reason to the contrary, the measurement frequency is 1 000 Hz
(see IEC 60268-1);
• the ambient pressure, relative humidity and ambient temperature are within the limits
given in IEC 60268-1, and shall be stated.

– 12 – IEC 60268-4:2018 © IEC 2018
Measurements may be made at a sound pressure of 0,3 Pa if this is necessary due to
limitations of the performance of the loudspeaker or other measurement equipment, and only
if any change in performance between the level used and the reference level is known with
the necessary accuracy for the relevant characteristics.
5 Particular conditions
5.1 Pre-conditioning
A microphone with preamplifier shall be switched on for the period of time specified by the
manufacturer, before measurements are made, to allow the components to reach the
stationary temperature for rated conditions. If the manufacturer specifies no period, a period
of 10 s shall be allowed for stabilization. If the microphone contains a vacuum tube or other
heating device the time shall be 10 min.
5.2 Sound source
The sound source shall be capable of producing at the microphone position the sound
pressure level as defined for rated conditions. The amplitude non-linearity of the sound
source shall be held to such a value that the effect on the measured response does not
exceed 0,5 dB. If the conditions of measurement preclude the possibility of securing
sufficiently low distortion, a narrow-band filter may be used at the microphone output
terminals, which allows the response at the fundamental frequency to be measured.
For free-field calibration and calibration of near-field microphones, the sound source shall be
contained in an enclosure which radiates sound from one well-defined opening fulfilling the
sound field requirements, preferably by only one being radially symmetrical with respect to the
axis of the reference direction of the microphone.
5.3 Measurement of sound pressure
A calibrated reference pressure microphone shall be used to measure the sound pressure.
The reference microphone shall be calibrated with an accuracy of ±1 dB at 1 kHz or better.
5.4 Voltage measuring system
The voltage generated by the microphone, when in a sound field, shall be determined by
using a voltmeter with an input resistance negligible to the specified microphone load. This
load is five times the rated impedance of the microphone, unless otherwise stated by the
manufacturer. If external equipment, such as a power supply, applies an impedance in parallel
with the microphone, its impedance shall be taken into account.
NOTE Microphones having a rated impedance of 200 Ω often have an actual internal impedance in the order of
50 Ω, and many of them perform best with a minimum load impedance around 1 000 Ω.
5.5 Acoustical environment
5.5.1 General
The microphone can be measured in different acoustical environments:
a) in a free field or similar with negligible boundary effects, e.g. by using special computer-
generated sound source signals:
– spherical waves, or
– plane waves, or
– waves produced by a specific sound source
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