IEC 60268-4:2004

INTERNATIONAL IEC
STANDARD
60268-4
Third edition
2004-02
Sound system equipment –
Part 4:
Microphones
Reference number
Publication numbering
As from 1 January 1997 all IEC publications are issued with a designation in the

60000 series. For example, IEC 34-1 is now referred to as IEC 60034-1.

Consolidated editions
The IEC is now publishing consolidated versions of its publications. For example,

edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the base publication, the

base publication incorporating amendment 1 and the base publication incorporating

amendments 1 and 2.
Further information on IEC publications
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology. Information relating to
this publication, including its validity, is available in the IEC Catalogue of
publications (see below) in addition to new editions, amendments and corrigenda.
Information on the subjects under consideration and work in progress undertaken
by the technical committee which has prepared this publication, as well as the list
of publications issued, is also available from the following:
• IEC Web Site (www.iec.ch)
• Catalogue of IEC publications
The on-line catalogue on the IEC web site (http://www.iec.ch/searchpub/cur_fut.htm)
enables you to search by a variety of criteria including text searches, technical
committees and date of publication. On-line information is also available on
recently issued publications, withdrawn and replaced publications, as well as
corrigenda.
• IEC Just Published
This summary of recently issued publications (http://www.iec.ch/online_news/
justpub/jp_entry.htm) is also available by email. Please contact the Customer
Service Centre (see below) for further information.
• Customer Service Centre
If you have any questions regarding this publication or need further assistance,
please contact the Customer Service Centre:
Email: custserv@iec.ch
Tel: +41 22 919 02 11
Fax: +41 22 919 03 00
INTERNATIONAL IEC
STANDARD
60268-4
Third edition
2004-02
Sound system equipment –
Part 4:
Microphones
 IEC 2004  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch  Web: www.iec.ch
PRICE CODE
Commission Electrotechnique Internationale X
International Electrotechnical Commission
МеждународнаяЭлектротехническаяКомиссия
For price, see current catalogue

– 2 – 60268-4  IEC:2004(E)
CONTENTS
FOREWORD.5

1 Scope.7

2 Normative references.7

3 General conditions.8

3.1 General .8

3.2 Measurement conditions .8

4 Particular conditions .9

4.1 Pre-conditioning .9
4.2 Sound source .10
4.3 Measurement of sound pressure.10
4.4 Voltage measuring system .10
4.5 Acoustical environment.10
4.6 Methods of measuring frequency response .13
4.7 Overall accuracy.14
4.8 Graphical presentation of results.14
5 Type description (acoustical behaviour).14
5.1 Principle of the transducer .14
5.2 Type of microphone .14
5.3 Type of directional response characteristics.14
6 Terminals and controls.15
6.1 Marking .15
6.2 Connectors and electrical interface values .15
7 Reference point and axis .15
7.1 Reference point .15
7.2 Reference axis .15
8 Rated power supply .15
8.1 Characteristic to be specified.15
8.2 Method of measurement .16
9 Electrical impedance.16
9.1 Internal impedance .16
9.2 Rated impedance.17
9.3 Minimum permitted load impedance .17

10 Sensitivity.17
10.1 General .17
10.2 Sensitivities with respect to acoustical environment.18
10.3 Sensitivities with respect to nature of signal .20
11 Response .21
11.1 Frequency response .21
11.2 Effective frequency range .21
12 Directional characteristics .22
12.1 Directional pattern .22
12.2 Directivity index .23
12.3 Front-to-rear sensitivity index (0° – 180°) .23
12.4 Noise-cancelling index .24
12.5 Special characteristics for stereo microphones.24

60268-4 © IEC:2004(E) – 3 –
13 Amplitude non-linearity .25

13.1 General .25

13.2 Total harmonic distortion .26
th
13.3 Harmonic distortion of the n order (n = 2, 3,.).26

13.4 Difference frequency distortion of second order.27

14 Limiting characteristics .28

14.1 Rated maximum permissible peak sound pressure.28

14.2 Overload sound pressure.28

15 Balance .28

15.1 Balance of the microphone output.28
15.2 Balance under working conditions .28
16 Equivalent sound pressure level due to inherent noise.29
16.1 Characteristic to be specified.29
16.2 Method of measurement .29
17 Ambient conditions .29
17.1 General .29
17.2 Pressure range.29
17.3 Temperature range .29
17.4 Relative humidity range .29
18 External influences .30
18.1 General .30
18.2 Equivalent sound pressure due to external magnetic fields.30
18.3 Equivalent sound pressure due to mechanical vibration.31
18.4 Equivalent sound pressure due to wind .32
18.5 Transient equivalent sound pressure due to "pop" effect .33
18.6 Equivalent sound pressure due to electromagnetic interference.34
18.7 Electrostatic discharge.34
19 Magnetic stray field.34
19.1 Characteristic to be specified.34
19.2 Method of measurement .35
20 Physical characteristics.35
20.1 Dimensions .35
20.2 Weight.35
20.3 Cables and connectors .35

