EN 60268-4:2010

SLOVENSKI STANDARD
01-november-2010
1DGRPHãþD
SIST EN 60268-4:2004
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Sound system equipment - Part 4: Microphones (IEC 60268-4:2010)
Elektroakustische Geräte - Teil 4: Mikrofone (IEC 60268-4:2010)
Equipements pour systèmes électroacoustiques - Partie 4: Microphones (CEI 60268-
4:2010)
Ta slovenski standard je istoveten z: EN 60268-4:2010
ICS:
33.160.50 Pribor Accessories
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 60268-4
NORME EUROPÉENNE
July 2010
EUROPÄISCHE NORM
ICS 33.160.50 Supersedes EN 60268-4:2004

English version
Sound system equipment -
Part 4: Microphones
(IEC 60268-4:2010)
Equipements pour systèmes Elektroakustische Geräte -
électroacoustiques - Teil 4: Mikrofone
Partie 4: Microphones (IEC 60268-4:2010)
(CEI 60268-4:2010)
This European Standard was approved by CENELEC on 2010-07-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2010 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60268-4:2010 E
Foreword
The text of document 100/1678/FDIS, future edition 4 of IEC 60268-4, prepared by IEC TC 100, Audio,
video and multimedia systems and equipment, was submitted to the IEC-CENELEC parallel vote and was
approved by CENELEC as EN 60268-4 on 2010-07-01.
This European Standard supersedes EN 60268-4:2004.
The main changes with respect to EN 60268-4:2004 are the following:
– correction of noise measurement;
– added annex for digital microphones;
– added requirement for tolerances in data to be specified.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2011-04-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2013-07-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 60268-4:2010 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 61606 series NOTE  Harmonized in EN 61606 series (not modified).
IEC 61672-1:2002 NOTE  Harmonized as EN 61672-1:2003 (not modified).
__________
- 3 - EN 60268-4:2010
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

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.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

IEC 60065 (mod) 2001 Audio, video and similar electronic EN 60065 2002
+ A1 (mod) 2005 apparatus - Safety requirements + corr. August 2007
- - + A1 2006
+ A11 2008
IEC 60268-1 1985 Sound system equipment - HD 483.1 S2 1989
+ A1 1988 Part 1: General - -
1)
+ A2 1988 - -
IEC 60268-2 1987 Sound system equipment - HD 483.2 S2 1993
2)
+ A1 1991 Part 2: Explanation of general terms and - -
calculation methods
IEC 60268-3 2000 Sound system equipment - EN 60268-3 2000
Part 3: Amplifiers + corr. January 2002

IEC 60268-5 2003 Sound system equipment - EN 60268-5 2003
+ A1 2007 Part 5: Loudspeakers + A1 2009

IEC 60268-11 1987 Sound system equipment - HD 483.11 S3 1993
+ A1 1989 Part 11: Application of connectors for the - -
3)
+ A2 1991 interconnection of sound system components - -

IEC 60268-12 1987 Sound system equipment - EN 60268-12 1995
+ A1 1991 Part 12: Application of connectors for - -
4)
+ A2 1994 broadcast and similar use + A2 1995

5)
IEC 61000-4-2 1995 Electromagnetic compatibility (EMC) - EN 61000-4-2 1995
Part 4-2: Testing and measurement
techniques - Electrostatic discharge immunity
test
IEC 61000-4-3 2006 Electromagnetic compatibility (EMC) - EN 61000-4-3 2006
+ A1 2007 Part 4-3: Testing and measurement + A1 2008
techniques - Radiated, radio-frequency,
electromagnetic field immunity test

1)
HD 483.1 S2 includes A1 to IEC 60268-1.
2)
HD 483.2 S2 includes A1 to IEC 60268-2.
3)
HD 483.11 S3 includes A1 + A2 to IEC 60268-11.
4)
EN 60268-12 includes A1 to IEC 60268-12.
5)
EN 61000-4-2 is superseded by EN 61000-4-2:2009, which is based on IEC 61000-4-2:2008.

