SIST EN 61094-6:2005

Measurement microphones -- Part 6: Electrostatic actuators for determination of frequency response

This part of EN 61094 - gives guidelines for the design of actuators for microphones equipped with electrically conductive diaphragms; - gives methods for the validation of electrostatic actuators; - gives a method for determining the electrostatic actuator response of a microphone. The applications of electrostatic actuators are not fully described within this standard but may include - a technique for detecting changes in the frequency response of a microphone, - a technique for determining the environmental influence on the response of a microphone, - a technique for determining the free field or pressure response of a microphone without specific acoustical test facilities, by the application of predetermined correction values specific to the microphone model and actuator used, - a technique applicable at high frequencies not typically covered by calibration methods using sound excitation.

Messmikrofone -- Teil 6: Elektrostatische Anregeelektroden zur Ermittlung des Frequenzgangs

Microphones de mesure -- Partie 6: Grilles d'entraînement pour la détermination de la réponse en fréquence

La présente partie de la EN 61094 - donne les lignes directrices pour la réalisation de grilles d'entraînement destinées aux microphones possédant une membrane faite dans un matériau électriquement conducteur; - donne des méthodes pour la validation des grilles d'entraînement; - donne une méthode pour déterminer la réponse d'un microphone à la grille d'entraînement. Les applications des grilles d'entraînement ne sont pas complètement décrites dans cette norme mais peuvent comprendre - une technique pour détecter les variations de la réponse en fréquence d'un microphone, - une technique pour déterminer l'influence des conditions ambiantes sur la réponse d'un microphone, - une technique de détermination de la réponse en fréquence en champ libre ou en pression d'un microphone sans nécessiter un équipement acoustique spécifique, par application de valeurs de corrections prédéterminées spécifiques des modèles de grille d'entraînement et de microphone utilisés, - une technique d'étalonnage applicable aux fréquences élevées non couvertes spécifiquement par des méthodes d'étalonnage utilisant une excitation acoustique.

Meritev mikrofonov – 6. del: Elektrostatični aktuatorji za ugotavljanje frekvenčnega odziva (IEC 61094-6:2004)

General Information

• Status: Published
• Publication Date: 30-Nov-2005
• ICS: 17.140.50 - Electroacoustics
• ICS: 33.160.50 - Accessories
• Technical Committee: SS EIT - Technical Board for the electrotechnical, information technology and telecommunications fields
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 01-Dec-2005
• Due Date: 01-Dec-2005
• Completion Date: 01-Dec-2005
• Ref Project: EN 61094-6:2005 - Measurement microphones - Part 6: Electrostatic actuators for determination of frequency response

Overview

EN 61094-6:2005 (CLC adoption of IEC 61094-6:2004) specifies requirements and methods for electrostatic actuators used with measurement microphones to determine their frequency response. The standard provides design guidelines for actuators paired with electrically conductive diaphragms, procedures to validate actuators, and a method to measure the electrostatic actuator response of a microphone. It is intended to supplement traditional acoustic calibration, especially at frequencies or conditions where sound-excitation methods are limited.

Key topics

• Actuator design guidance
  • Principles of operation for electrostatic actuators placed near microphone diaphragms.
  • Mechanical and electrical design considerations to simulate a uniformly distributed pressure over the diaphragm.
• Validation and performance
  • Methods to assess repeatability, uniformity between actuators of the same model, and consistency of actuator vs. pressure response levels.
• Measurement of actuator response
  • Recommended measurement setups, instrumentation and procedures for determining the actuator-induced frequency response.
  • Identification of uncertainty components and guidance on uncertainty analysis (referenced annexes include typical uncertainty examples).
• Environmental and reference conditions
  • Definition of reference environmental parameters and procedures to evaluate environmental influences on microphone response.
• Supporting material
  • Informative annexes with example actuator designs, measurement set-ups, uncertainty tables and model-dependent differences between pressure, free-field and actuator responses.

Applications & practical value

EN 61094-6:2005 supports practical tasks where conventional acoustic calibration is difficult or insufficient:

• Detecting changes in microphone frequency response over time (diagnostic checks).
• Assessing environmental influences (temperature, humidity) on measurement systems without full acoustic facilities.
• Estimating free-field or pressure response using actuator measurements plus model-specific correction factors - useful when anechoic or calibrated sound sources are unavailable.
• Extending calibration to very high frequencies where sound excitation methods are less effective.

