IEC 61094-2:2009

IEC 61094-2 ®
Edition 2.0 2009-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Measurement microphones –
Part 2: Primary method for pressure calibration of laboratory standard
microphones by the reciprocity technique

Electroacoustique – Microphones de mesure –
Partie 2: Méthode primaire pour l’étalonnage en pression des microphones
étalons de laboratoire par la méthode de réciprocité

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IEC 61094-2 ®
Edition 2.0 2009-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Measurement microphones –
Part 2: Primary method for pressure calibration of laboratory standard
microphones by the reciprocity technique

Electroacoustique – Microphones de mesure –
Partie 2: Méthode primaire pour l’étalonnage en pression des microphones
étalons de laboratoire par la méthode de réciprocité

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
X
CODE PRIX
ICS 17.140.50 ISBN 978-2-88910-410-9
– 2 – 61094-2 © IEC:2009
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 reciprocity .7
5.1 General principles .7
5.1.1 General .7
5.1.2 General principles using three microphones .7
5.1.3 General principles using two microphones and an auxiliary sound
source .7
5.2 Basic expressions .8
5.3 Insert voltage technique .9
5.4 Evaluation of the acoustic transfer impedance.9
5.5 Heat-conduction correction.11
5.6 Capillary tube correction.11
5.7 Final expressions for the pressure sensitivity .12
5.7.1 Method using three microphones .12
5.7.2 Method using two microphones and an auxiliary sound source .12
6 Factors influencing the pressure sensitivity of microphones.13
6.1 General .13
6.2 Polarizing voltage.13
6.3 Ground-shield reference configuration.13
6.4 Pressure distribution over the diaphragm .13
6.5 Dependence on environmental conditions .14
6.5.1 Static pressure .14
6.5.2 Temperature.14
6.5.3 Humidity .14
6.5.4 Transformation to reference environmental conditions .15
7 Calibration uncertainty components .15
7.1 General .15
7.2 Electrical transfer impedance .15
7.3 Acoustic transfer impedance .15
7.3.1 General .15
7.3.2 Coupler properties.15
7.3.3 Microphone parameters .16
7.4 Imperfection of theory.17
7.5 Uncertainty on pressure sensitivity level.18
Annex A (normative) Heat conduction and viscous losses in a closed cavity .20
Annex B (normative) Acoustic impedance of a capillary tube.23
Annex C (informative) Examples of cylindrical couplers for calibration of microphones .26
Annex D (informative) Environmental influence on the sensitivity of microphones .31
Annex E (informative) Methods for determining microphone parameters .34
Annex F (informative) Physical properties of humid air.37

61094-2 © IEC:2009 – 3 –
Figure 1 – Equivalent circuit for evaluating the acoustic transfer impedance Z .9
a,12
Figure 2 – Equivalent circuit for evaluating Z’ when coupler dimensions are small
a,12
compared with wavelength.10
Figure 3 – Equivalent circuit for evaluating Z’ when plane wave transmission in the
a,12
coupler can be assumed .10
Figure C.1 – Mechanical configuration of plane-wave couplers .27
Figure C.2 – Mechanical configuration of large-volume couplers .29
Figure D.1 – Examples of static pressure coefficient of LS1P and LS2P microphones
relative to the low-frequency value as a function of relative frequency f/f .32
o
Figure D.2 – General frequency dependence of that part of the temperature coefficient
for LS1P and LS2P microphones caused by the variation in the impedance of the
enclosed air .33

Table 1 – Uncertainty components .19
Table A.1 – Values for E .21
V
Table B.1 – Real part of Z in gigapascal-seconds per cubic metre (GPa⋅s/m ).24
a,C
Table B.2 – Imaginary part of Z in gigapascal-seconds per cubic metre (GPa⋅s/m ).25
a,C
Table C.1 – Nominal dimensions for plane-wave couplers.28
Table C.2 – Nominal dimensions and tolerances for large-volume couplers .29
Table C.3 – Experimentally determined wave-motion corrections for the air-filled large-
volume coupler used with type LS1P microphones.
Table F.1 – Calculated values of the quantities in Clauses F.1 to F.5 for two sets of
environmental conditions .40
Table F.2 – Coefficients used in the equations for humid air properties.41