Publication Year Title EN/HD Year

IEC 61265 1995 Electroacoustics - Instruments for EN 61265 1995
measurement of aircraft noise - Performance
requirements for systems to measure one-
third-octave band sound pressure levels in
noise certification of transport-category
aeroplanes
IEC 61938 1996 Audio, video and audiovisual systems - EN 61938 1997
Interconnections and matching values - + corr. February 1997
Preferred matching values of analogue signals

ISO 354 2003 Acoustics - Measurement of sound EN ISO 354 2003
absorption in a reverberation room

ITU-T 1996 Artificial mouth - -
Recommendation
P.51
IEC 60268-4 ®
Edition 4.0 2010-06
INTERNATIONAL
STANDARD
Sound system equipment –
Part 4: Microphones
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
X
ICS 33.160.50 ISBN 978-2-88910-994-4
– 2 – 60268-4 © IEC:2010(E)
CONTENTS
FOREWORD.6
1 Scope.8
2 Normative references.8
3 General conditions.9
3.1 General .9
3.2 Measurement conditions .9
3.2.1 General .9
3.2.2 Rated conditions .10
4 Particular conditions .11
4.1 Pre-conditioning .11
4.2 Sound source .11
4.3 Measurement of sound pressure.11
4.4 Voltage measuring system .11
4.5 Acoustical environment.11
4.5.1 General .11
4.5.2 Free-field conditions .11
4.5.3 Diffuse field conditions.13
4.5.4 Microphone coupled to a sound source by means of a small cavity
coupler .14
4.6 Methods of measuring frequency response .14
4.6.1 Point-by-point and continuous sweep frequency methods .14
4.6.2 Calibration methods .15
4.7 Overall accuracy.15
4.8 Graphical presentation of results.15
5 Type description (acoustical behaviour).16
5.1 Principle of the transducer .16
5.2 Type of microphone .16
5.3 Type of directional response characteristics.16
6 Terminals and controls.16
6.1 Marking .16
6.2 Connectors and electrical interface values .16
7 Reference point and axis .16
7.1 Reference point .16
7.2 Reference axis .16
8 Rated power supply .17
8.1 Characteristic to be specified.17
8.2 Method of measurement .17
9 Electrical impedance.17
9.1 Internal impedance .17
9.1.1 Characteristic to be specified .17
9.1.2 Methods of measurement.17
9.2 Rated impedance.18
9.3 Minimum permitted load impedance .18
10 Sensitivity.18
10.1 General .18
10.2 Sensitivities with respect to acoustical environment.19

60268-4 © IEC:2010(E) – 3 –
10.2.1 Free-field sensitivity.19
10.2.2 Diffuse-field sensitivity .19
10.2.3 Close-talking sensitivity.20
10.2.4 Pressure sensitivity.21
10.3 Sensitivities with respect to nature of signal .21
10.3.1 Rated sensitivity .21
10.3.2 Characteristic sensitivity for speech .21
11 Response .22
11.1 Frequency response .22
11.1.1 Characteristic to be specified .22
11.1.2 Method of measurement .23
11.2 Effective frequency range .23
11.2.1 Characteristic to be specified .23
11.2.2 Method of measurement .23
12 Directional characteristics .23
12.1 Directional pattern .23
12.1.1 Characteristic to be specified .23
12.1.2 Methods of measurement.23
12.2 Directivity index .25
12.2.1 Characteristic to be specified .25
12.2.2 Method of measurement .25
12.3 Front-to-rear sensitivity index (0° – 180°) .25
12.3.1 Characteristic to be specified .25
12.3.2 Method of measurement .25
12.4 Noise-cancelling index .25
12.4.1 Characteristic to be specified .25
12.4.2 Method of measurement .26
12.5 Special characteristics for stereo microphones.26
12.5.1 General .26
12.5.2 Included angle of an XY (left-right) microphone .26
12.5.3 Acceptance angle .26
12.5.4 Threshold angle.27
13 Amplitude non-linearity .27
13.1 General .27
13.2 Total harmonic distortion .27
13.2.1 Characteristics to be specified .27
13.2.2 Method of measurement .27
th
13.3 Harmonic distortion of the n order (n = 2, 3,.) .28
13.3.1 Characteristic to be specified .28
13.3.2 Method of measurement .28
13.4 Difference frequency distortion of second order.29
13.4.1 Characteristic to be specified .29
13.4.2 Method of measurement .29
14 Limiting characteristics .29
14.1 Rated maximum permissible peak sound pressure.29
14.2 Overload sound pressure.30
14.2.1 Characteristic to be specified .30
14.2.2 Method of measurement .30
15 Balance .30

