IEC TR 61094-10:2022

IEC TR 61094-10 ®
Edition 1.0 2022-08
TECHNICAL
REPORT
colour
inside
Electroacoustics – Measurement microphones –
Part 10: Absolute pressure calibration of microphones at low frequencies using
calculable pistonphones
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IEC TR 61094-10 ®
Edition 1.0 2022-08
TECHNICAL
REPORT
colour
inside
Electroacoustics – Measurement microphones –

Part 10: Absolute pressure calibration of microphones at low frequencies using

calculable pistonphones
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.140.50 ISBN 978-2-8322-4374-9

– 2 – IEC TR 61904-10:2022 © IEC 2022
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Reference environmental conditions . 7
5 Principles of absolute pressure calibration of microphones using a calculable
pistonphone . 7
5.1 General principle . 7
5.2 Basic expressions . 7
5.3 Heat conduction correction . 9
5.4 Operating frequency range . 9
6 General characteristics . 9
6.1 The pistonphone . 9
6.2 Measuring the piston volume velocity . 10
6.3 Test signals . 10
6.4 Mounting the microphone and pressure-equalizing tube . 10
6.5 Measuring the output voltages of the microphones . 10
7 Factors influencing the pressure sensitivity. 10
7.1 General . 10
7.2 Polarizing voltage . 11
7.3 Shield configuration . 11
7.4 Dependence on environmental conditions . 11
7.4.1 General . 11
7.4.2 Static pressure . 11
7.4.3 Temperature . 11
7.4.4 Humidity . 12
7.5 Vibration . 12
7.6 Distortion . 12
8 Calibration uncertainty components . 12
8.1 General . 12
8.2 Measurements of microphone output voltage . 12
8.3 Piston . 12
8.3.1 Frequency . 12
8.3.2 Measurement of the volume velocity . 12
8.4 Acoustic transfer impedance . 13
8.4.1 Cavity properties . 13
8.4.2 Physical quantities . 13
8.5 Microphone parameters . 13
8.5.1 Front cavity. 13
8.5.2 Acoustic impedance . 13
8.5.3 Polarizing voltage . 14
8.6 Uncertainty on pressure sensitivity level . 14
Annex A (informative) Example designs of pistonphones using laser interferometry . 16

IEC TR 61904-10:2022 © IEC 2022 – 3 –
Annex B (informative) Measurement uncertainties . 18
B.1 General . 18
B.2 Example of a typical uncertainty analysis . 18
B.2.1 General . 18
B.2.2 Uncertainty budget . 18
B.2.3 Combined and expanded uncertainties . 19
Bibliography . 20

Figure 1 – Equivalent circuit for evaluating the sound pressure over the exposed
surface of the diaphragm of the microphone . 8
Figure A.1 – Schematic cross-section of a laser pistonphone . 16
Figure A.2 – Example of laser pistonphone . 17
Figure A.3 – Example of laser pistonphone . 17

Table 1 – Uncertainty components . 14
Table B.1 – Example of uncertainty budget at 1 Hz . 19

– 4 – IEC TR 61904-10:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS –
MEASUREMENT MICROPHONES –
Part 10: Absolute pressure calibration of microphones
at low frequencies using calculable pistonphones

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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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.
IEC TR 61094-10 has been prepared by IEC technical committee 29: Electroacoustics. It is a
Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
29/1113/DTR 29/1124/RVDTR
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.

IEC TR 61904-10:2022 © IEC 2022 – 5 –
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 61094 series, published under the general title Electroacoustics –
Measurement microphones, can be found on the IEC website.
Future documents in this series will carry the new general title as cited above. Titles of existing
documents in this series will be updated at the time of the next edition.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

– 6 – IEC TR 61904-10:2022 © IEC 2022
ELECTROACOUSTICS –
MEASUREMENT MICROPHONES –
Part 10: Absolute pressure calibration of microphones
at low frequencies using calculable pistonphones

1 Scope
This part of IEC 61094
• is applicable to laboratory standard microphones meeting the requirements of IEC 61094-1
and other types of measurement microphones,
• describes one possible absolute method for determining the complex pressure sensitivity,
based on a device capable of generating a known sound pressure, especially at low
frequencies, and
• provides a reproducible and accurate basis for the measurement of sound pressure at low
frequencies.
All quantities are expressed in SI units.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61094-1:2000, Measurement microphones – Part 1: Specifications for laboratory standard
microphones
IEC 61094-2:2009, Electroacoustics – Measurement microphones – Part 2: Primary method for
pressure calibration of laboratory standard microphones by the reciprocity technique
IEC 61094-2:2009/AMD1:2022
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61094-1 and
IEC 61094-2 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
pistonphone
device in which sound pressure is generated in a fixed sealed volume of air, by the motion of
one or more pistons creating a well-defined volume velocity