Who uses it:

• Calibration laboratories and metrology institutes working with measurement microphones.
• Acoustic engineers and test houses needing non-acoustic techniques for microphone calibration or verification.
• Manufacturers of measurement microphones and electrostatic actuators designing or validating actuator hardware.

Related standards

• EN/IEC 61094-1 - Specifications for laboratory standard microphones
• EN/IEC 61094-2, -3 - Primary reciprocity calibration methods (pressure and free-field)
• EN/IEC 61094-5 - Comparison methods for working standard microphones
• ISO/IEC Guide (GUM) - Guide to the expression of uncertainty in measurement

Keywords: EN 61094-6:2005, electrostatic actuator, measurement microphones, frequency response, microphone calibration, actuator validation, uncertainty.

Standards Content (Sample)

SLOVENSKI SIST EN 61094-6:2005

STANDARD
december 2005
Meritev mikrofonov – 6. del: Elektrostatični aktuatorji za ugotavljanje
frekvenčnega odziva (IEC 61094-6:2004)
Measurement microphones – Part 6: Electrostatic actuators for determination of
frequency response (IEC 61094-6:2004)
ICS 17.140.50; 33.160.50 Referenčna številka
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

EUROPEAN STANDARD EN 61094-6
NORME EUROPÉENNE
EUROPÄISCHE NORM January 2005

ICS 17.140.50
English version
Measurement microphones
Part 6: Electrostatic actuators for determination of frequency response
(IEC 61094-6:2004)
Microphones de mesure Messmikrofone
Partie 6: Grilles d'entraînement Teil 6: Elektrostatische Anregeelektroden
pour la détermination zur Ermittlung des Frequenzgangs
de la réponse en fréquence (IEC 61094-6:2004)
(CEI 61094-6:2004)
This European Standard was approved by CENELEC on 2004-12-01. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration.

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

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

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

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

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2005 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 61094-6:2005 E
Foreword
The text of document 29/562/FDIS, future edition 1 of IEC 61094-6, prepared by IEC TC 29,
Electroacoustics, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC
as EN 61094-6 on 2004-12-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2005-09-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2007-12-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61094-6:2004 was approved by CENELEC as a European
Standard without any modification.

- 3 - EN 61094-6:2005
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
NOTE Where an international publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
1) 2)
IEC 61094-1 - Measurement microphones EN 61094-1 2000
Part 1: Specifications for laboratory
standard microphones
1) 2)
IEC 61094-2 - Part 2: Primary method for pressure EN 61094-2 1993
calibration of laboratory standard
microphones by the reciprocity technique

1) 2)
IEC 61094-3 - Part 3: Primary method for free-field EN 61094-3 1995
calibration of laboratory standard
microphones by the reciprocity technique

1) 2)
IEC 61094-5 - Part 5: Methods for pressure calibration EN 61094-5 2001
of working standard microphones by
comparison
ISO/IEC Guide 1995 Guide to the expression of uncertainty in - -
Express measurement (GUM)
1)
Undated reference.
2)
Valid edition at date of issue.

NORME CEI
INTERNATIONALE
IEC
61094-6
INTERNATIONAL
Première édition
STANDARD
First edition
2004-11
Microphones de mesure –
Partie 6:
Grilles d'entraînement pour la détermination
de la réponse en fréquence
Measurement microphones –
Part 6:
Electrostatic actuators for determination
of frequency response
 IEC 2004 Droits de reproduction réservés  Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in any
utilisée sous quelque forme que ce soit et par aucun procédé, form or by any means, electronic or mechanical, including
électronique ou mécanique, y compris la photocopie et les photocopying and microfilm, without permission in writing from
microfilms, sans l'accord écrit de l'éditeur. the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: [phone] Telefax: [phone] E-mail: [e-mail] Web: www.iec.ch
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Pour prix, voir catalogue en vigueur
For price, see current catalogue

