Electroacoustics - Simulators of human head and ear - Part 8: Acoustic coupler for highfrequency measurements of hearing aids and earphones coupled to the ear by means of ear inserts (IEC 60318-8:2022)

This part of IEC 60318 describes an acoustic coupler for loading a hearing aid or insert
earphone with a specified acoustic impedance when testing its acoustic performance, in the
frequency range up to 16 kHz. It is suitable for air-conduction hearing aids and earphones,
coupled to the ear by means of ear inserts, earmoulds or similar devices.
The acoustic coupler does not simulate the human ear. However, it has an effective volume of
only 0,4 cm3, which is small enough not to produce significant resonances in the coupler in the
frequency range below 16 kHz. Therefore, it will load the earphone with a known acoustic
impedance, which allows repeatable measurements with low uncertainty to be obtained on
earphones used in extended high-frequency audiometry.

Akustik - Simulatoren des menschlichen Kopfes und Ohres - Teil 8: Akustischer Kuppler zur Hochfrequenzmessung von Hörgeräten und Kopfhörern die mit dem Ohr mittels Ohreinsätzen gekoppelt sind (IEC 60318-8:2022)

Electroacoustique - Simulateurs de tête et d'oreille humaines - Partie 8: Coupleur acoustique pour les mesurages à hautes fréquences des appareils de correction auditive et des écouteurs couplés à l’oreille par des embouts (IEC 60318-8:2022)

L’IEC 60318-8:2022 décrit un coupleur acoustique permettant de charger un appareil de correction auditive ou un écouteur à insérer avec une impédance acoustique spécifiée lors de l'essai de ses performances acoustiques, dans la plage de fréquences allant jusqu'à 16 kHz. Il convient aux appareils de correction auditive et aux écouteurs à conduction aérienne, couplés à l'oreille par des inserts auriculaires, des embouts ou des appareils similaires. Le coupleur acoustique ne simule pas l'oreille humaine. Il a cependant un volume effectif de seulement 0,4 cm3, ce qui est suffisamment petit pour ne pas produire de résonances significatives dans le coupleur dans la plage de fréquences inférieure à 16 kHz. Il charge donc l'écouteur avec une impédance acoustique connue, ce qui permet d'obtenir des mesurages répétables avec une faible incertitude sur les écouteurs utilisés en audiométrie haute fréquence étendue.

Elektroakustika - Simulatorji človeške glave in ušes - 8. del: Akustični spojnik za visokofrekvenčne meritve slušnih pripomočkov in slušalk, ki so na ušesa pritrjeni z ušesnimi vstavki (IEC 60318-8:2022)

General Information

Status
Published
Publication Date
23-May-2022
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
20-May-2022
Due Date
25-Jul-2022
Completion Date
24-May-2022

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SLOVENSKI STANDARD
SIST EN IEC 60318-8:2022
01-julij-2022

Elektroakustika - Simulatorji človeške glave in ušes - 8. del: Akustični spojnik za

visokofrekvenčne meritve slušnih pripomočkov in slušalk, ki so na ušesa pritrjeni

z ušesnimi vstavki (IEC 60318-8:2022)

Electroacoustics - Simulators of human head and ear - Part 8: Acoustic coupler for

highfrequency measurements of hearing aids and earphones coupled to the ear by
means of ear inserts (IEC 60318-8:2022)

Akustik - Simulatoren des menschlichen Kopfes und Ohres - Teil 8: Akustischer Kuppler

zur Hochfrequenzmessung von Hörgeräten und Kopfhörern die mit dem Ohr mittels
Ohreinsätzen gekoppelt sind (IEC 60318-8:2022)

Electroacoustique - Simulateurs de tête et d'oreille humaines - Partie 8: Coupleur

acoustique pour les mesurages à hautes fréquences des appareils de correction auditive

et des écouteurs couplés à l’oreille par des embouts (IEC 60318-8:2022)
Ta slovenski standard je istoveten z: EN IEC 60318-8:2022
ICS:
13.140 Vpliv hrupa na ljudi Noise with respect to human
beings
17.140.50 Elektroakustika Electroacoustics
SIST EN IEC 60318-8:2022 en