– 4 – 61094-2 © IEC:2009
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS –
MEASUREMENT MICROPHONES –
Part 2: Primary method for pressure calibration of laboratory
standard microphones by the reciprocity technique

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
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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.
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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 61094-2 has been prepared by IEC technical committee 29:
Electroacoustics.
This second edition cancels and replaces the first edition published in 1992. This second
edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
• an update of Clause 6 to fulfil the requirements of ISO/IEC Guide 98-3;
• an improvement of the heat conduction theory in Annex A;
• a revision of Annex F: Physical properties of humid air.

61094-2 © IEC:2009 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
29/671/FDIS 29/676/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 61094 series, published under the general title Electroacoustics –
Measurement microphones, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result 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.
– 6 – 61094-2 © IEC:2009
ELECTROACOUSTICS –
MEASUREMENT MICROPHONES –
Part 2: Primary method for pressure calibration of laboratory
standard microphones by the reciprocity technique

1 Scope
This part of International Standard IEC 61094
– is applicable to laboratory standard microphones meeting the requirements of
IEC 61094-1 and other types of condenser microphone having the same mechanical
dimensions;
– specifies a primary method of determining the complex pressure sensitivity so as to
establish a reproducible and accurate basis for the measurement of sound pressure.
All quantities are expressed in SI units.
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 61094-1:2000, Measurement microphones – Part 1: Specifications for laboratory standard
microphones
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61094-1 and
ISO/IEC Guide 98-3 as well as the following apply.
3.1
reciprocal microphone
linear passive microphone for which the open circuit reverse and forward transfer impedances
are equal in magnitude
3.2
phase angle of pressure sensitivity of a microphone
for a given frequency, the phase angle between the open-circuit voltage and a uniform sound
pressure acting on the diaphragm
NOTE Phase angle is expressed in degrees or radians (° or rad).
___________
ISO/IEC Guide 98-3:2008 is published as a reissue of the Guide to the expression of uncertainty in
measurement (GUM), 1995.
61094-2 © IEC:2009 – 7 –
3.3
electrical transfer impedance
for a system of two acoustically coupled microphones the quotient of the open-circuit voltage
of the microphone used as a receiver by the input current through the electrical terminals of
the microphone used as a transmitter
NOTE 1 Electrical transfer impedance is expressed in ohms (Ω).
NOTE 2 This impedance is defined for the ground-shield configuration given in 7.2 of IEC 61094-1:2000.
3.4
acoustic transfer impedance
for a system of two acoustically coupled microphones the quotient of the sound pressure
acting on the diaphragm of the microphone used as a receiver by the short-circuit volume
velocity produced by the microphone used as a transmitter
NOTE Acoustic transfer impedance is expressed in pascal-seconds per cubic metre (Pa⋅s/m ).
3.5
coupler
device which, when fitted with microphones, forms a cavity of predetermined shape and
dimensions acting as an acoustic coupling element between the microphones
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 reciprocity
5.1 General principles
5.1.1 General
A reciprocity calibration of microphones may be carried out by means of three microphones,
two of which shall be reciprocal, or by means of an auxiliary sound source and two
microphones, of which one shall be reciprocal.
NOTE If one of the microphones is not reciprocal it can only be used as a sound receiver.
5.1.2 General principles using three microphones
Let two of the microphones be connected acoustically by a coupler. Using one of them as a
sound source and the other as a sound receiver, the electrical transfer impedance is
measured. When the acoustic transfer impedance of the system is known, the product of the
pressure sensitivities of the two coupled microphones can be determined. Using pair-wise
combinations of three microphones marked (1), (2) and (3), three such mutually independent
products are available, from which an expression for the pressure sensitivity of each of the
three microphones can be derived.
5.1.3 General principles using two microphones and an auxiliary sound source
First, let the two microphones be connected acoustically by a coupler, and the product of the
pressure sensitivities of the two microphones be determined (see 5.1.2). Next, let the two
microphones be presented to the same sound pressure, set up by the auxiliary sound source.
The ratio of the two output voltages will then equal the ratio of the two pressure sensitivities.