61094-6  IEC:2004 – 3 –
CONTENTS
FOREWORD.7

1 Scope.11
2 Normative references .11
3 Terms and definitions .13
4 Reference environmental conditions .13
5 Principle of electrostatic actuator operation .13
5.1 General .13
5.2 Electrostatic pressure.15
5.3 Electrostatic actuator response .21
6 Actuator design .23
6.1 General .23
6.2 Design.23
7 Validation .25
7.1 General .25
7.2 Repeatability of measurements .25
7.3 Uniformity of actuators of a given model.25
7.4 Uniformity of the difference between actuator and pressure response levels .25
8 Measurement of electrostatic actuator response.27
8.1 System for measurement of electrostatic actuator response .27
8.2 Uncertainty components .29
9 Applications of an electrostatic actuator .33
9.1 General .33
9.2 Verification of the frequency response of a measurement system.33
9.3 Determination of the environmental characteristics of microphone
measurement systems.33
9.4 Determination of free-field and pressure frequency responses.35
9.5 Measurement of actuator response at very high frequencies .35

Annex A (informative) Examples of electrostatic actuator designs.37
Annex B (informative) Set-up for measuring electrostatic actuator response .43
Annex C (informative) Typical uncertainty analysis .45
Annex D (informative) Difference between free-field-, pressure- and actuator
responses for typical models of measurement microphones.51

Figure 1 – Principle of microphone and electrostatic actuator .17
Figure 2 – Lumped parameter model of a measurement microphone excited by an
electrostatic actuator .21
Figure A.1 – Example of electrostatic actuator for type WS1 microphones .37
Figure A.2 – Example of an electrostatic actuator for type WS2 microphones .39
Figure A.3 – Examples of electrostatic actuators forming integral parts of the
microphone protection grids.41
Figure A.4 – Example of an electrostatic actuator combined with  weather-resistant
protection .41

61094-6  IEC:2004 – 5 –
Figure B.1 – Typical set-up for measuring the electrostatic actuator response of a
microphone.43
Figure D.1 – Examples of differences between relative pressure and actuator
frequency responses for four different type of measurement microphone: WS1P (a),
WS1F (b) of nominal sensitivities –26 dB re 1V/Pa and WS2P (c) and WS2F (d) of
nominal sensitivities –38 dB re 1V/Pa .51
Figure D.2 – Examples of differences between relative free-field and actuator
frequency responses for type WS1, WS2 and WS3 microphones when used without
protection grids.51
Figure D.3 – Example of model dependent difference between relative free field and
actuator frequency responses for a type WS2 microphone when used with its
protection grid.53
Figure D.4 – Example on the determination of a relative free-field frequency response b)
by adding the model dependent free-field to actuator difference as shown in Figure D.3
to the electrostatic actuator response of a microphone a) .53

Table C.1 – Uncertainties .49

61094-6  IEC:2004 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASUREMENT MICROPHONES –
Part 6: Electrostatic actuators for determination
of frequency response
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 61094-6 has been prepared by IEC technical committee 29:
Electroacoustics.
The text of this standard is based on the following documents:
FDIS Report on voting
29/562/FDIS 29/565/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.