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

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SIST EN IEC 60318-8:2022
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SIST EN IEC 60318-8:2022
EUROPEAN STANDARD EN IEC 60318-8
NORME EUROPÉENNE
EUROPÄISCHE NORM May 2022
ICS 17.140.50
English Version
Electroacoustics - Simulators of human head and ear - Part 8:
Acoustic coupler for high-frequency measurements of hearing
aids and earphones coupled to the ear by means of ear inserts
(IEC 60318-8:2022)

Electroacoustique - Simulateurs de tête et d'oreille Akustik - Simulatoren des menschlichen Kopfes und Ohres

humaines - Partie 8: Coupleur acoustique pour les - Teil 8: Akustischer Kuppler zur Hochfrequenzmessung

mesurages à hautes fréquences des appareils de correction von Hörgeräten und Kopfhörern die mit dem Ohr mittels

auditive et des écouteurs couplés à l'oreille par des Ohreinsätzen gekoppelt sind

embouts (IEC 60318-8:2022)
(IEC 60318-8:2022)

This European Standard was approved by CENELEC on 2022-05-12. 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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the

Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, 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: Rue de la Science 23, B-1040 Brussels

© 2022 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.

Ref. No. EN IEC 60318-8:2022 E
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SIST EN IEC 60318-8:2022
EN IEC 60318-8:2022 (E)
European foreword

The text of document 29/1111/FDIS, future edition 1 of IEC 60318-8, prepared by IEC/TC 29

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

EN IEC 60318-8:2022.
The following dates are fixed:

• latest date by which the document has to be implemented at national (dop) 2023-02-12

level by publication of an identical national standard or by endorsement

• latest date by which the national standards conflicting with the (dow) 2025-05-12

document have to be withdrawn

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

patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.

Any feedback and questions on this document should be directed to the users’ national committee. A

complete listing of these bodies can be found on the CENELEC website.
Endorsement notice

The text of the International Standard IEC 60318-8:2022 was approved by CENELEC as a European

Standard without any modification.

In the official version, for Bibliography, the following notes have to be added for the standards

indicated:
IEC 60318-5 NOTE Harmonized as EN 60318-5
IEC 60318-4 NOTE Harmonized as EN 60318-4
IEC 60118-0 NOTE Harmonized as EN 60118-0
IEC 61094-4 NOTE Harmonized as EN 61094-4
IEC 61094-2:2009 NOTE Harmonized as EN 61094-2:2009 (not modified)
IEC 60318-1:2009 NOTE Harmonized as EN 60318-1:2009 (not modified)
IEC 61094-6 NOTE Harmonized as EN 61094-6
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SIST EN IEC 60318-8:2022
IEC 60318-8
Edition 1.0 2022-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Electroacoustics – Simulators of human head and ear –
Part 8: Acoustic coupler for high-frequency measurements of hearing aids and
earphones coupled to the ear by means of ear inserts
Électroacoustique – Simulateurs de tête et d'oreille humaines –
Partie 8: Coupleur acoustique pour les mesurages à hautes fréquences des
appareils de correction auditive et des écouteurs couplés à l’oreille par des
embouts
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.140.50 ISBN 978-2-8322-1092-5

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 IEC 60318-8:2022
– 2 – IEC 60318-8:2022 © IEC 2022
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms, definitions and abbreviated terms ........................................................................ 7

3.1 Terms and definitions .............................................................................................. 7

3.2 Abbreviated terms ................................................................................................... 8

4 Mechanical design of the 0,4 cm coupler ....................................................................... 8

4.1 General ................................................................................................................... 8

4.1.1 Overall design guidelines ................................................................................. 8

4.1.2 Acceptance limits............................................................................................. 9

4.2 Cavity dimensions ................................................................................................... 9

4.2.1 Critical dimensions .......................................................................................... 9

4.2.2 Effective coupler volume and cavity dimensions .............................................. 9

4.3 Verification procedure of the effective coupler volume ............................................ 9

4.4 Microphone ............................................................................................................. 9