– 8 – 61094-2 © IEC:2009
Thus, from the product and the ratio of the pressure sensitivities of the two microphones, an
expression for the pressure sensitivity of each of the two microphones can be derived.
NOTE In order to obtain the ratio of pressure sensitivities, a direct comparison method may be used, and the
auxiliary sound source may be a third microphone having mechanical or acoustical characteristics which differ from
those of the microphones being calibrated.
5.2 Basic expressions
Laboratory standard microphones and similar microphones are considered reciprocal and thus
the two-port equations of the microphones can be written as:
z i + z q = U
11 12
(1)
z i + z q = p
21 22
where
p is the sound pressure, uniformly applied, at the acoustical terminals
(diaphragm) of the microphone in pascals (Pa);
U is the signal voltage at the electrical terminals of the microphone in volts
(V);
q is the volume velocity through the acoustical terminals (diaphragm) of the
microphone in cubic metres per second (m /s);
i is the current through the electrical terminals of the microphone in
amperes (A);
z = Z is the electrical impedance of the microphone when the diaphragm is
11 e
blocked in ohms (Ω);
z = Z is the acoustic impedance of the microphone when the electrical
22 a
–3
terminals are unloaded in pascal-seconds per cubic metre (Pa⋅s⋅m ),
z = z = M Z is equal to the reverse and forward transfer impedances in volt-seconds
12 21 p a
–3
per cubic metre (V⋅s⋅m ), M being the pressure sensitivity of the
p
–1
microphone in volts per pascal (V⋅Pa ).
NOTE Underlined symbols represent complex quantities.
Equations (1) may then be rewritten as:
Z i + M Z q = U
e p a
(1a)
M Z i + Z q = p
p a a
which constitute the equations of reciprocity for the microphone.
Let microphones (1) and (2) with the pressure sensitivities M and M be connected
p,1 p,2
acoustically by a coupler. From Equations (1a) it is seen that a current i through the
electrical terminals of microphone (1) will produce a short-circuit volume velocity (p = 0 at the
diaphragm) of M i and thus a sound pressure p = Z M i at the acoustical
p,1 1
a,12 p,1 1
terminals of microphone (2), where Z is the acoustic transfer impedance of the system.
a,12
The open-circuit voltage of microphone (2) will then be:
U = M ⋅ p = M M Z i
2 p,2 p,1 p,2 a,12 1
61094-2 © IEC:2009 – 9 –
Thus the product of the pressure sensitivities is given by:
U
M M = (2)
p,1 p,2
Z i
a,12 1
5.3 Insert voltage technique
The insert voltage technique is used to determine the open-circuit voltage of a microphone
when it is electrically loaded.
Let a microphone having a certain open-circuit voltage and internal impedance be connected
to a load impedance. To measure the open-circuit voltage, an impedance, small compared to
the load impedance, is connected in series with the microphone and a calibrating voltage
applied across it.
Let a sound pressure and a calibrating voltage of the same frequency be applied alternately.
When the calibrating voltage is adjusted until it gives the same voltage drop across the load
impedance as results from the sound pressure on the microphone, the open-circuit voltage
will be equal in magnitude to the calibrating voltage.
5.4 Evaluation of the acoustic transfer impedance
The acoustic transfer impedance Z = p /(M i ) can be evaluated from the equivalent
a,12 p,1 1
circuit in Figure 1, where Z and Z are the acoustic impedances of microphones (1) and
a,1 a,2
(2) respectively.
M i
p,1 1
Z Z p
a,1 a,2 2
IEC  260/09
Key
1 Coupler
Figure 1 – Equivalent circuit for evaluating the acoustic transfer impedance Z
a,12
In several cases, Z can be evaluated theoretically. Assume the sound pressure to be the
a,12
same at any point inside the coupler (this will take place when the physical dimensions of the
coupler are very small compared to the wavelength). The gas in the coupler then behaves as
a pure compliance and, from the equivalent circuit in Figure 2, Z is given by Z'
a,12 a,12
(assuming adiabatic compression and expansion of the gas):