21 Classification of the characteristics to be specified.35
21.1 General .35
21.2 Classification.36
Annex A (normative) Sound insulation device .41
Annex B (informative) Simplified procedure for “pop” measurements .42
Bibliography .45
Figure 1a – Balance of the output.37
Figure 1b – Balance under working conditions .37
Figure 2 – Measurement set-up for wind influence .37
Figure 3a – Wind generator with radial fan (front and side view) .38

– 4 – 60268-4  IEC:2004(E)
Figure 3b – Wind generator with axial fan.38

Figure 4 – Electrical and mechanical set-up for the measuring of the "pop" effect .39

Figure 5 – Reference signal and characteristics.40

Figure A.1 – Sound insulation device.41

Figure B.1 – Measurement set-up.43

Figure B.2 – Test fixture for the sound field sensitivity .44

Table 1 – Reverberation time of the empty room.12

Table 2 – Speech power weighting factor at octave-band centre frequencies .20

Table 3 – Classification of characteristics .36

60268-4 © IEC:2004(E) – 5 –
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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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 third edition cancels and replaces the second edition published in 1997, and constitutes a
technical revision.
The text of this standard is based on the following documents:
FDIS Report on voting
100/721/FDIS 100/750/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.

– 6 – 60268-4  IEC:2004(E)
The committee has decided that the contents of this publication will remain unchanged until
2008. At this date, the publication will be

• reconfirmed;
• withdrawn;
• replaced by a revised edition, or

• amended.
60268-4 © IEC:2004(E) – 7 –
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 gives recommendations as to characteristics to be specified.
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 use 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
IEC 60268-1:1985, Sound system equipment – Part 1: General
IEC 60268-2:1987, Sound system equipment – Part 2: Explanation of general terms and
calculation methods
IEC 60268-3:2000, Sound system equipment – Part 3: Amplifiers
IEC 60268-5:2003, Sound system equipment – Part 5: Loudspeakers
IEC 60268-11:1987, Sound system equipment – Part 11: Application of connectors for the
interconnection of sound system components
IEC 60268-12:1987, Sound system equipment – Part 12: Application of connectors for
broadcast and similar use
IEC 60574-3:1983, Audiovisual, video and television equipment and systems – Part 3:
Connectors for the interconnection of equipment in audiovisual systems
IEC 60914:1988, Conference systems – Electrical and audio requirements

– 8 – 60268-4  IEC:2004(E)
IEC 61000-4-2:1995, Electromagnetic compatibility (EMC) – Part 4-2: Testing and measure-

ment techniques – Electrostatic discharge immunity test

IEC 61000-4-3:2002, Electromagnetic compatibility (EMC) – Part 4: Testing and measurement

techniques – Section 3: Radiated, radio-frequency, electromagnetic field immunity test

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 coefficients in a reverberant room
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 Introduction
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:
– 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.

60268-4 © IEC:2004(E) – 9 –
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 at no-load condition (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 free sound field,
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 0,3 Pa (84 dB SPL)
– for close-talking microphones, the microphone shall be placed at a stated distance, no
more than 25 mm from the artificial mouth, 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 1000 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.
NOTE 1 The ITU/T has published Recommendation P.51 which includes the specification of an artificial mouth. An
artificial mouth conforming to that Recommendation should be used wherever possible.
NOTE 2 An artificial voice which emits a signal simulating that emitted by noise 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 3 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.
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.
– 10 – 60268-4  IEC:2004(E)
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.
NOTE If external equipment, such as a power supply, places a load on the microphone, the true e.m.f. should 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 without boundaries:
– 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
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.

60268-4 © IEC:2004(E) – 11 –
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, 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.
4.5.2.1 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.
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
– 12 – 60268-4  IEC:2004(E)
where
r is the distance from the source to the measuring point;

d is the effective diameter of the sound source;

λ is the sound wavelength.
NOTE It is advisable for the distance from the source to the measuring point to exceed three times the largest
dimension of the radiating surface of the source.

