EN 61094-3:2016

Electroacoustics - Measurement microphones - Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique

IEC 61094-3:2016 specifies a primary method of determining the complex free-field sensitivity of laboratory standard microphones so as to establish a reproducible and accurate basis for the measurement of sound pressure under free-field conditions. It is applicable to laboratory standard microphones meeting the requirements of IEC 61094-1. It is intended for use by laboratories with highly experienced staff and specialized equipment. This second edition cancels and replaces the first edition published in 1995. This edition constitutes a technical revision.

Messmikrofone - Teil 3: Primärverfahren zur Freifeld-Kalibrierung von Laboratoriums-Normalmikrofonen nach der Reziprozitätsmethode

Électroacoustique - Microphones de mesure - Partie 3: Méthode primaire pour l’étalonnage en champ libre des microphones étalons de laboratoire par la méthode de réciprocité

L'IEC 61094-3:2016 spécifie une méthode primaire de détermination de l'efficacité en champ libre complexe des microphones étalons de laboratoire de manière à établir une base reproductible et exacte pour la mesure de la pression acoustique dans des conditions de champ libre. Elle est applicable aux microphones étalons de laboratoire satisfaisant aux exigences de l'IEC 61094-1. Elle est destinée à être utilisée dans des laboratoires ayant du personnel hautement expérimenté et un équipement spécialisé. Cette deuxième édition annule et remplace la première édition parue en 1995. Cette édition constitue une révision technique.

Merilni mikrofoni - 3. del: Primarna metoda za kalibriranje laboratorijskih standardnih mikrofonov v prostem polju z recipročno tehniko (IEC 61094-3:2016)

Ta del standarda IEC 61094
• določa primarno metodo določanja zapletene občutljivosti laboratorijskih standardnih mikrofonov v prostem polju za vzpostavitev ponovljive in natančne podlage za merjenje zvočnega tlaka v pogojih prostega polja,
• se uporablja za laboratorijske standardne mikrofone, ki izpolnjujejo zahteve standarda IEC 610941,
• je namenjen za uporabo v laboratorijih z visoko usposobljenim osebjem in specializirano opremo.
OPOMBA: Načelo kalibracije, opisano v tem delu standarda IEC 61094, se uporablja tudi za delujoče standardne mikrofone, ki se po možnosti uporabljajo brez zaščitne mrežice.

General Information

• Status: Published
• Publication Date: 18-Oct-2016
• Withdrawal Date: 18-Jul-2019
• ICS: 17.140.50 - Electroacoustics
• ICS: 33.160.50 - Accessories
• Technical Committee: CLC/SR 29 - Electroacoustics
• Drafting Committee: IEC/TC 29 - IEC_TC_29
• Current Stage: 6060 - Document made available - Publishing
• Start Date: 02-Sep-2016
• Completion Date: 02-Sep-2016
• Ref Project: SIST EN 61094-3:2016 - Measurement microphones - Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique (IEC 61094-3:2016)

Relations

• Corrected By: EN 61094-3:2016/AC:2017-01 - Electroacoustics - Measurement microphones - Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique
• Effective Date: 27-Dec-2016
• Replaces: EN 61094-3:1995 - Measurement microphones - Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique
• Effective Date: 29-Jan-2023

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN 61094-3:2016
01-november-2016
1DGRPHãþD
SIST EN 61094-3:2002
0HULOQLPLNURIRQLGHO3ULPDUQDPHWRGD]DNDOLEULUDQMHODERUDWRULMVNLK
VWDQGDUGQLKPLNURIRQRYYSURVWHPSROMX]UHFLSURþQRWHKQLNR,(&
Measurement microphones - Part 3: Primary method for free-field calibration of
laboratory standard microphones by the reciprocity technique (IEC 61094-3:2016)

Microphones de mesure - Partie 3 : Méthode de base pour l'étalonnage en champ libre

des microphones étalons de laboratoire par la méthode de réciprocité (IEC 61094-
3:2016)
Ta slovenski standard je istoveten z: EN 61094-3:2016
ICS:
17.140.50 Elektroakustika Electroacoustics
33.160.50 Pribor Accessories
SIST EN 61094-3:2016 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 61094-3:2016
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SIST EN 61094-3:2016
EUROPEAN STANDARD EN 61094-3
NORME EUROPÉENNE
EUROPÄISCHE NORM
September 2016
ICS 17.140.50; 33.160.50 Supersedes EN 61094-3:1995
English Version
Electroacoustics - Measurement microphones - Part 3: Primary
method for free-field calibration of laboratory standard
microphones by the reciprocity technique
(IEC 61094-3:2016)