– 10 – 61094-2 © IEC:2009
M i
p,1 1
Z Z Z p
a,1 a,V a,2 2
IEC  261/09
Figure 2 – Equivalent circuit for evaluating Z’ when coupler
a,12
dimensions are small compared with wavelength
V V
⎛ ⎞
1 1 1 1 V
e,1 e,2
⎜ ⎟
= + + = jω + + (3)
'
⎜ ⎟
Z Z Z κ p κ p κ p
Z s r s,r r s,r
a,V a,1 a,2
⎝ ⎠
a,12
where
V is the total geometrical volume of the coupler in cubic metres (m );
V is the equivalent volume of microphone (1) in cubic metres (m );
e,1
V is the equivalent volume of microphone (2) in cubic metres (m );
e,2
κ p
s
Z = is the acoustic impedance of the gas enclosed in the coupler in pascal-seconds
a,V
jωV
per cubic metre (Pa⋅s/m );
ω is the angular frequency in radians per second (rad/s);
p is the static pressure in pascals (Pa);
s
p is the static pressure at reference conditions in pascals (Pa);
s,r
κ is the ratio of the specific heat capacities at measurement conditions;
κ is κ at reference conditions.
r
Values for κ and κ in humid air can be derived from equations given in Annex F.
r
At higher frequencies, when the dimensions are not sufficiently small compared with the
wavelength, the evaluation of Z generally becomes complicated. However, if the shape of
a,12
the coupler is cylindrical and the diameter the same as that of the microphone diaphragms,
then, at frequencies where plane-wave transmission can be assumed, the whole system can
be considered as a homogeneous transmission line (see Figure 3).

M i
p,1 1
Z , y , l
Z Z p
a,0 0
a,1 a,2 2
IEC  262/09
’ when plane wave
Figure 3 – Equivalent circuit for evaluating Z
a,12
transmission in the coupler can be assumed
Z is then given by Z' (assuming adiabatic compression and expansion of the gas):
a,12 a,12
61094-2 © IEC:2009 – 11 –
⎡ ⎤
⎛ Z Z ⎞ ⎛ Z Z ⎞
1 1
a,0 a,0 a,0 a,0
⎜ ⎟ ⎜ ⎟
⎢ ⎥
coshγ l 1 sinhγ l (4)
= + + +
0 0
ٛ ⎜ ⎟ ⎜ ⎟
Z ⎢ Z Z Z Z ⎥
′
Z
a,0 a,1 a,2 a,1 a,2
⎝ ⎠ ⎝ ⎠
⎣ ⎦
a,12
where
Z is the acoustic impedance of plane waves in the coupler. If losses in the
a,0
coupler are neglected, then Z = ρcS/ ;
a,0 0
–3
ρ  is the density of the gas enclosed in kilograms per cubic metre (kg⋅m );
–1
c  is the free-space speed of sound in the gas in metres per second (m⋅s );
S  is the cross-sectional area of the coupler in square metres (m );
l is the length of the coupler, i.e. the distance between the two diaphragms in
metres (m);
–1
γ = α + jβ is the complex propagation coefficient in metres to power minus one (m ).
Values for ρ and c in humid air can be derived from equations given in Annex F.
The real part of γ accounts for the viscous losses and heat conduction at the cylindrical
surface and the imaginary part is the angular wave number.
If losses are neglected, γ may be approximated by putting α equal to zero and β equal to ω/c
in Equation (4).
Allowance shall be made for any air volume associated with the microphones that is not
enclosed by the circumference of the coupler and the two diaphragms (see 7.3.3.1).
5.5 Heat-conduction correction
The evaluation of Z' in the preceding subclause assumes adiabatic conditions in the
a,12
coupler. However, in practice, the influence of heat conduction at the walls of the coupler
causes departure from purely adiabatic conditions, especially for small couplers and low
frequencies.
At low frequencies, where the sound pressure can be considered the same at any point and
under the assumption that the walls remain at a constant temperature, the influence of the
heat conduction losses can be calculated and expressed in terms of a complex correction
factor Δ to the geometrical volume V in Equation (3). Expressions for the correction factor Δ
H H
are given in Annex A.
At high frequencies, wave-motion will be present inside the coupler and the sound pressure
will no longer be the same at all points. For right-cylindrical couplers where the transmission
line theory can be applied (see 5.4), the combined effect of heat conduction and viscous
losses along the cylindrical surface can be accounted for by the complex propagation
coefficient and acoustic impedance for plane-wave propagation in the coupler. The additional
heat conduction at the end surfaces of the coupler, the microphone diaphragms, can be
accounted for by including further components in the acoustic impedances of the
microphones. Expressions for the complex propagation coefficient and acoustic impedance for
plane-wave propagation are given in Annex A.
5.6 Capillary tube correction
The coupler is usually fitted with capillary tubes in order to equalize the static pressure inside
and outside the coupler. Two such capillary tubes also permit the introduction of a gas other
than air.
The acoustic input impedance of an open capillary tube is given by:

– 12 – 61094-2 © IEC:2009
Z = Z tanh γ l (5)
C
a,C a,t
where
Z is the complex acoustic wave impedance of an infinite tube in pascal-seconds per cubic
a,t
–3
metre (Pa⋅s⋅m );
l is the length of the tube in metres (m).
C
The shunting effect of the capillary tubes can be taken into account by introducing a complex
correction factor Δ to the acoustic transfer impedances given in Equations (3) and (4):
C
ٛ
′′
Z
a,12
Δ = 1+ n (6)
C
Z
a,C
where
n is the number of identical capillary tubes used;
”
Z is the acoustic transfer impedance Z' corrected for heat conduction according
a,12 a,12
to 5.5.
An expression for the acoustic input impedance Z of an open capillary tube is given in
a,C
Annex B.
5.7 Final expressions for the pressure sensitivity
5.7.1 Method using three microphones
Let the electrical transfer impedance U /i (see 5.2) be denoted by Z with similar
2 1 e,12
expressions for other pairs of microphones.
Taking into account the corrections given in 5.5 and 5.6, the final expression for the modulus
of the pressure sensitivity of microphone (1) is:
"
⎧⎫
ZZ Z ΔΔ
⎪⎪
e,12 e,31 a,23 C,12 C,31
M = (7)
⎨⎬
p,1
""
Z Δ
ZZ
e,23 C,23
⎪⎪
a,12 a,31
⎩⎭
Similar expressions apply for microphones (2) and (3).
The phase angle of the pressure sensitivity for each microphone is determined by a similar
procedure from the phase angle of each term in the above expression.
NOTE When complex quantities are expressed in terms of modulus and phase, the phase information should be
referred to the full four-quadrant phase range, i.e. 0 - 2π rad or 0 – 360°.
5.7.2 Method using two microphones and an auxiliary sound source
If only two microphones and an auxiliary sound source are used, the final expression for the
modulus of the pressure sensitivity is:
M Z
p,1 e,12
M=Δ (8)
p,1 '' C
M
Z
p,2
a,12
61094-2 © IEC:2009 – 13 –
where the ratio of the two pressure sensitivities is measured by comparison against the
auxiliary source, see 5.1.3.
6 Factors influencing the pressure sensitivity of microphones
6.1 General
The pressure sensitivity of a condenser microphone depends on polarizing voltage and
environmental conditions.
The basic mode of operation of a polarized condenser microphone assumes that the electrical
charge on the microphone is kept constant at all frequencies. This condition cannot be
maintained at very low frequencies and the product of the microphone capacitance and the
polarizing resistance determines the time constant for charging the microphone. While the
open-circuit sensitivity of the microphone, as obtained using the insert voltage technique, will
be determined correctly, the absolute output from an associated preamplifier to the
microphone will decrease at low frequencies in accordance with this time constant.
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 so that the resulting uncertainty components are small.
6.2 Polarizing voltage
The sensitivity of a condenser microphone is approximately proportional to the polarizing
voltage and thus the polarizing voltage actually used during the calibration shall be reported.
To comply with IEC 61094-1 a polarizing voltage of 200,0 V is recommended.
6.3 Ground-shield reference configuration
According to 3.3 of IEC 61094-1:2000, the open-circuit voltage shall be measured at the
electrical terminals of the microphone when it is attached to a specified ground-shield
configuration using the insert voltage technique described in 5.3 above. Specifications for
ground-shield configurations for laboratory standard microphones are given in
IEC 61094-1:2000.
The appropriate ground-shield configuration shall apply to both transmitter and receiver
microphones during the calibration, and the shield should be connected to ground potential.
If any other arrangement is used, the results of a calibration shall be referred to the reference
ground-shield configuration.
If the manufacturer specifies a maximum mechanical force to be applied to the central
electrical contact of the microphone, this limit shall not be exceeded.
6.4 Pressure distribution over the diaphragm
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 usually the microphone is more sensitive to a sound pressure at the centre of
the diaphragm. This difference will vary for microphones with various different non-
uniformities of tension distribution on the diaphragm.
For cylindrical couplers, as described in Annex C, both longitudinal and radial wave motions
(symmetric as well as asymmetric) will be present. The radial wave motion will result in a

