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
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite ni
utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et
les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.

IEC Secretariat Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.

IEC publications search - webstore.iec.ch/advsearchform IEC Products & Services Portal - products.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always have
committee, …). It also gives information on projects, replaced access to up to date content tailored to your needs.
and withdrawn publications.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 300 terminological entries in English
details all new publications released. Available online and once
and French, with equivalent terms in 19 additional languages.
a month by email.
Also known as the International Electrotechnical Vocabulary

(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc
If you wish to give us your feedback on this publication or need
further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.

Recherche de publications IEC - Découvrez notre puissant moteur de recherche et consultez
webstore.iec.ch/advsearchform gratuitement tous les aperçus des publications. Avec un
La recherche avancée permet de trouver des publications IEC abonnement, vous aurez toujours accès à un contenu à jour
en utilisant différents critères (numéro de référence, texte, adapté à vos besoins.
comité d’études, …). Elle donne aussi des informations sur les
projets et les publications remplacées ou retirées. Electropedia - www.electropedia.org

Le premier dictionnaire d'électrotechnologie en ligne au monde,
IEC Just Published - webstore.iec.ch/justpublished
avec plus de 22 300 articles terminologiques en anglais et en
Restez informé sur les nouvelles publications IEC. Just
français, ainsi que les termes équivalents dans 19 langues
Published détaille les nouvelles publications parues.
additionnelles. Egalement appelé Vocabulaire
Disponible en ligne et une fois par mois par email.
Electrotechnique International (IEV) en ligne.

Service Clients - webstore.iec.ch/csc
Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC Products & Services Portal - products.iec.ch

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

– 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

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

– 4 – IEC 60318-8:2022 © IEC 2022
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

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.

– 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.

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

– 8 – IEC 60318-8:2022 © IEC 2022
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.
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 verified with an expanded uncertainty that is less
than the maximum permitted uncertainty specified in Table 2.
One means of verifying the effective volume is given in Annex A.
4.4 Microphone
4.4.1 General
A calibrated microphone shall be fitted with the diaphragm mounted centrally in the base of the
coupler. The effective volume of the coupler shall include the contribution from the microphone,
and any protection grid fitted to it.
In the frequency range from 100 Hz to 16 kHz, the overall pressure sensitivity level of the
microphone and associated measuring system shall be known with an uncertainty not exceeding
0,5 dB for a coverage probability of 95 %. The microphone shall be removeable for calibration.
4.4.2 Microphone type
A type WS3P microphone as specified in IEC 61094-4 [4] is preferred.
NOTE The equivalent volume of a WS3P microphone is approximately 0,25 mm .
Other types of microphones may be used, provided they fulfil the requirements of 4.4.1.

– 10 – IEC 60318-8:2022 © IEC 2022
4.5 Static pressure equalisation vent
Any change in the static pressure within the cavity caused by coupling the hearing aid or the
earphone to the coupler and microphone shall decay toward the ambient static-pressure with a
time constant of less than 1,5 s. If this necessitates the introduction of a controlled leak in the
coupler, it shall have the following characteristics:
a) it shall not alter the cavity volume by more than 4 mm ,
b) it shall attenuate external sound reaching the cavity, with the coupling plate aperture sealed,
by at least 16 dB at 125 Hz, increasing by 6 dB per octave for increasing frequencies up to
2 kHz.
NOTE 1 Equalisation can be realised, for example, by a capillary tube.
NOTE 2 The time constant can be measured as the time it takes for the static pressure inside the coupler to fall by
64 % after a sudden increase in static pressure inside the coupler.
4.6 Acoustic transfer impedance level
The transfer impedance level of the coupler and the associated acceptance intervals shall be
as specified in Table 1.
Table 1 – The acoustic transfer impedance level modulus and the associated
acceptance intervals
Nominal frequency
Acoustic transfer impedance level in dB re 1 Ns/m
Hz Level Acceptance interval
250 167,0 ±0,5
1 000 155,3 ±0,7
2 000 149,2 ±0,7
4 000 143,5 ±1,0
8 000 138,7 ±1,2
16 000 136,5 ±2,5
NOTE Measurements made at intermediate frequencies, while not part of the
specification, can nevertheless provide additional information.