4.4.1 General ........................................................................................................... 9

4.4.2 Microphone type .............................................................................................. 9

4.5 Static pressure equalisation vent .......................................................................... 10

4.6 Acoustic transfer impedance level ......................................................................... 10

5 Calibration ..................................................................................................................... 10

5.1 Reference environmental conditions ..................................................................... 10

5.2 Method of calibration ............................................................................................ 10

6 Coupling of receivers and hearing aids to the coupler .................................................... 11

6.1 Coupling to a hearing aid receiver by means of tubing .......................................... 11

6.2 Coupling to a hearing aid embedded in or connected to an earmould .................... 11

6.3 Coupling to a receiver in the canal (RIC) ............................................................... 12

6.4 Coupling to a BTE hearing aid with 2 mm continuous internal diameter

tubing ................................................................................................................... 13

6.5 Coupling to a BTE hearing aid with earmould simulator ......................................... 14

6.6 Coupling to a BTE hearing aid with thin acoustic coupling tubing .......................... 15

7 Maximum permitted expanded uncertainty for coupler conformance testing ................... 16

Annex A (informative) Example of a method for verification of the effective volume of

the coupler .................................................................................................................... 18

A.1 Method for verification of the effective volume of the coupler ................................ 18

A.2 Measurement uncertainty ...................................................................................... 19

Annex B (informative) Example of a method for the measurement of the acoustic

transfer impedance of the 0,4 cm coupler .................................................................... 20

B.1 Measurement procedure ....................................................................................... 20

B.1.1 Acoustic transfer impedance .......................................................................... 20

B.1.2 Setup for measuring the coupler acoustic transfer impedance ....................... 21

B.2 Typical coupler acoustic transfer impedance ......................................................... 22

B.3 Measurement uncertainty ...................................................................................... 24

Annex C (informative) Example of one specific design of the coupler ................................... 25

Annex D (informative) Electrical analogue representation of the coupler as a tube

model ............................................................................................................................ 26

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IEC 60318-8:2022 © IEC 2022 – 3 –

Annex E (informative) Example assessments of conformance to specifications of this

document ...................................................................................................................... 31

E.1 General ................................................................................................................. 31

E.2 Conformance criteria ............................................................................................. 31

E.3 Example test results ............................................................................................. 31

Bibliography .......................................................................................................................... 34

Figure 1 – Coupling to a hearing aid receiver by means of coupling tubing ........................... 11

Figure 2 – Coupling to an ITE, ITC, or CIC ............................................................................ 12

Figure 3 – Coupling to a receiver in the canal (RIC) .............................................................. 13

Figure 4 – Coupling to a BTE hearing aid with 2 mm continuous internal diameter

coupling tubing ..................................................................................................................... 14

Figure 5 – Coupling to a BTE hearing aid with earmould simulator ........................................ 15

Figure 6 – Coupling to a BTE hearing aid with thin coupling tubing ....................................... 16

Figure 7 – Relationship between tolerance interval, corresponding acceptance interval

and the maximum permitted uncertainty of measurement ...................................................... 17

Figure A.1 – Measurement setup for coupler volume measurement ....................................... 19

Figure B.1 – Test set-up for measuring the coupler acoustic transfer impedance .................. 22

Figure B.2 – Typical coupler acoustic transfer impedance ..................................................... 22

Figure B.3 – Typical acoustic transfer impedance times frequency ........................................ 23

Figure C.1 – Example of a specific design of the 0,4 cm coupler, shown with

removable coupling plate with a nipple for the attachment of coupling tubing ........................ 25

Figure D.1 – Electrical analogue of the coupler as a lossy transmission line tube with

lossy terminations ................................................................................................................. 26

Figure D.2 – Transfer impedance model vs. measurements .................................................. 29

Figure D.3 – Equivalent volume calculated from the model vs. measurements ...................... 29

Figure D.4 – Model predictions of coupler impedance ........................................................... 30

Figure E.1 – Examples of assessment of conformance ......................................................... 33