61094-6  IEC:2004 – 9 –
IEC 61094 consists of the following parts, under the general title Measurement microphones:
Part 1: Specifications for laboratory standard microphones
Part 2: Primary method for pressure calibration of laboratory standard microphones by the
reciprocity technique
Part 3: Primary method for free-field calibration of laboratory standard microphones by the
reciprocity technique
Part 4: Specifications for working standard microphones
Part 5: Methods for pressure calibration of working standard microphones by comparison
Part 6: Electrostatic actuators for determination of frequency response
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.
61094-6  IEC:2004 – 11 –
MEASUREMENT MICROPHONES –
Part 6: Electrostatic actuators for determination
of frequency response
1 Scope
This part of IEC 61094
– gives guidelines for the design of actuators for microphones equipped with electrically
conductive diaphragms;
– gives methods for the validation of electrostatic actuators;
– gives a method for determining the electrostatic actuator response of a microphone.
The applications of electrostatic actuators are not fully described within this standard but may
include
– a technique for detecting changes in the frequency response of a microphone,
– a technique for determining the environmental influence on the response of a microphone,
– a technique for determining the free-field or pressure response of a microphone without
specific acoustical test facilities, by the application of predetermined correction values
specific to the microphone model and actuator used,
– a technique applicable at high frequencies not typically covered by calibration methods
using sound excitation.
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, Measurement microphones – Part 1: Specifications for laboratory standard
microphones
IEC 61094-2, Measurement microphones – Part 2: Primary method for pressure calibration of
laboratory standard microphones by the reciprocity technique
IEC 61094-3, Measurement microphones – Part 3: Primary method for free-field calibration of
laboratory standard microphones by the reciprocity technique
IEC 61094-5, Measurement microphones – Part 5: Methods for pressure calibration of working
standard microphones by comparison
ISO/IEC GUIDE EXPRESS: 1995, Guide to the expression of uncertainty in measurement
(GUM)
61094-6  IEC:2004 – 13 –
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61094-1 as well as
the following apply.
3.1
electrostatic actuator
device for determination of microphone frequency response comprising an electrically
conductive stiff plate placed near the microphone diaphragm such that a time-varying voltage,
applied between the plate and the diaphragm, produces an electrostatic force that simulates a
sound pressure uniformly distributed over the surface of the diaphragm
3.2
electrostatic actuator response of a microphone
microphone output as a function of frequency measured using a specified design of electro-
static actuator driven by a voltage that is of uniform amplitude with frequency, relative to the
output at a specified frequency
NOTE Electrostatic actuator response is expressed in decibels (dB).
3.3
acoustic radiation impedance
acoustic impedance loading the microphone diaphragm on its outer surface
–
NOTE 1 Acoustic radiation impedance is expressed in pascal-second per cubic meter (Pa⋅s⋅ m ).
NOTE 2 The radiation impedance depends on the presence and design of the actuator.
4 Reference environmental conditions
The reference environmental conditions are:
temperature 23,0 °C
static pressure 101,325 kPa
relative humidity 50 %
5 Principle of electrostatic actuator operation
5.1 General
In practice, measurements of sound are made in many different environments where different
types of sound fields exist. The sensitivity and frequency response of measurement
microphones depend on the type of sound field, so ideally the microphone should be
calibrated using a similar type of field to that which exists on the measurement site. The
various types of sound fields are generally approximated by three idealized fields: free field,
diffuse-field and pressure-field.
However, the establishment of such idealized sound fields, which are suitable for calibration
of measurement microphones over the frequency ranges of interest is technically difficult and
requires costly acoustical laboratory facilities. Therefore, the electrostatic actuator method is
used for determining a relative frequency response of measurement microphones. This
method, which accounts for the type of sound field by using specific predetermined
corrections, requires no such facilities.

61094-6  IEC:2004 – 15 –
At higher frequencies, the free-field sensitivity of a microphone is determined by the
behaviour of its diaphragm and the sound diffraction and reflection caused by the microphone.
The effect of the diaphragm behaviour, which may cause significant differences in the relative
frequency responses between individual microphones of the same model, requires specific
determination. This frequency response determination is performed using the electrostatic
actuator method.
The effect of the diffraction and reflection depends on the type of sound field and on the
shape and dimensions of the microphone. As these parameters are essentially the same for
all microphones of the same model, the influence of diffraction and reflection does not differ
significantly between individual microphones of the same model.
Therefore, corrections for specific types of sound field may be determined once for a model of
microphone and subsequently applied to the electrostatic actuator response of any
microphones of that model.
Free-field and pressure-field corrections are calculated by determining the respective
frequency responses of one or more microphones of the same model by using acoustical
calibration methods, for example, those in IEC 61094-2 and IEC 61094-3, and by subtracting
the respective electrostatic actuator responses.
In principle, the electrostatic actuator calibration method may be used from very low to very
high frequencies. However, the actuator excites the microphone diaphragm only and not the
static pressure equalisation vent, which is generally exposed to sound when measurements
are made in a free-field. The actuator excitation corresponds to that of a pressure-field and
thus cannot be used for determination of the lower limiting frequency under free-field
conditions. Free-field response determinations by electrostatic actuator should only be made
at frequencies which are at least 10 times greater than the lower limiting frequency derived
from the time constant of the venting system of the microphone. At low frequencies, a small
perforation in the microphone diaphragm will exhibit different effects in the actuator response
and in the acoustic responses in a pressure field or a free field. At high frequencies, the
degree to which the actuator excitation approximates that of a pressure field depends on the
relation between the acoustic impedance of the microphone diaphragm and the acoustic
radiation impedance of the microphone diaphragm with the actuator in place. This relation is
described in 5.3, while 9.3 describes some practical consequences for the determination of
the environmental characteristics of a microphone.
5.2 Electrostatic pressure
The rigid and electrically conductive plate of the actuator is placed close to and parallel to the
microphone diaphragm, see Figure 1. It forms an electrical capacitor together with the
microphone diaphragm, which shall also be electrically conductive. When a voltage is applied
between the capacitor plates, the actuator produces a force F distributed over the diaphragm
surface; see Equation (1) below.
The corresponding electrostatically produced pressure p is defined by Equation (2). Edge
act
effects are neglected. The ratio between the effective actuator area and the active diaphragm
area gives a constant, which is generally less than unity because the actuator is perforated
for acoustic reasons.
61094-6  IEC:2004 – 17 –
+
d
F
U
-
IEC  1507/04
Key
1 Microphone housing
2 Microphone diaphragm. Area S
dia
3 Electrostatic actuator. Area S
act
4 Holes
Figure 1 – Principle of microphone and electrostatic actuator
ε S
gas act
F =− U (1)
2d
ε
F
gas
p==− aU (2)
act
S
2d
dia
where
F is the electrostatic force produced on diaphragm (a pushing or pulling force is
considered to be positive or negative respectively), in newtons (N);
p is the electrostatically produced pressure on the diaphragm, in pascals (Pa);