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non-uniform pressure distribution over the diaphragm. It will be generated when the source
differs from a true piston source covering the whole end surface of the coupler or when the
combined microphone/coupler geometry is not a perfect right angle cylinder. In addition
asymmetric radial wave motion is also generated by the transmitter microphone by
imperfections in the backplate/diaphragm geometry or in the diaphragm tension and
homogeneity.
It is recommended that the sound pressure distribution during a calibration should be uniform
to better than ± 0,1 dB over the surface of the diaphragm. However, it is difficult to control this
condition in an actual calibration set-up due to the geometrical imperfection of real
microphones and couplers. Although radial wave motion can never be avoided because the
velocity distribution of the transmitter microphone differs from that of a true piston, couplers
having the same diameter as that of the microphone diaphragm will exhibit the smallest
amount of radial wave motion and be less sensitive to geometrical imperfections than
couplers with larger diameters.
However, when a calibration at high frequencies with a high accuracy is necessary, it may be
preferable to use more than one coupler with different dimensions to assess the true
sensitivity of the microphones and to apply a theoretically based correction for the radial
wave-motion effects.
6.5 Dependence on environmental conditions
6.5.1 Static pressure
The acoustic resistance and mass of the gas between the diaphragm and backplate, the
compliance of the cavity behind the diaphragm and thus the pressure sensitivity of the
microphone, depend on the static pressure. This dependence is a function of frequency. It can
be determined for a microphone under test by making reciprocity calibrations at different
static pressures.
Annex D contains information on the influence of static pressure on the pressure sensitivity of
laboratory standard condenser microphones.
6.5.2 Temperature
The acoustic resistance and mass of the gas between diaphragm and backplate and thus the
pressure sensitivity of the microphone, depend on the temperature. In addition the mechanical
dimensions of the microphone depend on the temperature and the sensitivity of the
microphone depends on the mechanical tension in the diaphragm and on the spacing between
diaphragm and backplate. The total effect of these dependencies is a function of frequency.
The combined dependence can be determined for a microphone under test by making
reciprocity calibrations at different temperatures.
Annex D contains information on the influence of temperature on the pressure sensitivity of
laboratory standard condenser microphones.
NOTE If a microphone is exposed to excessive temperature variations a permanent change in sensitivity may
result.
6.5.3 Humidity
Although the thermodynamic state of the air enclosed in the cavity behind the diaphragm of
the microphone depends slightly on humidity, an influence on the sensitivity has not been
observed for laboratory standard microphones, provided condensation does not take place.
NOTE Certain conditions can influence the stability of polarizing voltage and backplate charge and therefore
influence the sensitivity. For example the surface resistance of the insulation material between the backplate and
the housing of the microphone may deteriorate under excessively humid conditions, particularly if the material is
contaminated (see also 7.3.3.3). The surface resistance has a noticeable effect on the sensitivity of the
microphone at low frequencies, especially on the phase response.

