IEC 61094-5:2016

IEC 61094-5 ®
Edition 2.0 2016-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Measurement microphones –
Part 5: Methods for pressure calibration of working standard microphones
by comparison
Électroacoustique – Microphones de mesure –
Partie 5: Méthodes pour l'étalonnage en pression par comparaison des
microphones étalons de travail

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IEC 61094-5 ®
Edition 2.0 2016-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Measurement microphones –

Part 5: Methods for pressure calibration of working standard microphones

by comparison
Électroacoustique – Microphones de mesure –

Partie 5: Méthodes pour l'étalonnage en pression par comparaison des

microphones étalons de travail

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.140.50 ISBN 978-2-8322-3434-1

– 2 – IEC 61094-5:2016 © IEC 2016
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references. 6
3 Terms and definitions . 6
4 Reference environmental conditions . 7
5 Principles of pressure calibration by comparison . 7
5.1 Principles . 7
5.1.1 General principle . 7
5.1.2 General principles using simultaneous excitation . 7
5.1.3 General principles using sequential excitation . 8
5.2 Measuring the output voltages of the microphones . 8
6 Factors influencing the pressure sensitivity . 8
6.1 General . 8
6.2 Microphone pressure equalization mechanism . 8
6.3 Polarising voltage . 9
6.4 Reference shield configuration . 9
6.5 Pressure distribution over the diaphragms . 9
6.6 Dependence on environmental conditions . 10
6.7 Validation . 10
7 Calibration uncertainty components . 10
7.1 General . 10
7.2 Sensitivity of the reference microphone . 10
7.3 Measurements of microphone output . 11
7.4 Differences between the sound pressure at the test microphone and that at
the reference microphone . 11
7.5 Acoustic impedances of the microphones . 11
7.6 Microphone separation distance . 11
7.7 Microphone capacitance . 11
7.8 Microphone configuration during calibration . 11
7.9 Uncertainty on pressure sensitivity level . 12
Annex A (informative)  Examples of couplers and jigs for simultaneous excitation . 13
A.1 A coupler for use with WS2 microphones at frequencies up to 10 kHz . 13
A.2 A jig for use with WS2 or WS3 microphones at frequencies up to 20 kHz . 14
Annex B (informative) Examples of couplers for sequential excitation . 16
B.1 A coupler for use with LS1 microphones at frequencies up to 8 kHz . 16
B.2 A coupler for use with WS2 microphones at frequencies up to 16 kHz . 16
Annex C (informative) Determining the open-circuit sensitivity of a measurement
microphone without using the insert-voltage method . 18
Annex D (informative) Typical uncertainty analysis . 19
D.1 Introduction . 19
D.2 Analysis . 19
D.3 Combined and expanded uncertainties . 21
Bibliography . 22

Figure A.1 – A coupler for use with WS2 microphones . 13

Figure A.2 – A jig fitted with an LS2 and WS2 microphone . 14
Figure A.3 – Example arrangement of LS2 and WS2 microphones in a jig . 14
Figure A.4 – Example arrangement of LS2 and WS3 microphones in a jig . 14
Figure B.1 – A coupler for use with LS1 microphones . 16
Figure B.2 – A coupler for use with WS2 microphones . 17

Table A.1 – Calculated corrections to be added to the sensitivity level of the WS3
microphone when using the arrangement in Figure A.4 . 15
Table D.1 – Example uncertainty budget . 20

– 4 – IEC 61094-5:2016 © IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS – MEASUREMENT MICROPHONES –

Part 5: Methods for pressure calibration of working
standard microphones by comparison

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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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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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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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 61094-5 has been prepared by IEC technical committee 29:
Electroacoustics.
This edition cancels and replaces the first edition published in 2001. This edition constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) details of additional components of uncertainty;
b) revised corrections for type WS3 microphones;
c) provision for the calibration of microphones in driven shield configuration.