act
ε is the dielectric constant of gas in space between actuator and diaphragm, in
gas
–12
farads per meter (F/m) (in air: ε = 8,85 × 10 F/m);
gas
d is the effective distance between actuator and diaphragm, in meters (m);
S is the active diaphragm area, in square meters (m );
dia
S is the effective surface area of actuator above the active diaphragm area, in
act
square meters (m );
S
act
a = is the ratio between effective actuator area and active diaphragm area;
S
dia
U is the voltage applied between actuator and microphone diaphragm, in volts (V).
Actuators are generally operated with a d.c voltage and a superimposed sinusoidal a.c
voltage. Equation (3) describes the instantaneous electrostatic pressure on the diaphragm for
this mode of operation.
61094-6  IEC:2004 – 19 –
ε a 2
gas
p()tU=− +u 2sin()ωt (3)
()0
2d
The Equations (4), (5) and (6) describe the resulting electrostatic pressure components, which
include the desired equivalent sound pressure p at the fundamental frequency and two non-
nd
desired components, a 2 harmonic pressure p and a static pressure p .
d stat
ε a
gas
p = Uu (4)
d
ε a
gas
p = u (5)
d
22 d
ε a
gas
p =− Uu+ (6)
( )
stat 0
2d
where
p(t) is the equivalent instantaneous sound pressure, in pascals (Pa);
p is the r.m.s. value of the sound pressure at the fundamental frequency, in pascals (Pa);
nd
p is the r.m.s. value of the sound pressure at the 2 harmonic frequency, in pascals (Pa);

d
p is the static pressure, in pascals (Pa);

stat
t is the time, in seconds (s);
U is the d.c. voltage applied between actuator and microphone diaphragm, in volts (V);
u is the r.m.s. value of the a.c. voltage applied between actuator and microphone
diaphragm, in volts (V);
ω is the angular frequency, in radians per second (rad/s).
The equation below defines the fraction of distortion, i.e. the ratio between the magnitudes of
the second harmonic and the fundamental frequency components:
u
D = 100 % (7)
2 2U
Examples of the design of electrostatic actuators are given in Annex A and an example of a
measurement set-up in Annex B.
NOTE 1 Although Equation (4) describes the absolute value of the equivalent sound pressure produced on the
microphone diaphragm, the actuator method is usually only used for measurement of relative frequency response.
The method might be used for determination of absolute microphone sensitivity but the resulting uncertainty would
generally be too large for most applications. Relatively large uncertainty is associated with the determination of the
distance d and with the ratio of areas a.
NOTE 2 Actuators may also be operated with a.c. voltage only. Equations (3), (4), (5) and (6) are also valid for
this mode of operation (U = 0). It should be noticed that the frequency of the electrostatically produced equivalent
pressure becomes twice the frequency of the supplied electrical signal, and that any variation of voltage input level
causes a change in this equivalent sound pressure level that is twice as large.
NOTE 3 The influence of the distortion of the excitation signal, see equation (7), on the microphone output signal
depends on the frequency response of the microphone. This influence can be eliminated by using a selective
measurement technique to measure the fundamental frequency component only.