Électroacoustique - Microphones de mesure - Partie 3: Messmikrofone - Teil 3: Primärverfahren zur Freifeld-

Méthode primaire pour l'étalonnage en champ libre des Kalibrierung von Laboratoriums-Normalmikrofonen nach der

microphones étalons de laboratoire par la méthode de Reziprozitätsmethode
réciprocité (IEC 61094-3:2016)
(IEC 61094-3:2016)

This European Standard was approved by CENELEC on 2016-07-19. 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 CEN-CENELEC

Management Centre 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 CEN-CENELEC Management Centre has the

same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,

Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,

Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and the United Kingdom.
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2016 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.

Ref. No. EN 61094-3:2016 E
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SIST EN 61094-3:2016
EN 61094-3:2016
European foreword

The text of document 29/873/CDV, future edition 2 of IEC 61094-3, prepared by IEC TC 29,

Electroacoustics, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as

EN 61094-3:2016.
The following dates are fixed:
(dop) 2017-04-19
• latest date by which the document has
to be implemented at national level by
publication of an identical national
standard or by endorsement
• latest date by which the national (dow) 2019-07-19
standards conflicting with the
document have to be withdrawn
This document supersedes EN 61094-3:1995.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent

rights.
Endorsement notice

The text of the International Standard IEC 61094-3:2016 was approved by CENELEC as a European

Standard without any modification.

In the official version, for Bibliography, the following note has to be added for the standard indicated:

IEC 61094-8:2012 NOTE Harmonized as EN 61094-8:2012.
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SIST EN 61094-3:2016
EN 61094-3:2016
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

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.

NOTE 1 When an International Publication has been modified by common modifications, indicated by (mod), the relevant EN/HD

applies.

NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available here:

www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 61094-1 2000 Measurement microphones -- Part 1: EN 61094-1 2000
Specifications for laboratory standard
microphones
IEC 61094-2 2009 Electroacoustics - Measurement EN 61094-2 2009
microphones -- Part 2: Primary method for
the pressure calibration of laboratory
standard microphones by the reciprocity
technique
ISO 9613-1 - Acoustics; attenuation of sound during - -
propagation outdoors; part_1: calculation of
the absorption of sound by the atmosphere
IEC/TS 61094-7 - Measurement microphones -- Part 7: Values - -
for the difference between free-field and
pressure sensitivity levels of laboratory
standard microphones
ISO/IEC Guide 98-3 - Uncertainty of measurement - Part 3: Guide - -
to the expression of uncertainty in
measurement (GUM:1995)
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SIST EN 61094-3:2016
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SIST EN 61094-3:2016
IEC 61094-3
Edition 2.0 2016-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Electroacoustics – Measurement microphones –
Part 3: Primary method for free-field calibration of laboratory standard
microphones by the reciprocity technique
Électroacoustique – Microphones de mesure –
Partie 3: Méthode primaire pour l’étalonnage en champ libre des microphones
étalons de laboratoire par la méthode de réciprocité
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.140.50; 33.160.50 ISBN 978-2-8322-3478-5

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
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SIST EN 61094-3:2016
– 2 – IEC 61094-3:2016 © 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 free-field 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 ........................................................................................................... 8

5.2 Basic expressions ................................................................................................. 8

5.3 Insert voltage technique ........................................................................................ 9

5.4 Free-field receiving characteristics of a microphone ............................................... 9

5.5 Free-field transmitting characteristics of a microphone ......................................... 10

5.6 Reciprocity procedure ......................................................................................... 11

5.7 Final expressions for the free-field sensitivity ....................................................... 11

5.7.1 Method using three microphones .................................................................. 11

5.7.2 Method using two microphones and an auxiliary sound source ...................... 12

6 Factors influencing the free-field sensitivity .................................................................. 12

6.1 General ............................................................................................................... 12

6.2 Polarizing voltage ................................................................................................ 12