The text of this standard is based on the following documents:
CDV Report on voting
29/870/CDV 29/887A/RVC
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 in the IEC 61904 series, published under the general title Measurement
microphones, 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 website 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.
– 6 – IEC 61094-5:2016 © IEC 2016
ELECTROACOUSTICS – MEASUREMENT MICROPHONES –

Part 5: Methods for pressure calibration of working
standard microphones by comparison

1 Scope
This part of IEC 61094-5 is applicable to working standard microphones with removable
protection grids meeting the requirements of IEC 61094-4 and to laboratory standard micro-
phones meeting the requirements of IEC 61094-1.
This part of IEC 61094 describes methods of determining the pressure sensitivity by
comparison with either a laboratory standard microphone or another working standard
microphone with known sensitivity in the respective frequency range.
Alternative comparison methods based on the principles described in IEC 61094-2 are
possible but beyond the scope of this part of IEC 61094.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61094-1, Measurement microphones – Part 1: Specifications for laboratory standard
microphones
IEC 61094-4, Measurement microphones – Part 4: Specifications for working standard
microphones
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61094-1 and the
following apply.
3.1
reference microphone
laboratory standard microphone or working standard microphone with known pressure
sensitivity
3.2
test microphone
laboratory standard microphone or working standard microphone to be calibrated by
comparison with a reference microphone
3.3
monitor microphone
microphone used to measure changes in sound pressure

3.4
coupler
device which, when fitted with microphones, forms a cavity of predetermined shape and
dimensions and provides an acoustic coupling element between the microphones and
between the microphones and the sound source
3.5
jig
device which, when fitted with microphones, holds them with their diaphragms face to face
separated by a small distance but does not enclose the space between them
4 Reference environmental conditions
The reference environmental conditions are:
• temperature 23,0 °C
• static pressure 101,325 kPa
• relative humidity 50 %
5 Principles of pressure calibration by comparison
5.1 Principles
5.1.1 General principle
The pressure sensitivity of a measurement microphone is defined in terms of a sound
pressure applied uniformly over the diaphragm. Consequently, the pressure sensitivity can
only be realised in principle for microphones from which the protection grid can be removed
and the diaphragm exposed to the sound pressure stimulus.
The principle of these comparison methods is that when the reference microphone and the
test microphone are exposed to the same sound pressure either simultaneously or
sequentially, the ratio of their pressure sensitivities is given by the ratio of their open-circuit
output voltages. The sensitivity (both modulus and phase) of the test microphone can then be
calculated from the sensitivity of the reference microphone.
The principle of the method allows the test microphone to be attached to a particular
preamplifier and the sensitivity of the system may be referred to the output of that
preamplifier.
Multi-frequency measurements can be performed particularly rapidly if a wideband sound
source is used and the output voltages of the microphones are analysed in narrow bands.
NOTE If the reference and test microphones have significantly different frequency response characteristics, e.g.
around resonance frequencies, or when a pressure response microphone is compared with a free-field response
microphone, this approach can lead to errors when the intention is to determine the pressure sensitivity at the test
frequency, rather than the test frequency band. Due consideration of the analysis bandwidth is advised to avoid
th
such errors. Typically, a bandwidth of 1/6 -octave or narrower will be sufficient to constrain any error to less than
0,01 dB. However further caution is advised on reducing the bandwidth too severely, as can be possible with FFT
(fast Fourier transform) analysers, as this can highlight deficiencies such as standing waves in the sound field,
which can also lead to errors (see [1] for further details).
5.1.2 General principles using simultaneous excitation
In order for the two microphones to be simultaneously exposed to essentially the same sound
pressure it is usually necessary for the front surfaces of the two microphones to be separated
___________
Numbers in brackets refer to the bibliography.