61094-6  IEC:2004 – 21 –
5.3 Electrostatic actuator response
The electrostatic actuator method uses an electrostatically produced pressure to excite the
microphone diaphragm. A constant electrostatic pressure may in practice be produced on a
microphone diaphragm over a wide frequency range by keeping the driving a.c. voltage
constant while its frequency is varied.
The movement of the microphone diaphragm caused by the electrostatic excitation produces
a sound pressure on the diaphragm in addition to the electrostatic pressure. This pressure is
a function of frequency as it depends on both the diaphragm impedance and on the radiation
impedance.
The difference between the pressure response and the electrostatic actuator response can be
described by the equivalent circuit model in Figure 2.
Z
a,r
+
p p
act Z a,d
a,d
-
IEC  1508/04
Components
Z acoustic impedance of the microphone diaphragm for unloaded electrical terminals, in pascal-seconds per
a,d
-
cubic meter (Pa⋅s⋅ m );
Z acoustic radiation impedance of the diaphragm with the actuator in place, in pascal-second per cubic meter
a,r
-
(Pa⋅s⋅ m );
p Resulting equivalent sound pressure acting on the diaphragm, in pascals (Pa).
a,d
Figure 2 – Lumped parameter model of a measurement microphone
excited by an electrostatic actuator
The resulting influence on the pressure acting on the diaphragm is then given by the ratio
pZ
a,d a,d
= (8)
pZ +Z
act a,d a,r
For microphones having high diaphragm impedance, the additional sound pressure becomes
relatively low and the measured response becomes essentially equal to the pressure
response of the microphone.
The radiation impedance and the measured response are influenced by the mechanical
configuration of the electrostatic actuator itself. To keep the influence of the electrostatic
actuator as low as possible, actuators are generally perforated by either holes or slots. A high
percentage of perforation will reduce the influence of the actuator on the radiation impedance
but could also result in a lower pressure and less uniform distribution across the diaphragm.

61094-6  IEC:2004 – 23 –
To determine the frequency responses valid for free-field and pressure-field conditions,
microphone and actuator model-specific corrections shall be added to the measured actuator
response.
6 Actuator design
6.1 General
The actuator shall be designed such that the microphone is not damaged and the sensitivity is
not unduly affected when the actuator is applied.
6.2 Design
The difference between the actuator response of a microphone and its free-field, pressure
and diffuse-field responses respectively are essentially the same for all microphones of the
same model. Therefore, once these differences have been determined and made available,
either one of the three responses can be calculated after measurement of the actuator
response.
An electrostatic actuator shall be designed to measure a response, which for all microphones
of the same model forms essentially fixed differences to the free-field and pressure responses
respectively.
The above design criteria lead to the following requirements:
a) measurements made with a given actuator shall be reproducible;
b) measurements made with actuators of a given design shall be reproducible with the same
microphone;
c) the acoustic influence of the actuator itself on the measured response shall be essentially
the same for all microphones of the same model.
To obtain reproducible results with the actuator, no significant change should occur in the
measured response by rotating the actuator relative to the microphone. This means that
the actuator should produce an essentially uniform and rotationally symmetric distribution of
the electrostatic pressure over the diaphragm. This may be obtained by making the
dimensions of pattern details (hole diameters or slot widths) small compared to typical details
of microphone backplate designs. It is, therefore, recommended to keep the dimensions of
any such details less than 15 % of the microphone diaphragm diameter and the percentage of
perforation of 40 % or more. Typical degrees of perforation are between 40 % and 50 %.
To obtain the same results with different actuators of the same model, the actuators should be
produced with narrow tolerances. The variations for a given model of actuator on the distance
between actuator and diaphragm, the percentage of perforation and the thickness of the
actuator should be within ± 5 % of the nominal value.
The presence of the electrostatic actuator changes the radiation impedance of the micro-
phone, and thus also affects the resulting pressure acting on the diaphragm; see Figure 2 and
Equation (8). The resulting radiation impedance, which is in series with the microphone
impedance, should be low to ensure that the influence of the actuator becomes essentially the
same for all microphones of the same model, even if their diaphragm impedance varies within
the limits associated with the model of microphone. This means that actuators generally need
to be designed with a high degree of perforation as mentioned above.