– 4 – 60268-4 © IEC:2010(E)
15.1 Balance of the microphone output.30
15.2 Balance under working conditions .30
16 Equivalent sound pressure level due to inherent noise.31
16.1 Characteristic to be specified.31
16.2 Method of measurement .31
17 Ambient conditions .32
17.1 General .32
17.2 Pressure range.32
17.3 Temperature range .32
17.4 Relative humidity range .32
18 External influences .32
18.1 General .32
18.1.1 Specification and measuring methods .32
18.1.2 Other external interferences.32
18.2 Equivalent sound pressure due to external magnetic fields.33
18.2.1 Characteristic to be specified .33
18.2.2 Method of measurement .33
18.3 Equivalent sound pressure due to mechanical vibration.33
18.3.1 Characteristic to be specified .33
18.3.2 Method of measurement .34
18.4 Equivalent sound pressure due to wind .34
18.4.1 Characteristic to be specified .34
18.4.2 Method of measurement .34
18.5 Transient equivalent sound pressure due to "pop" effect .37
18.5.1 Characteristic to be specified .37
18.5.2 Method of measurement .38
18.6 Equivalent sound pressure due to electromagnetic interference.39
18.6.1 General .39
18.6.2 Characteristic to be specified .39
18.6.3 Method of measurement .39
18.7 Electrostatic discharge.40
19 Magnetic stray field.40
19.1 Characteristic to be specified.40
19.2 Method of measurement .40
20 Physical characteristics.40
20.1 Dimensions .40
20.2 Weight.40
20.3 Cables and connectors .40
21 Classification of the characteristics to be specified.40
21.1 General .40
Annex A (normative) Sound insulation device .43
Annex B (informative) Simplified procedure for “pop” measurements .44
Annex C (informative) Supplement for digital microphones .47
Bibliography .49

Figure 1 – Balance of the output.30
Figure 2 – Balance under working conditions .31

60268-4 © IEC:2010(E) – 5 –
Figure 3 – Measurement set-up for wind influence .35
Figure 4 – Wind generators, type 1 (Figure 4a) and type 2 (Figure 4b)).36
Figure 5 – Electrical and mechanical set-up for the measuring of the "pop" effect .38
Figure A.1 – Sound insulation device.43
Figure B.1 – Measurement set-up.45
Figure B.2 – Test fixture for the sound field sensitivity .46

Table 1 – Reverberation time of the empty room.14
Table 2 – Speech power weighting factor at octave-band centre frequencies .22
Table 3 – Reference signal and characteristics.39
Table 4 – Classification of characteristics .42
Table C.1 – Classification of the characteristics to be specified .47
Table C.2 – Additional digital characteristics to be specified .48

– 6 – 60268-4 © IEC:2010(E)
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 fourth edition cancels and replaces the third edition published in 2004, and constitutes a
technical revision.
The main changes with respect to the previous edition are the following:
– correction of noise measurement,
– added annex for digital microphones,
– added requirement for tolerances in data to be specified.

60268-4 © IEC:2010(E) – 7 –
The text of this standard is based on the following documents:
FDIS Report on voting
100/1678/FDIS 100/1707/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.
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 publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual edition of this document may be issued at a later date.

– 8 – 60268-4 © IEC:2010(E)
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.
NOTE The characteristics specified in this standard 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 objective characteristics of at least some of the subjective descriptions used to describe microphone
performance.
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.
IEC 60065:2001, Audio, video and similar electronic apparatus – Safety requirements
Amendment 1 (2005)
IEC 60268-1:1985, Sound system equipment – Part 1: General
Amendment 1 (1988)
Amendment 2 (1988)
IEC 60268-2:1987, Sound system equipment – Part 2: Explanation of general terms and
calculation methods
Amendment 1 (1991)
IEC 60268-3:2000, Sound system equipment – Part 3: Amplifiers
IEC 60268-5:2003, Sound system equipment – Part 5: Loudspeakers
Amendment 1 (2007)
IEC 60268-11:1987, Sound system equipment – Part 11: Application of connectors for the
interconnection of sound system components
Amendment 1 (1989)
Amendment 2 (1991)
60268-4 © IEC:2010(E) – 9 –
IEC 60268-12:1987, Sound system equipment – Part 12: Application of connectors for
broadcast and similar use
Amendment 1 (1991)
Amendment 2 (1994)
IEC 61000-4-2:1995, Electromagnetic compatibility (EMC) – Part 4-2: Testing and measure-
ment 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
Amendment 1 (2007)
IEC 61265:1995, Electroacoustics – Instruments for measurement of aircraft noise – Perform-
ance requirements for systems to measure one-third-octave-band sound pressure levels in
noise certification of transport-category aeroplanes
IEC 61938:1996, Audio, video and audiovisual systems – Interconnections and matching
values – Preferred matching values of analogue signals
ISO 354:2003, Acoustics – Measurement of sound absorption in a reverberation room
ITU-T Recommendation P.51:1996, Artificial mouth
3 General conditions
3.1 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 4.7);
• marking (see also 6.1);
• 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.
3.2 Measurement conditions
3.2.1 General
For convenience in specifying how microphones shall be set up for measurement, a set of
conditions has been defined in this recommendation under the title of "rated conditions".
Three ratings are basic to the formulation of these concepts:

– 10 – 60268-4 © IEC:2010(E)
– rated impedance (see 9.2);
– rated power supply (see 8.1);
– rated sensitivity (see 10.3.1).
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 standard for electrical impedance, sensitivity,
directional pattern, dynamic range and external influences. Where alternative methods are
given, the chosen method shall be specified.
3.2.2 Rated conditions
The microphone is understood to be working under rated conditions when the following
conditions are fulfilled:
– the microphone shall operate under no-load conditions (see 9.2);
– if the microphone needs a power supply, this shall be the rated power supply;
– the microphone (except a close-talking microphone) shall be placed in a sound field
meeting the free-field conditions in 4.5.2, the waves having zero degree incidence with
respect to the reference direction;
– the undisturbed sound pressure (in the absence of the microphone) in the sound field at the
reference point of the microphone shall be sinusoidal and set at a level of 1 Pa (94 dB SPL);
– for close-talking microphones, the microphone shall be placed at a stated distance, no
more than 25 mm from the artificial mouth complying with ITU-T P.51, and the undisturbed
sound pressure in the sound field at the reference point of microphone shall be sinusoidal
and set at a level of 3 Pa (104 dB SPL);
– if a special microphone needs a different measurement level, this 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, shall be set to the position recommended by the manufacturer;
– in the absence of a clear reason to the contrary, the measurement frequency shall
be 1 000 Hz (see IEC 60268-1);
– the ambient pressure, the relative humidity and the ambient temperature shall be 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.
NOTE 1 An artificial voice which emits a signal simulating that emitted by the nose should be used for measuring
pressure-gradient close-talking microphones to ensure that nasal sounds are adequately reproduced. The absence
of such sounds in the reproduction may give rise to unnatural speech quality.
NOTE 2 Limitations of the measurement site or the measurement equipment may also require the use of other
than the given measurement sound pressure levels. This is acceptable only if any change in performance between
the level used and the reference level are known with the necessary accuracy for the relevant characteristics.

60268-4 © IEC:2010(E) – 11 –
4 Particular conditions
4.1 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.
4.2 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 performance 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 radially symmetrical with respect to the axis of the reference direction
of the microphone.
4.3 Measurement of sound pressure
A calibrated reference pressure microphone shall be used to measure the sound pressure. The
reference microphone should be calibrated with an accuracy of ±1 dB or better.
4.4 Voltage measuring system
The electromotive force (e.m.f.) generated by the microphone, when in a sound field, shall be
determined by measuring the open-circuit voltage of the microphone using a voltmeter with an
input impedance of at least 100 times the rated impedance of the microphone. If external
equipment, such as a power supply, places a load on the microphone, the true e.m.f. shall be
calculated by correcting for the effect of this load.
4.5 Acoustical environment
4.5.1 General
The microphone can be measured in different acoustical environments:
a) in a free field or similar with neglectable 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).
4.5.2 Free-field conditions
4.5.2.1 General
A free-field sound wave is normally divergent in character. In certain circumstances it can
approximate an ideal plane wave.

– 12 – 60268-4 © IEC:2010(E)
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 ⎞
(1−B) +B⎜1+ ⎟cosθ
⎜ ⎟
jkr
⎝ ⎠
where
1–B is the contribution of the pressure component;
B is the contribution of the pressure gradient component;
k = 2π/λ or 2πf/v;
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
the microphone.
4.5.2.2 Spherical waves
The sound pressure generated in a free field by an omnidirectional sound source varies
inversely with the distance from the acoustic centre of the sources.

60268-4 © IEC:2010(E) – 13 –
The output voltage of the microphone varies inversely with the distance between the source
and the microphone when the relevant dimensions of both are small compared with the
wavelength, allowing the results from the measurements made at a certain distance r to be
converted by calculation to results which would be obtained at the reference distance.
When either the circumference of the radiating surface of the source or the circumference of
the principal acoustic entry of the microphone exceeds the wavelength, this computation
applies only when the measuring distance conforms to:
r ≥d
r ≥ d / λ
where
r is the dis
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