– 8 – IEC 61094-5:2016 © IEC 2016
by a small fraction of the wavelength at the highest frequency of interest. For frequencies up
to 20 kHz, this can be achieved by mounting the two microphones face-to-face separated by
approximately 1 mm in either a coupler or a jig.
The optimum microphone separation is somewhat dependent on the acoustic environment and
should be determined for a particular set-up. Details of likely levels of performance can be
found in [1]
Couplers usually contain an integral sound source; jig mounted microphones are usually
exposed to an externally produced sound field. In order to reduce the effect of systematic
differences in sound pressure between the two microphone positions, for example caused by
some asymmetries, the following procedure shall be used: after the ratio of the microphone
pressure sensitivities is first determined, the microphones shall be interchanged, and the
measurement repeated. The sensitivity is then calculated from the mean of the two ratios.
Examples of practical arrangements and precautions to be taken are given in Annex A.
NOTE Avoiding asymmetry and standing waves in the sound field, especially in jig configurations, has a
significant beneficial impact on the reliability of the results.
5.1.3 General principles using sequential excitation
In order for the two microphones to be sequentially exposed to essentially the same sound
pressure, either the exchange of microphones shall not change the sound pressure
significantly or any significant change shall be detected and corrected. This can be achieved
by incorporating a sound source, a monitor microphone, and the test/reference microphone in
a coupler. In any design of coupler, the monitor microphone shall accurately sense changes in
the sound pressure at the test/reference microphone position. Examples of practical
arrangements are given in Annex B.
5.2 Measuring the output voltages of the microphones
The output of a test or reference microphone may be determined as the open-circuit voltage
by use of the insert voltage technique (see 5.3 of IEC 61094-2:2009) or by using a measuring
system consisting of a high input impedance microphone preamplifier and a voltmeter (see
Annex C).
The method used to measure the output voltage of the test microphone shall be stated on any
calibration certificate.
6 Factors influencing the pressure sensitivity
6.1 General
The pressure sensitivity of a measurement microphone can depend on environmental
conditions. Further, the definition of the pressure sensitivity implies that certain requirements
be fulfilled by the measurements. It is essential during a calibration that these conditions are
controlled sufficiently well if the resulting uncertainty components are to remain small.
6.2 Microphone pressure equalization mechanism
The normal construction of a measurement microphone has the cavity behind the diaphragm
fitted with a narrow pressure-equalizing tube to permit the static pressure to be the same on
both sides of the diaphragm. Consequently, at very low frequencies, this tube also partially
equalizes the sound pressure. If, during the calibration, the sound which is coherent with that
on the diaphragm is incident on the pressure-equalizing tube, then this could change the
apparent sensitivity at low frequencies and the result would not be the true pressure
sensitivity.
In a jig, where sound is incident on the pressure equalizing tube, the size of this change shall
be determined by comparing calibrations made in the jig with calibrations made in a coupler
that does not expose the pressure equalizing tube to the sound field.
In a coupler an "O" ring can be used to seal the gap between the coupler and the microphone.
If this is done, care shall be taken to ensure that the "O" ring does not exert undue force on
the microphone and cause a change in sensitivity.
6.3 Polarising voltage
If the test microphone requires an external polarising voltage, then the polarising voltage used
during the calibration shall be reported.
If the reference microphone requires an external polarising voltage, then any difference
between that applied when it was calibrated and that applied when it is used as the reference
microphone shall be allowed for in the uncertainty calculations (see Annex D).
6.4 Reference shield configuration
When the open-circuit voltage is measured, the shield configurations given in IEC 61094-1 or
IEC 61094-4 shall be used.
If a microphone is intended to be used with a preamplifier having a non-standard shield
configuration, then it shall be calibrated as a system along with its preamplifier.
When insert voltage calibrations are performed, it shall be stated whether output voltage from
the microphone is applied to the shield (driven shield configuration), or whether the shield is
grounded.
If the instruction manual specifies a maximum mechanical force to be applied to the central
electrical contact of the microphone, this limit shall not be exceeded.
6.5 Pressure distribution over the diaphragms
The definition of the pressure sensitivity assumes that the sound pressure over the diaphragm
is applied uniformly. The output voltage of a microphone presented with a non-uniform
pressure distribution over the surface of the diaphragm will differ from the output voltage of
the microphone when presented with a uniform pressure distribution having the same mean
value, because the microphone is usually more sensitive to a sound pressure at the centre of
the diaphragm.
Uniformity of sound pressure over the diaphragm of the microphone can be optimised by
maintaining the radial symmetry of the sound field around the circumferences of the
microphones. This can be achieved using a radially symmetric sound source positioned
coaxially with the microphones and, when the microphones are mounted in a jig, with the
microphones positioned in the far field of the sound source. Although pressure non-uniformity
over the surface of the diaphragm can be minimised by using a radially symmetric sound
source, some non-uniformity at high frequencies can remain even with a perfect source.
It is difficult to control the uniformity of the sound field in an actual calibration set-up.
However, the combined effect of asymmetries in the sound field and in the microphones
becomes evident when the microphones are rotated relative to each other about their axis of
symmetry. Thus, the related component of measurement uncertainty can be reduced by
averaging results from a number of such measurement configurations.
NOTE When comparing microphones of the same model, the requirement for uniformity of the sound field reduces
to a requirement of rotational symmetry of the sound field.