Table 1 – The acoustic transfer impedance level modulus and the associated

acceptance intervals ............................................................................................................. 10

Table 2 – Values of U for measurements ........................................................................ 17

max
Table A.1 – Typical components of measurement uncertainty in the measurement of

effective volume .................................................................................................................... 19

Table B.1 – Typical acoustic transfer impedance values ....................................................... 23

Table B.2 – Typical components of measurement uncertainty in the measurement of

acoustic transfer impedance ................................................................................................. 24

Table D.1 – List of constants ................................................................................................ 28

Table E.1 – Examples of assessment of conformance ........................................................... 32

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS – SIMULATORS OF HUMAN HEAD AND EAR –
Part 8: Acoustic coupler for high-frequency measurements of hearing
aids and earphones coupled to the ear by means of ear inserts
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.

IEC 60318-8 has been prepared by IEC technical committee 29: Electroacoustics. It is an

International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
29/1111/FDIS 29/1117/RVD

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 International Standard is English.

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

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SIST EN IEC 60318-8:2022
IEC 60318-8:2022 © IEC 2022 – 5 –

at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are

described in greater detail at www.iec.ch/standardsdev/publications.

A list of all parts in the IEC 60318 series, published under the general title Electroacoustics –

Simulators of human head and ear, can be found on the IEC website.

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.

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SIST EN IEC 60318-8:2022
– 6 – IEC 60318-8:2022 © IEC 2022
INTRODUCTION

Advancement in hearing aid design makes it possible to increase the bandwidth of hearing aids

up to 16 kHz.
3 1

The 2 cm coupler as described in IEC 60318-5 [1] is suitable for measurements up to 8 kHz.

At frequencies above 8 kHz, high measurement uncertainty will occur in earphone responses,

due to acoustic resonances in the coupler.

The occluded-ear simulator as described in IEC 60318-4 [2] simulates the human external ear

up to 10 kHz and can be used as an acoustic coupler up to 16 kHz. It is designed with a principal

cavity length which produces a half-wavelength resonance of the sound pressure at

approximately 13,5 kHz. This resonance, which is also present in a person’s ear canal but more

controlled by the tympanic membrane, can also cause measurement uncertainty in earphone

responses above 10 kHz.

Accordingly, there is a need for a well-defined and robust acoustic coupler to be used by

designers of transducers (receiver, earphone), and by the designer and dispensers of hearing

aids when making measurements on earphones in the frequency range 8 kHz to 16 kHz.

The sound pressure developed by an earphone is, in general, not the same in an acoustic

coupler as in a person's ear. However, results obtained with an acoustic coupler can be used

as a simple and ready means for the exchange of specifications and test data on hearing aids

and insert earphones used in audiometry.

This document describes an acoustic coupler for loading a hearing aid or insert earphone with

a specified acoustic impedance when testing acoustic performance, in the frequency range up

to 16 kHz, as required in IEC 60118-0 [3].
____________
Numbers in square brackets refer to the Bibliography.
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SIST EN IEC 60318-8:2022
IEC 60318-8:2022 © IEC 2022 – 7 –
ELECTROACOUSTICS –
SIMULATORS OF HUMAN HEAD AND EAR –
Part 8: Acoustic coupler for high-frequency measurements of hearing
aids and earphones coupled to the ear by means of ear inserts
1 Scope

This part of IEC 60318 describes an acoustic coupler for loading a hearing aid or insert

earphone with a specified acoustic impedance when testing its acoustic performance, in the

frequency range up to 16 kHz. It is suitable for air-conduction hearing aids and earphones,

coupled to the ear by means of ear inserts, earmoulds or similar devices.