4.5.2.2 Plane progressive waves

A plane progressive wave can be obtained either in a duct or in a free field.
a) In a duct
In designing a duct capable of producing useful results, there are many problems to be
solved such as the design of the terminating impedance, the avoidance of cross-modes, the
shape of the original wavefront and the relative dimensions of the duct and the microphone.
b) In a free field
A spherical wave at a distance of at least half the wavelength from the centre of curvature
at the lowest frequency of measurement is a practical approximation to a plane progressive
wave.
NOTE It should be understood that for measurement of "shotgun" types and pressure zone microphones,
determining the smallest permitted distance is complicated and no exact rules can be given. Therefore, in these
cases the measuring distance used should be stated.
4.5.2.3 Use of an artificial mouth
In order that the conditions of test may be similar to those of actual use, it is necessary to
introduce an obstacle in the shape of a human head when measuring close-talking
microphones by means of an artificial mouth (see note to 3.2.2).
4.5.3 Diffuse field conditions
Some measurements can be made in a diffuse field in which sound waves are propagated with
random incidence. In this case, bands of noise of third-octave width or broadband signals
together with suitable filtering shall be used.
A diffuse sound field can be approximately realized in a reverberant room characterized by a
sufficiently long duration of reverberation at a sufficiently large distance from the source and
the walls, and above a limiting frequency (see also ISO 354).

The reverberation time T of the empty room is specified in Table 1.
Table 1 – Reverberation time of the empty room
T > 5 s 5 s 5 s 4,5 s 3,5 s 2 s
at 125 Hz 250 Hz 500 Hz 1 000 Hz 2 000 Hz 4 000 Hz
For the determination of the lower frequency limit, the following equation can be used:
f ≥
1/ 3
V
where
V is the volume of the room in cubic metres;
f is the frequency in hertz.
60268-4 © IEC:2004(E) – 13 –
The region of measurement shall be chosen at such a distance from the source that the direct

sound of the source is negligible.

When an omnidirectional source is used, the minimum distance r (in metres) from the source

to the measuring points is given by:

1/ 2
r ≥ 0,06()V T
where
V is the volume of the room in cubic metres;

T is the Sabine reverberation time at the frequency f.

NOTE The preceding two formulae allow wider tolerance than those of the first edition of this standard. Though
this is unlikely to change the measurement results significantly, it is recommended to avoid the lower limits as far
as possible.
4.5.4 Microphone coupled to a sound source by means of a small cavity coupler
To determine the pressure sensitivity of a microphone, a rigid cavity is used to couple the
sound source to the microphone. This method is useful for obtaining the pressure sensitivity of
a microphone by comparison with the sensitivity of a calibrated reference microphone. In order
to obtain a sufficiently uniform sound pressure inside the cavity, this method shall only be used
within the limits of the frequency range where the linear dimensions of the cavity are less than
one-tenth of the wavelength. At low frequencies care shall be taken to eliminate air leakage.
4.6 Methods of measuring frequency response
4.6.1 Point-by-point and continuous sweep frequency methods
Response curves may be prepared point-by-point or automatically.
a) Point-by-point method
Great care shall be taken to ensure that all significant peaks and troughs of the frequency
response curve are explored.
NOTE The graph should clearly indicate the measurement points.
b) Continuous sweep frequency method
The rate of traversing the frequency range shall be slow enough to ensure that the resulting
curve does not deviate from that which would be obtained under steady state conditions.
NOTE 1 Stopping the trace at any instant should not change the indicated response by more than ±1 dB.
NOTE 2 The following additional apparatus may be used:
– equipment capable of automatically maintaining the requisite sound pressure level over the frequency range
concerned;
– an automatic level recorder as output indicator.
c) Special computer-based signals and procedures
Several computer algorithms are available to generate signals and to evaluate responses in
the time domain, as well as in the frequency domain. Some of them are just digital
procedures that replace their analogue ancestors, such as the Fast Fourier Transform for
spectral analysis. Other algorithms provide new types of test signals and responses. Most
of them are applicable if the user takes care of their inherent limitations and preconditions.
In cases where existing specified procedures are replaced by new ones for the evaluation
of the same characteristic, the user has to ensure that the result is at least as accurate as
with the old procedure. New techniques are considered for standardization when basic
matters of background and their relationship to known properties have been determined.

– 14 – 60268-4  IEC:2004(E)
4.6.2 Calibration methods
Irrespective of the choice of the point-by-point or automatic method, there are two methods of

conducting the calibration.
a) Substitution method
A method of measurement of the response of a microphone in which the microphone to be

calibrated and the standard microphone employed to measure the requisite sound pressure

are placed alternately at the same test points in the sound field.