– 10 – IEC 61094-5:2016 © IEC 2016
Alternatively, issues with sound field non-uniformity can be overcome if excitation is made
with a diffuse sound field, for example in a reverberation room. Care should be taken to avoid
creating standing waves in the sound field surrounding the microphones as these can cause
significant and unpredictable measurement errors. A broadband source, or repeated
measurements at different positions within the field, is also necessary to achieve a sufficiently
low measurement uncertainty.
The effect of a non-uniform pressure distribution over the surface of the diaphragm will be
significantly greater if the test and reference microphones are of different diameters. A
theoretical model which can be used to apply corrections and assess the uncertainties in this
case is given in the literature (for example [1]).
6.6 Dependence on environmental conditions
The sensitivity of a microphone can depend on static pressure, temperature or humidity. This
dependence can be determined by comparison with a well characterised laboratory standard
microphone over a range of conditions.
If the reference microphone and the test microphone are different manufacturer models, then
the sensitivity of the reference microphone shall be corrected to the actual environmental
conditions during the test. Alternatively, if they are of the same model, there can be an
advantage in assuming that they have the same dependence on environmental conditions so
that the calibration of the test microphone can be referred to the conditions at which the
calibration of the reference microphone is valid.
Alternatively, when reporting the results of a calibration, the pressure sensitivity can be
corrected to the reference environmental conditions if reliable correction data are available.
The actual conditions during the calibration shall be reported.
6.7 Validation
Calibrations performed in any particular jig or coupler shall be validated by comparison with
calibrations performed in other jigs and couplers and alternative sound sources. A separate
validation is necessary for each different type of microphone. If the test microphone is a
laboratory standard microphone, then the jig or coupler can be validated by comparing a
comparison calibration with a reciprocity calibration. For some microphones, it can be
necessary to use more than one jig and/or coupler to cover a full frequency range with low
uncertainty.
7 Calibration uncertainty components
7.1 General
In addition to the factors influencing the pressure sensitivity mentioned in Clause 6, further
uncertainty components are introduced by the method, the equipment and the degree of care
under which the calibration is carried out. Factors which affect the calibration in a known way
should be measured or calculated with an accuracy necessary to achieve the desired overall
measurement uncertainty, and with as high an accuracy as practicable if their influence is to
be minimised.
7.2 Sensitivity of the reference microphone
The uncertainty in the sensitivity of the reference microphone directly affects the uncertainty
in the sensitivity of the test microphone.

7.3 Measurements of microphone output
Uncertainties of random or time-varying nature in the measurement of the outputs of the
microphones directly affect the uncertainty in the sensitivity of the test microphone.
Uncertainties of systematic nature in the measurement of the outputs of the microphones can
affect the uncertainty in the sensitivity of the test microphone. The uncertainty can be reduced
if the same system is used for both the test and reference microphones.
If test and reference microphone are measured simultaneously, systematic uncertainty can be
reduced using the procedure described in Annex C.
7.4 Differences between the sound pressure at the test microphone and that at the
reference microphone
With simultaneous or sequential excitation, differences in the acoustic impedance between
the test and reference microphones can cause the sound pressure at the test and reference
microphones to differ. A theoretical model which may be used to assess the resulting
uncertainty can be found in the literature (for example [2]).
7.5 Acoustic impedances of the microphones
When the reference microphone and the test microphone have significantly different acoustic
impedances (for example, pressure and free-field response microphones at frequencies above
10 kHz), they can respond differently to the same sound field because of differing volume
velocities at the diaphragms. It is recommended that wherever possible a reference
microphone of similar acoustic impedance to that of the test microphone be used. If no
suitable reference microphone is available, the size of the error caused should be estimated
and added to the uncertainty budget.
7.6 Microphone separation distance
The ideal microphone separation distance used in simultaneous excitation measurements
should be established for each acoustic environment in which jig measurements are to be
carried out. The distance can be determined by making a series of measurements at different
separations and comparing the results with a primary pressure calibration for the same
microphone. Measurements made in some sound fields can be very sensitive to very small
changes in microphone separation distance and microphone position relative to the sound
field. In these cases it is preferable to improve the sound field rather than the positioning
system because a very reproducible positioning system can introduce repeatable systematic
errors that are not easily detected.
7.7 Microphone capacitance
In some calibration methods (for example the approach outlined in Annex C), the gain of the
microphone preamplifier(s) used is assumed to be constant when fitted with different
microphones. However the gain of the preamplifier is typically a function of the attached
microphone capacitance.
Therefore a correction should be made or a component of uncertainty allowed if the
capacitances of the reference microphone and test microphone are sufficiently different for
the influence on the preamplifier gain to be significant.
NOTE This effect is avoided if the insert voltage technique is used.
7.8 Microphone configuration during calibration
It may be necessary to fit a microphone with one or more adapters suiting a particular
calibration coupler or configuration. Such adapters may have an influence on the sensitivity of
the microphone, and this shall be included as an uncertainty component.