6.3 Shield configuration ............................................................................................ 12

6.4 Acoustic conditions ............................................................................................. 13

6.5 Position of the acoustic centre of a microphone ................................................... 13

6.6 Dependence on environmental conditions ............................................................ 14

6.6.1 General ....................................................................................................... 14

6.6.2 Static pressure ............................................................................................. 14

6.6.3 Temperature ................................................................................................ 14

6.6.4 Humidity ...................................................................................................... 14

6.6.5 Transformation to reference environmental conditions................................... 14

6.7 Considerations concerning measurement space ................................................... 15

7 Calibration uncertainty components .............................................................................. 15

7.1 General ............................................................................................................... 15

7.2 Electrical transfer impedance .............................................................................. 15

7.3 Deviations from ideal free-field conditions ............................................................ 15

7.4 Attenuation of sound in air ................................................................................... 16

7.5 Polarizing voltage ................................................................................................ 16

7.6 Physical properties of air ..................................................................................... 16

7.7 Imperfection of theory ......................................................................................... 16

7.8 Uncertainty on free-field sensitivity level .............................................................. 17

Annex A (informative) Values for the position of the acoustic centre .................................... 19

Annex B (normative) Values of the air attenuation coefficient .............................................. 20

B.1 General ............................................................................................................... 20

B.2 Calculation procedure ......................................................................................... 20

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Annex C (informative) Environmental influence on the sensitivity of microphones ................ 23

C.1 General ............................................................................................................... 23

C.2 Dependence on static pressure ........................................................................... 23

C.3 Dependence on temperature ............................................................................... 23

Annex D (informative) Application of time selective techniques for removal of

unwanted reflections and acoustic interference between microphones ................................. 25

D.1 General ............................................................................................................... 25

D.2 Practical considerations ...................................................................................... 25

D.2.1 Signal-to-noise ratio ..................................................................................... 25

D.2.2 Reflections from walls and measurement rig ................................................. 25

D.3 Frequency limitations .......................................................................................... 26

D.3.1 General ....................................................................................................... 26

D.3.2 Measurements based on frequency sweeps .................................................. 26

D.3.3 Measurements based on pure tones ............................................................. 26

D.4 Generating missing portions of the frequency response previous to

transforming to the time-domain. ......................................................................... 27

D.4.1 General ....................................................................................................... 27

D.4.2 Missing frequencies below the minimum measurement frequency .................. 27

D.4.3 Missing frequencies above the maximum measured frequency ...................... 27

D.4.4 Filtering the extended frequency response .................................................... 28

Bibliography ....................................................................................................................... 29

Figure 1 – Equivalent circuit for a receiving microphone under free-field conditions ............... 9

Figure 2 – Equivalent circuit for a transmitting microphone under free-field conditions ......... 10

Figure A.1 – Example of the estimated values of the acoustic centres of LS1P and

LS2aP microphones given in the bibliographical references for Annex A .............................. 19

Table 1 – Uncertainty components ...................................................................................... 17

Table B.1 – Values for attenuation of sound pressure in air (in dB/m) ................................... 22

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SIST EN 61094-3:2016
– 4 – IEC 61094-3:2016 © IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS – MEASUREMENT MICROPHONES –
Part 3: Primary method for free-field 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

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 itself does not provide any attestation of conformity. Independent certification bodies provide conformity

assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any

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

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 61094-3 has been prepared by IEC technical committee 29:

Electroacoustics.

This second edition cancels and replaces the first edition published in 1995. This edition

constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous

edition:

a) a new informative annex describing the use of time-selective techniques to minimize the

influence of acoustic reflections from the measurement setup;
b) provision for the calibration of microphones in driven shield configuration.
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SIST EN 61094-3:2016
IEC 61094-3:2016 © IEC 2016 – 5 –
The text of this standard is based on the following documents:
CDV Report on voting
29/873/CDV 29/892A/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 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 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.

IMPORTANT – The 'colour inside' logo on the cover page of this publication 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.
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SIST EN 61094-3:2016
– 6 – IEC 61094-3:2016 © IEC 2016
ELECTROACOUSTICS – MEASUREMENT MICROPHONES –
Part 3: Primary method for free-field calibration of laboratory
standard microphones by the reciprocity technique
1 Scope
This part of IEC 61094

• specifies a primary method of determining the complex free-field sensitivity of laboratory

standard microphones so as to establish a reproducible and accurate basis for the

measurement of sound pressure under free-field conditions,
• is applicable to laboratory standard microphones meeting the requirements of
IEC 61094-1,

• is intended for use by laboratories with highly experienced staff and specialized

equipment.