5 Calibration
5.1 Reference environmental conditions
The reference environmental conditions are the following:
– static pressure: 101,325 kPa
– temperature: 23 °C
– relative humidity: 50 %
5.2 Method of calibration
The manufacturer shall provide a method of calibration for the microphone system in an
instruction manual.
The manufacturer shall also provide:
• The effective volume of the coupler at 250 Hz. One method for performing the measurement
is given in Annex A.
• The product of the acoustic impedance and frequency, expressed as a level and as a
function of frequency. One method for performing the measurement is given in Annex B.
Ideally, the calibration should be performed at the reference environmental conditions given in
5.1. If the environmental conditions are different from those given in 5.1, the actual
environmental conditions at the time of test shall be stated.
6 Coupling of receivers and hearing aids to the coupler
6.1 Coupling to a hearing aid receiver by means of tubing
Figure 1 describes the coupling to a hearing aid receiver. A bore diameter in the coupling plate,
which fits the coupling tube external diameter shall be used. The tube shall be attached to the
coupler in a manner that does not change the acoustic properties of the coupler. The end of the
tube should be flush with the lower face of the coupling plate.
For measurements on hearing aid receivers, a tube with an internal diameter of
1 mm ± 0,06 mm and an effective length of a coupling tube of 5 mm ± 0,1 mm shall be used.
For more reliable sealing of the coupling tubing to the coupling plate, the thickness of the
coupling plate may be increased in the centre portion.

Key
a Hearing aid receiver
b Coupling tubing
c Coupling plate with central bore corresponding to the external diameter of coupling tubing
d Effective length of coupling tubing
e Length of receiver connection port
f Internal diameter of coupling tubing
g Effective diameter of receiver connection port
Figure 1 – Coupling to a hearing aid receiver by means of coupling tubing
6.2 Coupling to a hearing aid embedded in or connected to an earmould
This Subclause 6.2 is applicable to any style of hearing aid embedded in or attached to an
earmould.
– 12 – IEC 60318-8:2022 © IEC 2022
Examples of hearing aids fully embedded in the earmould are ITE (in-the-ear), ITC
(in-the-canal), CIC (completely-in-the-canal), IIC (invisible-in-the-canal).
An example of a partially embedded hearing aid is the receiver-in-the-canal (RIC) hearing aid.
Examples of hearing aids attached to an earmould are behind-the-ear (BTE) hearing aids either
coupled with standard coupling tubing or thin tubing.
The coupling arrangement is illustrated in Figure 2 where it is shown with an ITE. The coupling
is performed by attaching the ITE by means of a sealant to the concave shaped ITE ear-mould
simulator. Care shall be taken to avoid any gaps or slit leakages. Any vents in the instrument
shall be sealed at the coupler side. The end of the ITE sound port shall be flush with the coupler
reference plane. The coupling plate is removable from the 0,4 cm coupler. This allows the
inspection of the seal quality and the position of the opening of the sound port that will face the
inside volume of the coupler.
3 3
NOTE Owing to the smaller volume, the 0,4 cm coupler is more sensitive to gaps or slit leakages than the 2 cm
coupler.
Key
a ITE
b Sealant to seal ITE to mounting fixture
c ITE ear-mould simulator
Figure 2 – Coupling to an ITE, ITC, or CIC
6.3 Coupling to a receiver in the canal (RIC)
Figure 3 shows the coupling of a receiver-in-the-canal to the 0,4 cm coupler. This may be used
as an alternative to 6.2 provided that a coupling adapter is available. The coupling adapter is
considered a part of the receiver system, and therefore its lower face shall be aligned to within
± 0,5 mm with the coupler reference plane.
If the receiver is attached to an earmould, then coupling as described in 6.2 and Figure 2 shall
be used.
Key
a Receiver in the canal (RIC)
b RIC specific coupling adaptor
c Coupling plate
d Bore in RIC specific coupling adaptor
Figure 3 – Coupling to a receiver in the canal (RIC)
6.4 Coupling to a BTE hearing aid with 2 mm continuous internal diameter tubing
Figure 4 shows the coupling to a BTE hearing aid by means of coupling tubing with
2 mm ± 0,1 mm continuous internal diameter, which reflects the most common fitting practice.
The tubing is connected to the coupling plate with a nipple of 2 mm ± 0,1 mm internal diameter.
An effective coupling tubing length of 43 mm ± 1 mm shall be used.
NOTE The 43 mm length consists of 25 mm of standard tubing length plus 18 mm for the earmould and includes
the thickness of the coupling plate.