– 12 – IEC 61094-5:2016 © IEC 2016
NOTE Both the reference and test microphones can be influenced by the fitting of adapters.
7.9 Uncertainty on pressure sensitivity level
For determining the pressure sensitivity level of working standard microphones, when the
reference microphone has been calibrated in accordance with IEC 61094-2, it is estimated
that a comparison calibration of microphones of the same diameter can achieve an expanded
uncertainty with coverage factor 2 (see ISO/IEC Guide 98-3) of approximately 0,1 dB at low
and middle frequencies. The uncertainty increases to about 0,2 dB at 10 kHz and 20 kHz for
WS1P and WS2P working standard microphones, respectively. Annex D contains an example
of an uncertainty analysis.
Annex A
(informative)
Examples of couplers and jigs for simultaneous excitation
A.1 A coupler for use with WS2 microphones at frequencies up to 10 kHz
The coupler shown in Figure A.1 allows two microphones with exposed diaphragms to be
inserted face-to-face separated by about 2 mm. The coupler contains a radial sound source
that generates a radially symmetric acoustic field between the diaphragms. In this example
the grid of the test microphone has been removed and replaced with an adaptor ring to give
the configuration of an LS2 microphone. Variations on the principle could include a slightly
larger diameter coupler where the test microphone would be supported by other means.
Dimensions in millimetres
11,4
IEC
Key
1 Preamplifier A
2 Microphone A
3 Microphone B
4 Preamplifier B
5 Coupler cavity, diameter 9,3 mm
6 Sound inlet
7 Cylindrical source diaphragm
Figure A.1 – A coupler for use with WS2 microphones
This method may also be used without removing any protection grid from the test microphone
provided that the presence of the grid is allowed for in the uncertainty calculation. The grid
can cause an unacceptable level of measurement uncertainty at high frequencies, effectively
reducing the frequency range over which the coupler can be used.
0,5
1,8
– 14 – IEC 61094-5:2016 © IEC 2016
A.2 A jig for use with WS2 or WS3 microphones at frequencies up to 20 kHz
A simple arrangement for holding and positioning an LS2 microphone and a WS2 microphone
in a suitable position for a simultaneous calibration is shown in Figure A.2. The jig is enclosed
in an acoustic chamber with a loudspeaker providing the sound source. The preferred location
for the sound source is on the axis of symmetry of the microphones. The detailed positioning
for WS2 and WS3 microphones is shown in Figures A.3 and A.4 respectively. Note that the
protection grids have been removed.
IEC
Figure A.2 – A jig fitted with an LS2 and WS2 microphone
Dimensions in millimetres
1 0,5
IEC IEC
NOTE The dimension shown is the diaphragm-to-
NOTE The dimension shown is the diaphragm-to-
diaphragm separation
diaphragm separation. This separation distance is the
only one for which the corrections specified in Table
A.1 are valid
Figure A.3 – Example arrangement of LS2 Figure A.4 – Example arrangement of LS2
and WS2 microphones in a jig and WS3 microphones in a jig
When the arrangement of Figure A.4 is used, corrections are required to account for the radial
sensitivity of the microphones and the fact that the test microphone is smaller than the
reference microphone. Table A.1 gives corrections to be added to the sensitivity level of the
WS3 microphone assuming that the reference microphone is of type LS2aP (see [1]) and that
the sound field is radially symmetrical. The expanded uncertainty on the corrections is
estimated to be 10 % of their value (in dB) which is approximately the change observed by
doubling the distance between the microphones.