NOTE The calibration principle described in this part of IEC 61094 is also applicable to working standard

microphones, preferably used without their protection grid.
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 61094-1:2000, 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 TS 61094-7:2006, Measurement microphones – Part 7: Values for the difference between

free-field and pressure sensitivity levels of laboratory standard microphones

ISO 9613-1, Acoustics – Attenuation of sound during propagation outdoors – Part 1:

Calculation of the absorption of sound by the atmosphere

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,

IEC 61094-2, ISO/IEC Guide 98-3 and the following apply.
3.1
phase

phase angle between the open-circuit voltage and the

sound pressure that would exist at the position of the acoustic centre of the microphone in the

absence of the microphone, for a sinusoidal plane progressive wave of given frequency and

direction of sound incidence, and for given environmental conditions
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SIST EN 61094-3:2016
IEC 61094-3:2016 © IEC 2016 – 7 –
Note 1 to entry: Phase is expressed in degrees (°) or radians (rad).
3.2
acoustic centre

point from which approximately spherical wavefronts from a sound-emitting

transducer producing a sinusoidal signal at a given frequency appear to diverge with respect

to a small region around an observation point at a specified direction and distance from the

sound source

Note 1 to entry: The acoustic centre of a reciprocal transducer when used as a receiver is coincident with the

acoustic centre when used as a transmitter.

Note 2 to entry: This definition only applies to regions of the sound field where spherical or approximately

spherical wavefronts are observed.
3.3
equivalent point-transducer

notional transducer occupying a single point that, when located at the position of its acoustic

centre, simulates the transmitting or receiving characteristics of a microphone, for a

sinusoidal signal of given frequency, and for a given observation direction and distance

3.4
principal axis

line through the centre of and perpendicular to the diaphragm of the

microphone
3.5
free-field conditions
airborne sound-field environment where sound waves can propagate freely without
disturbances of any kind
4 Reference environmental conditions
The reference environmental conditions are:
• temperature 23,0 °C
• static pressure 101,325 kPa
• relative humidity 50 %
5 Principles of free-field 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, one of which shall be reciprocal.

NOTE 1 If one of the microphones is not reciprocal it can only be used as a sound receiver.

NOTE 2 Laboratory standard microphones are reciprocal when used within their linear operating range.

5.1.2 General principles using three microphones

Let two of the microphones be coupled acoustically under free-field conditions. 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 free-field sensitivities of the two coupled microphones can be determined. Using pair-wise

combinations of three microphones, three such mutually independent sensitivity products are

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available, from which an expression for the free-field 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 coupled acoustically under free-field conditions, and the

product of the free-field sensitivities of the two microphones be determined as described in

5.1.2. Next, let the two microphones be sequentially presented to the same sound pressure,

set up by the auxiliary sound source under identical free-field conditions. The ratio of the two

output voltages will then equal the ratio of the free-field sensitivities of the two microphones.

Thus, from the product and the ratio of the free-field sensitivities of the two microphones, an

expression for the free-field sensitivity of each of the two microphones can be derived.

NOTE In order to obtain the ratio of free-field sensitivities, a direct comparison method can be used, and the

auxiliary sound source can be another type of transducer or a third microphone having mechanical or acoustical

characteristics which differ from those of the microphones being calibrated.
5.2 Basic expressions

Laboratory standard microphones are considered reciprocal and thus the two-port formulae of

the microphones can be written as:
zi+=z q U
11 12
(1)
zi+ z q=p
21 22
where
p is the sound pressure, at the acoustical terminals 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
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
per cubic metre (V⋅s⋅m ), M being the pressure sensitivity of the
microphone in volts per pascal (V⋅Pa ).
NOTE Underlined symbols represent complex quantities.
Formula (1) may then be rewritten as:
Zi+=M Z q U
e pa
(2)
M Zi+ Z q=p
pa a
which constitute the formulae of reciprocity for the microphone.

When the sound pressure p is not uniform over the surface of the diaphragm, as will be the

case at high frequencies when the microphone is located in a plane progressive wave, the

location of the acoustic terminals is given through the equivalent point-transducer simulating

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SIST EN 61094-3:2016
IEC 61094-3:2016 © IEC 2016 – 9 –

the microphone. In this case, Formula (1) will also be valid for the real microphone through a

special interpretation of , see 5.4 and 5.5.
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 electrical impedance be

connected to an external electrical 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 voltag
...