61094-2 © IEC:2009 – 15 –
6.5.4 Transformation to reference environmental conditions
When reporting the results of a calibration, the pressure sensitivity should be referred to the
reference environmental conditions if reliable correction data are available.
The actual conditions during the calibration should be reported.
NOTE During a calibration, the temperature of the microphone can be different from the ambient air temperature.
7 Calibration uncertainty components
7.1 General
In addition to the factors mentioned in Clause 6 which affect the pressure sensitivity, 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, shall be measured or calculated with as high accuracy as practicable in order to
minimize their influence on the resulting uncertainty.
7.2 Electrical transfer impedance
Various methods are used for measuring the electrical transfer impedance with the necessary
accuracy, and no preference is given.
The current through the transmitter is usually determined by measuring the voltage across a
calibrated impedance in series with the transmitter microphone. To ensure a correct
determination of the current, the ground shield reference configuration, see 6.3, shall be
attached to the transmitter microphone. The calibration of the series impedance shall include
any cable capacitance and other load impedance present when measuring the voltage across
the impedance. This allows the electrical transfer impedance to be determined by a voltage
ratio and the calibrated series impedance.
The voltage used to excite the transmitter microphone shall be such that the effect of
harmonics, from this source or generated by the microphone, on the uncertainty in the
determination of the pressure sensitivity is small compared to the random uncertainty.
Noise or other interference such as cross-talk, whether of acoustical or other origin, shall not
unduly affect the determination of the pressure sensitivity.
NOTE 1 Frequency selective techniques can be used to improve the signal-to-noise ratio.
NOTE 2 Cross-talk can be measured by substituting the receiver microphone with a dummy microphone having
the same capacitance and external geometry as the receiver microphone and then determining the resulting
difference in the electric transfer impedance. The coupler and microphones should be positioned as during a
calibration. Alternatively, cross-talk can be determined by setting the polarizing voltage to zero volts during a
calibration. In both methods, frequency selective techniques are recommended.
7.3 Acoustic transfer impedance
7.3.1 General
Several factors influence the acoustic transfer impedance but the major source of uncertainty
in its determination is often the microphone parameters, especially for small couplers.
7.3.2 Coupler properties
7.3.2.1 Coupler dimensions
The shape and dimensions of the coupler cavity shall be chosen in such a way that 6.4 is
satisfied. As long as the greatest dimension of the coupler is small compared to the
wavelength of sound in the gas, the sound pressure will be substantially uniform in the

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coupler and independent of the shape. At high frequencies and for large couplers, this
requirement may be met by filling the cavity with helium or hydrogen.
The uncertainty on coupler dimensions affects the acoustic transfer impedance by different
amounts that vary with frequency. It also influences the heat conduction and capillary tube
corrections.
Examples of couplers are given in Annex C.
NOTE 1 Cylindrical couplers used in a frequency range where the dimensions are not small compared to the
wavelength should be manufactured with the utmost care so that asymmetric sound fields are not
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