If the sound arrives from a direction other than the axis of symmetry of the jig, measurements
should be made with the sound arriving from several different directions and an average
taken. A convenient means of achieving this is to use a diffuse sound field.
Table A.1 – Calculated corrections to be added to the sensitivity level
of the WS3 microphone when using the arrangement in Figure A.4
Frequency Correction
kHz dB
1 –0,004
1,25 –0,006
1,6 –0,009
2 –0,015
2,5 –0,023
3,15 –0,036
4 –0,059
5 –0,092
6,3 –0,146
8 –0,235
10 –0,367
12,5 –0,572
16 –0,933
20 –1,443
th
NOTE The expanded uncertainty is estimated to be 1/10 of the value of the
correction (in decibels).
– 16 – IEC 61094-5:2016 © IEC 2016
Annex B
(informative)
Examples of couplers for sequential excitation
B.1 A coupler for use with LS1 microphones at frequencies up to 8 kHz
A coupler for use with LS1 microphones is shown in Figure B.1. A WS1P microphone, used as
the sound source, is screwed directly into the upper port of the coupler without any protection
grid or adaptor. A probe tube microphone is inserted from the side of the coupler so that the
probe tip is one-third of the distance along a radius from the wall, and is used to control the
sound pressure in the coupler. The acoustic impedance of the probe tube microphone used
–3
can affect the results, but a tube with an acoustic impedance of 800 MPa∙s∙m has been
used successfully. The test and reference microphones are held in the coupler by a yoke and
spring arrangement.
If both test and reference microphones are WS1 microphones converted to the LS1
configuration with an adaptor ring, the same adaptor ring should be used on both
microphones.
Dimensions in millimetres
3 1 2
23,85
IEC
Key
1 Aperture for source microphone
2 Thread to fit source microphone
3 Position of source microphone diaphragm
4 Probe tube
5 Aperture for test and reference microphone
Figure B.1 – A coupler for use with LS1 microphones
B.2 A coupler for use with WS2 microphones at frequencies up to 16 kHz
Figure B.2 shows a coupler that can be used for sequential comparison calibrations of WS2
microphones. A cylindrical source diaphragm generates a radially symmetric sound field and a
19 3
1,2
monitor microphone detects the change in sound pressure when the test microphone is
replaced by the reference microphone.
Dimensions in millimetres
IEC
Key
1 Monitor microphone
2 Test/reference microphone
3 Coupler cavity, diameter 9,3 mm
4 Cylindrical source diaphragm
Figure B.2 – A coupler for use with WS2 microphones
This method may also be used without removing any protection grid from the test microphone
provided that the presence of the grid is allowed for in the uncertainty calculation.

0,5
1,3
– 18 – IEC 61094-5:2016 © IEC 2016
Annex C
(informative)
Determining the open-circuit sensitivity of a measurement
microphone without using the insert-voltage method
When a comparison calibration is being performed, it is possible to determine the open-circuit
sensitivity of the test microphone without using the insert-voltage method. It is necessary for
the open-circuit sensitivity of the reference microphone to be known and a correction (or
uncertainty) to be included for any difference due to the test and reference microphone
presenting a different electrical source impedance to the preamplifier. The principle is that by
interchanging the microphones between the two measuring channels and repeating the
measurements, any difference in the gains of the two channels (and some other systematic
effects) can be eliminated. This can be demonstrated by the following.
When two microphones with their diaphragms facing are at close proximity to each other, and
their outputs measured as levels on two measurement channels, then the level reading
difference, L , between the two channels (neglecting any influence of microphone
C12
capacitance) is
L = (L + L + L + L ) – (L + L + L + L ) (C.1)
C12 1 m1 d1 WA 2 m2 d1 WB
where
L and L are the pressure sensitivity levels of the microphones;
1 2
L and L are the gains of the measuring systems;
m1 m2
L is the sound pressure level that the excita
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