61094-6  IEC:2004 – 25 –
NOTE Designers and users of actuators should be aware that actuators and their possible fixtures may resonate
at certain frequencies and disturb frequency response measurements in narrow frequency ranges around the
resonances.
7 Validation
7.1 General
Validation of a model of electrostatic actuator is made by performing tests, which prove that
the actuator conforms to the requirements given in 6.2.
The performance of an actuator depends on the properties of the microphone to be measured.
Therefore, the actuator should be tested with all models of microphones for which it is to be
used.
Testing of a model of actuator with a model of microphone requires a minimum of 3 actuators
and 3 microphones of the selected models.
7.2 Repeatability of measurements
One of the actuators shall be tested with one of the microphones. Measurements of
electrostatic actuator response shall be replicated ten times. The actuator shall be fully
removed from and replaced on the microphone between the measurements. The specified
frequency of the electrostatic actuator responses (see 3.2) shall be the same for all
replications (250 Hz is recommended). The experimental standard deviation of the results
shall be calculated and not exceed 0,04 dB at any frequency.
NOTE The angle of rotation between the actuator and the microphone should be different for the 10 reproductions
of the measurements except for actuators which are an integral part of a microphone diaphragm protecting grid.
7.3 Uniformity of actuators of a given model
All the actuators shall be tested with a microphone that has been randomly selected. Five
repetitions of electrostatic actuator response measurement shall be performed with each
actuator. The recommended specified frequency for these measurements is 250 Hz, and the
average of the five repetitions shall be calculated for each actuator. None of these average
responses shall at any frequency deviate more than 0,06 dB from the average of all
measurements.
NOTE This test does not apply to actuators which are integral parts of microphone diaphragm protection grids.
7.4 Uniformity of the difference between actuator and pressure response levels
One of the actuators is randomly selected and used to measure the electrostatic actuator
response of each microphone. The recommended specified frequency for these measure-
ments is 250 Hz, and the average of five repetitions shall be calculated for each microphone.
The pressure response shall be measured for each microphone using the methods specified
in IEC 61094-2 or IEC 61094-5.
The difference between the average actuator response and the pressure response level shall
be calculated for each microphone. None of these differences shall deviate from their mean
value by more than 0,1 dB.
61094-6  IEC:2004 – 27 –
8 Measurement of electrostatic actuator response
8.1 System for measurement of electrostatic actuator response
Systems for measurement of electrostatic actuator response of a microphone consist of two
parts: a system for electrostatic excitation of the microphone diaphragm and a system for
determination of the microphone output voltage. Elements of a typical measurement system
are shown in Annex B, Figure B.1.
The system may either measure the response of a microphone with its associated preamplifier
or the open circuit response of the microphone itself. In the latter case the insert voltage
technique shall be used to correct for the influence of loading of the microphone.
The applied model of actuator shall fulfil the requirements given in Clause 7. The actuator
shall be operated such that the microphone is not damaged and such that its sensitivity is not
unduly affected when the actuator is positioned on the microphone.
The electrostatic actuator response of a microphone is influenced by environmental
conditions. Ambient pressure, temperature and relative humidity shall thus be measured and
stated together with the measured microphone response.
When setting up a measurement system it shall be ensured that cross-talk, which may occur
between the excitation part and the measuring part of the measurement system, does not
significantly influence the measurement result. It shall also be ensured that the static pressure
component of the actuator-generated pressure is so small that it does not significantly modify
the frequency response of the microphone by displacing its diaphragm.
8.1.1 Cross-talk
Typically the a.c. voltage that is applied to an electrostatic actuator is 30 V. This voltage leads
to an electrostatically produced pressure of about 1 Pa and to output voltages of 0,3 mV to
100 mV, depending on the frequency and on the model of measurement microphone. This
results in a level difference of 50 dB to 100 dB between the excitation voltage on the actuator
and the output voltage of the microphone. For example, to ensure that a cross-talk signal
arising from the excitation voltage does not influence the measured output voltage from the