– 14 – IEC 60318-8:2022 © IEC 2022

Key
a BTE
b Ear hook
c Coupling tubing with 2 mm internal diameter
d Coupling plate with a nipple of 2 mm internal diameter
e Effective length of coupling tubing (coupler reference plane to tip of ear-hook)
Figure 4 – Coupling to a BTE hearing aid with 2 mm
continuous internal diameter coupling tubing
6.5 Coupling to a BTE hearing aid with earmould simulator
Figure 5 describes coupling to a BTE by means of coupling tubing with an effective coupling
length of 25 mm and internal diameter of 2 mm connected to an earmould simulator with a
length of 18 mm and an internal diameter of 3 mm.
NOTE This configuration is used in IEC 60118-0 [3] as the standard coupling test.

Key
a BTE
b Ear hook
c Coupling tubing
d Effective length of coupling tubing
e Length of earmould simulator
f Earmould simulator
Figure 5 – Coupling to a BTE hearing aid with earmould simulator
6.6 Coupling to a BTE hearing aid with thin acoustic coupling tubing
Figure 6 describes coupling of a BTE by means of a thin acoustic coupling tubing. Typically,
thin coupling tubing is ready-made in various shapes and lengths and has a coupling adapter
for attachment of ear-domes. This adapter can be used to insert the tubing into a corresponding
bore in the coupling plate. The adapter aligns flush with the coupler reference plane.
If the thin tubing is firmly attached to a custom-made earmould, then coupling according to
Figure 2 shall be used.
– 16 – IEC 60318-8:2022 © IEC 2022

Key
a BTE
b Thin coupling tubing
c Coupling plate prepared to accept thin tubing coupling adaptor
d Thin tubing coupling adaptor
Figure 6 – Coupling to a BTE hearing aid with thin coupling tubing
7 Maximum permitted expanded uncertainty for coupler conformance testing
Conformance to a performance specification is demonstrated when a measured deviation from
a design goal does not exceed the corresponding acceptance limit(s) and the laboratory has
demonstrated that the actual associated uncertainty of measurement (for a probability of 95 %,
equivalent to a coverage factor of k = 2) does not exceed the maximum permitted uncertainty
U , stated in Table 2.
max
See also ISO/IEC Guide 98-3 [5] and ISO/IEC Guide 98-4 [6].

Table 2 – Values of U for measurements
max
Measured quantity Relevant U
max
subclause number
(k = 2)
Effective coupler volume 4.2.2
3,5 mm
Diameter of cylindrical cavity 4.2.2 0,01 mm
Microphone pressure sensitivity level 4.4.1 0,5 dB
Sound attenuation 4.5 3 dB
Transfer impedance 250 Hz to 8 kHz 4.6 0,3 dB
Transfer impedance 16 kHz 4.6 0,5 dB
Ambient pressure 5.2 0,1 kPa
Temperature 5.2 0,5 °C
Relative humidity 5.2 5 %
Internal diameter of coupling tube for receiver 6.1 0,02 mm
Effective length of coupling tube for receiver 6.1 0,05 mm
Internal diameter of coupling tubing or ear-mould substitute 6.4, 6.5 0,2 mm
Length of coupling tubing or earmould substitute 6.4, 6.5 0,5 mm