This method leads to the highest accuracy.

b) Simultaneous comparison method

For reasons of convenience an alternative method for measuring the response of a
microphone is sometimes employed in which the microphone to be calibrated and the
standard microphone employed to measure the requisite sound pressure are placed
simultaneously at two different points normally not widely separated. Care shall be taken
that one microphone is not placed at a more favourable point in the sound field than the
other. The points chosen shall be such that the results of a response test carried out by the
comparison method agree within ±1 dB with the corresponding results obtained by the
substitution method.
The simultaneous method shall be used only after checking that this requirement is met.
Compliance with this requirement can be checked by the following:
– the sound pressures, measured at the two different points in the free sound field by means
of a calibrated microphone, shall correspond within ±1 dB;
– the distance between the microphones shall be such that the sound pressure at each of the
two microphone points is independent within ±1 dB of the presence of the second
microphone at the other point.
4.7 Overall accuracy
An overall accuracy of ±2 dB or better shall be obtained for the calibration of all types of
microphones.
4.8 Graphical presentation of results
The graphical presentation of measurement results should follow the recommendations of
IEC 60268-1.
5 Type description (acoustical behaviour)

5.1 Principle of the transducer
The manufacturer shall specify the principle of the transducer, for example electrostatic
(condenser), electrodynamic, electromagnetic or piezoelectric.
5.2 Type of microphone
The manufacturer shall specify the type of microphone, for example pressure, pressure-
gradient (with acoustical phase shift network, if any), or combination of a pressure and
pressure-gradient microphone, or velocity microphone.
5.3 Type of directional response characteristics
The manufacturer shall specify the type of directional response characteristics of the
microphone, for example omnidirectional, unidirectional, bidirectional, (sphere, cardioid,
hypercardioid, hemisphere or half-cardioid of revolution, etc.).

60268-4 © IEC:2004(E) – 15 –
6 Terminals and controls
6.1 Marking
Recommendations for marking the terminals and controls are given in IEC 60268-1, clause 5

and IEC 61938, 7.4.4 and 7.5.5, with the addition of the following requirement, if the

microphone conforms to the requirements of IEC 61938, clause 7:

The polarity shall be indicated by a mark, preferably a coloured dot or a connector pin number

designated in the instruction manual, at that output terminal at which a positive instantaneous

voltage is produced by an inward movement of the diaphragm or equivalent, that is an increase

in sound pressure at the principal entry. Marking for safety shall be in accordance with IEC
60065 or other appropriate safety standard.
Marking of the polarity is recommended if the microphone conforms to the requirements of
IEC 61938. However, it is a requirement of this standard if it does not.
6.2 Connectors and electrical interface values
Connectors and their wiring shall be in accordance with IEC 60268-11 or IEC 60268-12.
Interface values (voltages and impedances) shall be in accordance with IEC 61938.
7 Reference point and axis
7.1 Reference point
In the absence of clear reason to the contrary, the reference point shall be the centre of the
principal sound entry. Otherwise it should be stated.
NOTE In order to allow unambiguous specification of the reference point, reference axis and polarity, the
manufacturer should designate a principal sound entry even for a bidirectional microphone.
7.2 Reference axis
The reference axis is a line passing through the reference point indicating a recommended
direction of sound incidence specified by the manufacturer. The microphone shall be so
designed that the recommended direction of sound incidence is obvious to the user.
NOTE The reference axis should preferably be perpendicular to the plane of the principal acoustic entry of the
microphone and should pass through the centre of the entry.
8 Rated power supply
8.1 Characteristic to be specified
The following information shall be specified by the manufacturer for each pair of microphone
terminals to be connected to the power supply and for each position of the power supply
adaptor, if any:
– the type of power supply (phantom, A-B, etc.; see IEC 61938);
– power supply voltage and its upper and lower limits;
– apparent current drawn from the power supply, expressed in amperes;
– for multi-voltage microphones, the voltage-current characteristic.

– 16 – 60268-4  IEC:2004(E)
8.2 Method of measurement
a) The microphone is operated under rated conditions.

b) The current drawn from the power supply is measured in amperes.

9 Electrical impedance
9.1 Internal impedance
9.1.1 Characteristic to be specified

The modulus of the internal impedance of the microphone measured between the output
terminals.
NOTE If the impedance can be satisfactorily represented by that of a simple network, the values of the network
components may be given. If this is not applicable, the impedance should be specified as a function of frequency.
9.1.2 Methods of measurement
The internal impedance may be measured by the comparison method or by applying a sound
pressure and measuring the output voltage under different load conditions. Both methods are
indicated below.
a) Method 1
The impedance can be measured by means of a measuring bridge. An alternative method
is that of comparison with a known impedance. In the latter case a constant current from a
high impedance source is passed through the microphone and the voltage across its
terminals is measured.
The microphone is then replaced by a known resistance and the procedure repeated.
Comparison of the two values give
...