microphone by more than 0,03 dB, the cross-talk signal needs to be 50 dB below the
microphone output signal. Thus, differences in level as high as 100 dB to 150 dB between
actuator signal and microphone cross-talk signal may be necessary, depending on the
required uncertainty and model of microphone. Further information is given in Annex B.
8.1.2 Static attraction force
The presence of a d.c. voltage between actuator and microphone diaphragm results in a static
attraction force which can be derived from Equation (6). This force results in a change of the
diaphragm to backplate distance in the microphone and consequently a small change in the
sensitivity of the microphone, in particular around the resonance frequency. The estimated
influence of this effect is less than 0,05 dB if the following criterion is fulfilled:
Mp⋅
pstat
−3
<⋅210 (9)
U
pol
61094-6  IEC:2004 – 29 –
where
M is the pressure sensitivity of the microphone, in volts per pascal (V/Pa);
p
p is the static attraction force per unit area given by Equation (6), in pascals (Pa);
stat
U is the external or equivalent internal polarization voltage of the microphone, in
pol
volts (V).
For a nominal distance of 0,5 mm between actuator and diaphragm the d.c. voltage applied to
the actuator is typically about 800 V.
The a.c. and d.c. voltage applied to the actuator shall be chosen such that the distortion as
given by Equation (7) does not influence the measured response significantly. Particular care
should be taken if the microphone itself or the surrounding environment introduces high peaks
in the resulting response.
NOTE 1 It should be ascertained that the electrical field strength between the actuator and the microphone
diaphragm created by the applied d.c. and a.c. voltage is well below the breakdown voltage for the gas in use in
order to avoid ionic discharges. For many gases, it should be noted that the breakdown voltage is lower than for
air. An excessive amount of dust or other deposits on the diaphragm may increase the risk of ionic discharges.
NOTE 2 Actuators are generally not fully insulated which represents a risk to the operator when a high d.c. and
a.c. voltage is applied to the actuator. This means that the electrical safety requirements, which are valid for the
laboratory or other site of use, must be followed. Such requirements generally set upper limits for the current,
which might inadvertently be drawn from the voltage supply for the actuator.
8.2 Uncertainty components
8.2.1 General
In addition to the factors that influence the response of the microphone, further uncertainty
components are introduced by the measurement method, the equipment and the degree of
care under which the measurement is carried out. Factors, which affect the measurement in a
known way, shall be measured or calculated with as high an accuracy as practicable in order
to minimise their influence on the resulting uncertainty.
8.2.2 Electrical frequency response of measurement equipment
The frequency response of the entire measurement system that generates the electrical
excitation signal for the actuator and measures the microphone output signal, should either be
essentially constant or accounted for by correcting the measurement result. The overall
frequency response may be measured by applying a fraction of the a.c. excitation signal to
the input of the system that measures the microphone output signal. Where the open-circuit
response is to be determined, the signal shall be applied as an insert voltage signal in series
with the microphone itself. During the test the system settings shall be the same as those
applied for the actuator response measurement and the order of magnitude of the test signal
shall be equal to that of the microphone output signal.
8.2.3 Cross-talk of measurement system
Signals due to cross-talk are correlated with the true measurement signal and adds linearly to
the microphone output signal. For example, to ensure an influence of less than 0,03 dB, the
magnitude of the cross-talk signal needs to be at least 50 dB below the microphone output
signal.
61094-6  IEC:2004 – 31 –
8.2.4 Inherent electrical and environmental acoustic noise
Noise that is non-correlated with the true measurement signal adds to the output signal on
r.m.s. basis. For example, to ensure an influence of less than 0,03 dB the magnitude of the
noise needs to be at least 25 dB below the signal.
8.2.5 Distortion
The electrostatic pressure produced by an electrostatic actuator is distorted; see 5.2,
Equation (3). The influence of the distortion on the measured response depends on the
applied measurement principle. In case of frequency-selective measurements the influence
may be eliminated. In case of non-selective measurements some influence on the measured
frequency response may occur if a significant difference is present between the microphone
sensitivity at the measurement frequency and at
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