The measurement uncertainty is composed of several factors:
– uncertainty associated with the equipment used, such as sound generators, sound level
meters, measuring microphones, coupler, etc.;
– uncertainties arising from the acoustic coupling of the hearing aid to the coupler. Such
uncertainties could be related to diameter and length of coupling tubing;
– uncertainty associated with the accuracy and care in positioning the hearing aid in the test
space.
Figure 7 illustrates the relationship between acceptance interval (as stated in this document),
guard bands and how the tolerance interval may be calculated.

Key
AI acceptance interval
TI tolerance interval
U guard band for the maximum permitted uncertainty of measurement for a 95 % coverage interval
max
A lower acceptance limit
L
A upper acceptance limit
U
T lower tolerance limit
L
T upper tolerance limit
U
Figure 7 – Relationship between tolerance interval, corresponding acceptance
interval and the maximum permitted uncertainty of measurement

– 18 – IEC 60318-8:2022 © IEC 2022
Annex A
(informative)
Example of a method for verification of the effective volume of the coupler
A.1 Method for verification of the effective volume of the coupler
One procedure for verifying the effective volume of the coupler is based on an acoustic
comparison method using a reference cavity with a known volume and same nominal
dimensions as the coupler under test. The procedure is carried out at 250 Hz.
The method uses an additional high acoustic impedance sound source and a monitor
microphone, both fitted into a flat plate which can be attached to the reference cavity or to the
coupler under test, for generating and measuring the sound pressure in the cavity
(see Figure A.1).
A sound source with high acoustic impedance provides a constant volume velocity for a given
electrical input voltage, regardless of the volume it is driving. Since the acoustic impedance of
the closed cavity can be represented by a simple acoustic compliance, the resulting sound
pressure is inversely proportional to the effective volume of the cavity. Note that the effective
volume of the cavity in this configuration includes the contribution V from the sound source
s
and monitor microphone.
Assuming short successive sound pressure measurements in the reference cavity and in the
coupler under test, such that the ambient temperature and atmospheric air pressure are
unchanged, then
p (V + V ) = p (V + V )
coupler coupler s ref ref s
where
V is the known volume of reference cavity
ref
V is the volume of the coupler under test
coupler
p is the sound pressure in the reference cavity
ref
p is the sound pressure in the coupler under test.
coupler
Then, if V is assumed to be negligible
s
p
ref
VV=
coupler ref
p
coupler
where p /p is given by the corresponding ratio of output voltages from the monitor
ref coupler
microphone.
The error, due to neglecting V is:
s
p
ref
V = V - V
err s s
p
coupler
This should be accounted for in the measurement uncertainty.

The error V will be minimized if V and V are equal, and therefore the nominal volume
err ref coupler
of V shall be close to the nominal volume of V .
ref coupler
Key
1 reference cavity 4 probe sound source
2 measurement probe 5 coupler cavity for test
3 probe microphone 6 coupler microphone
Figure A.1 – Measurement setup for coupler volume measurement
A.2 Measurement uncertainty
Table A.1 lists components of measurement uncertainty and their typical value. Table A.1 is
intended to be a guide only and should not be used as a substitute for an uncertainty analysis
based on a specific measurement set up.
Table A.1 – Typical components of measurement uncertainty
in the measurement of effective volume
Component of uncertainty Typical Contribution to
uncertainty uncertainty on
effective volume
(semi-range,
unless stated
otherwise – see
NOTE)
mm
Diameter of reference volume 0,03 mm 1,2
Length of reference volume 0,01 mm 0,5
Assumption that the reference volume is a perfect cylinder negligible 0
Assumption that reference volume is a pure acoustic compliance negligible 0
Voltage ratio measurement
...