The acoustic coupler does not simulate the human ear. However, it has an effective volume of

only 0,4 cm , which is small enough not to produce significant resonances in the coupler in the

frequency range below 16 kHz. Therefore, it will load the earphone with a known acoustic

impedance, which allows repeatable measurements with low uncertainty to be obtained on

earphones used in extended high-frequency audiometry.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1 Terms and definitions
3.1.1
acoustic coupler

device for measuring the acoustic output of sound sources where the sound pressure is

measured by a calibrated microphone coupled to the source by a cavity of predetermined shape

and volume which does not necessarily approximate the acoustic impedance of the normal

human ear
3.1.2
earmould simulator
ear insert simulator

insert which terminates the entrance of the acoustic coupler and provides for passage of sound

into the acoustic coupler through an opening on its axis
3.1.3
reference plane

plane perpendicular to the axis of the cavity of the acoustic coupler, chosen to pass through

the position normally occupied by the tip of an earmould
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3.1.4
acoustic transfer impedance

quotient of sound pressure at the diaphragm of the acoustic coupler's microphone by the volume

velocity through the reference plane
3.1.5
reference cavity

cylindrical cavity with the same nominal diameter and volume as the acoustic coupler under

test, establishing a volume that can be measured using precision dimensional measurements

3.1.6
effective coupler volume

equivalent volume of air of the acoustic compliance of the coupler formed by the cavity and the

microphone at a frequency of 250 Hz
3.1.7
effective length of coupling tubing

length of the coupling tubing that extends from the output of the receiver or BTE ear hook to

the coupler reference plane

Note 1 to entry: The actual length of tubing used can deviate from the effective length of coupling tubing, for

example, (a) the overlap resulting from the connection to the ear hook or hearing aid receiver can increase the actual

length of tubing used, whereas (b) connection to the nipple of the coupling plate or earmould simulator, which is

considered part of the effective length of coupling tubing, can reduce the actual length of tubing used accordingly.

See Figure 1, Figure 4 and Figure 5.
3.2 Abbreviated terms
CIC completely-in-the-canal
ITC in-the-canal
IIC invisible-in-the-canal
ITE in-the-ear
BTE behind-the-ear
RIC receiver-in-the-canal
SPL sound pressure level
4 Mechanical design of the 0,4 cm coupler
4.1 General
4.1.1 Overall design guidelines

The coupler consists essentially of a cylindrical cavity whose effective coupler volume is

nominally 400 mm . A microphone with a diaphragm having high acoustic impedance is located

in the base of the cylindrical cavity. A protection grid can be fitted but it is not required. The

microphone measures the sound pressure level (SPL) in the coupler.

The coupler shall be made of a material that has no negative influences on its performance.

For example, it should be acoustically hard and dimensionally stable. The general construction

of the coupler and mounting of the microphone shall be designed to reduce the response to

vibration of any earphone or to sound outside the cavity.

The external diameter of the coupler should be kept as small as possible in order to minimise

diffraction errors which can affect the measurements when the coupler is placed in a sound

field. See Annex C.
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NOTE Due to the small effective volume, the 0,4 cm coupler produces a sound pressure level output at 1 kHz that

is approximately 14 dB higher than data obtained with the 2 cm coupler, under the same conditions using an

earphone with high source impedance and small coupling volume.
4.1.2 Acceptance limits

Acceptance limits in this document include allowances for design, manufacturing and ageing.

In subsequent subclauses, acceptance limits are provided for allowable values of measured

deviations from design goals. All specifications concerning conformity with the requirements of

this standard are given as acceptance limits. Annex E describes example assessments of

conformance to specifications of this document.
4.2 Cavity dimensions
4.2.1 Critical dimensions

The critical dimensions of the coupler are those which determine the shape and the volume of

the cavity terminated by a measurement microphone, and the static pressure equalisation vent.

4.2.2 Effective coupler volume and cavity dimensions
3 3
The effective coupler volume shall be 400 mm ± 6 mm .

Any contributions to the cavity volume arising from sources other than the cavity itself, such as

the front cavity and impedance of the measurement microphone, the static-pressure

equalisation vent and any microphone insertion stop fitted, shall be included in the effective

coupler volume. Therefore, the height of the cylindrical cavity should be designed such that the

effective coupler volume conforms to the requirement for all microphone models intended for

use with the coupler.
The diameter d of the cylindrical coupler cavity shall be 9,45 mm ± 0,04 mm.
4.3 Verification procedure of the effective coupler volume
The effective volume of the coupler shall be ve
...

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