IEC TR 61904-10:2022 © IEC 2022 – 7 –
3.2
calculable pistonphone
pistonphone where the generated sound pressure can be calculated from physical principles
4 Reference environmental conditions
The reference environmental conditions are the following:
• temperature 23,0 °C;
• static pressure 101,325 kPa;
• relative humidity 50 %.
5 Principles of absolute pressure calibration of microphones using a
calculable pistonphone
5.1 General principle
The microphone to be calibrated is exposed to a known or calculable sound pressure produced
within the sealed cavity (or coupler) of a pistonphone, without the need for a prior measurement
with another microphone. The dimensions of the cavity are constrained to allow the assumption
to be made that the sound pressure is uniformly distributed within.
A sound generator consisting of a sealed cavity (or coupler) of known volume that is driven by
a piston or similar mechanism capable of producing a known volume velocity (e.g. an
electrodynamic loudspeaker) has the potential to generate a known sound pressure. If the
piston is assumed to be rigid and of known frontal area, laser interferometry or other
displacement measurement techniques can be used to determine the piston displacement and
thereby derive the volume displacement.
The pressure sensitivity M of the microphone is then determined directly from its open-circuit
p
output voltage U and the applied sound pressure p .
m,0 m
U
m0,
M =
(1)
p
p
m
Alternatively, a microphone system comprising of a microphone, a preamplifier and optionally
and amplifier stage, can be calibrated by the same principle, except that the system output
voltage replaces the open-circuit output voltage of the microphone in Formula (1).
5.2 Basic expressions
The generated sound pressure p that is applied to the diaphragm of the microphone is
m
calculated from an evaluation of the acoustic transfer impedance Z of the cavity and a
T
measurement of the piston displacement 𝛿𝛿𝛿𝛿.
The acoustic transfer impedance is the constant of proportionality between the sound pressure
at the microphone diaphragm and the volume velocity driving the cavity. In the case of a
sinusoidally driven rigid piston, the volume velocity is given by the product of the piston area
S , the piston displacement and a factor jω, where ω is the angular frequency:
p
pjω Sδx⋅Z
(2)
m pT
=
– 8 – IEC TR 61904-10:2022 © IEC 2022
If the piston is not rigid, calculation of the volume velocity requires the surface integral of
displacement to be determined, for example with scanning interferometry.
The acoustic transfer impedance can be calculated when the cavity has a simple geometry
enabling its volume, V to be determined. When the characteristic cavity dimensions are
significantly smaller than the acoustic wavelength, λ (typically when V λ ), then the sound
pressure can be assumed to be uniformly distributed within the cavity. Then, assuming adiabatic
compression and expansion of the gas and that the cavity is perfectly sealed, the acoustic
impedance of the cavity Z is κ P /(jωV), where κ is the ratio of specific heats for air and P is
c s s
the static pressure inside the cavity. From the equivalent circuit in Figure 1, Z is then given by:
T

V
1 11 V
e,m
= += jω +
 (3)

Z ZZ κP κP
T C m S r S,r

where
V is the equivalent volume of microphone to be calibrated;
e,m
κ and κ are the ratio of the specific heats at measurement conditions and at reference
r
conditions respectively;
P is the reference static pressure.
s,r
Values for κ and κ in humid air can be determined from formulas given in IEC 61094-2:2009,
r
Annex F.
Key
𝜔𝜔 angular frequency
S piston surface area
p
δx piston displacement
q and q volume velocities
m
Z and Z acoustic impedances of the cavity and microphone respectively
c m
p sound pressure acting on the microphone
m
Figure 1 – Equivalent circuit for evaluating the sound pressure over
the exposed surface of the diaphragm of the microphone
At higher frequencies, where the wavelength can no longer be considered sufficiently large
compared to the cavity dimensions, the evaluation of Z generally becomes more complicated
T
and requires the specific geometry of the cavity to be accounted for. The onset of such
behaviour is generally considered to be the upper frequency limit for the operation of the
pistonphone within the scope of this document.

IEC TR 61904-10:2022 © IEC 2022 – 9 –
5.3 Heat conduction correction
The evaluation of Z in Formula (3) assumes adiabatic conditions in the cavity. However, in
T
practice, the influence of heat conduction at the walls of the cavity causes increasing departure
from purely adiabatic conditions as the frequency is reduced, especially for small cavities.
At frequencies where the sound pressure can be considered to be uniformly distributed within
the cavity 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 ΔH to the geometrical volume V in Formula (3). The formulation for the
influence of the heat conduction losses and expressions for a correction factor ΔH, when the
cavity shape is a perfect right circular cylinder, are given in IEC 61094-2:2009 and
IEC 61094-2:2009/AMD1:2022.
5.4 Operating frequency range
The upper frequency limit of operation is likely to be determined by the onset of sound pressure
non-uniformity. This is normally assessed by modelling the sound field within the pistonphone
cavity. The model can be used to determine a correction to account for the non-uniform sound
pressure distribution for the specific cavity geometry, but a point will be reached where the
magnitude of this correction, and therefore the associated uncertainty, becomes unacceptable.
Each cavity geometry will require individual treatment, but an upper frequency limit of around
200 Hz is typically possible when characteristic dimensions are no greater than 60 mm.
It is also possible that the volume velocity source determines the upper frequency of operation.
As the frequency increases, a gr
...