Electroacoustics - Hearing aids - Part 8: Methods of measurement of performance characteristics of hearing aids under simulated in situ working conditions

Describes methods for a test which simulates the acoustical effects of a median adult wearer on the performance of a hearing aid. Establishes certain guidelines for simulated in situ measurements of hearing aids; it describes a simplified method for simulated in situ measurements of hearing aids and a description for determination of the directivity index (DI) of directional microphones in hearing aids in the horizontal plane. In addition this second edition now specifies tolerances. Conformance to the specifications in this International Standard is demonstrated only when the result of a measurement, extended by the actual expanded uncertainty of measurement of the testing laboratory, lies fully within the tolerances specified in this International Standard extended by the values for Umax

Akustik - Hörgeräte - Teil 8: Verfahren zur Messung der Übertragungseigenschaften von Hörgeräten unter simulierten In-Situ-Bedingungen

Electroacoustique - Appareils de correction auditive - Partie 8: Méthodes de mesure des caractéristiques fonctionnelles des appareils de correction auditive dans des conditions simulées de fonctionnement in situ

Décrit les méthodes pour un essai qui simule les effets acoustiques d'un adulte moyen, porteur d'un appareil de correction auditive, sur les caractéristiques de l'appareil. Etablit un certain nombre de directives pour la mesure des appareils de correction auditive dans des conditions simulées de fonctionnement in situ; elle décrit une méthode simplifiée pour la mesure des appareils de correction auditive, dans des conditions simulées de fonctionnement in situ, et une méthode permettant de déterminer l'indice de directivité (ID) des microphones directionnels utilisés dans les appareils de correction auditive, dans le plan horizontal. De plus, cette deuxième édition de la présente partie de la EN 60118 spécifie désormais les tolérances. Dans la présente Norme Internationale, la conformité aux spécifications n'est démontrée que lorsque les résultats de mesure, augmentés de l'incertitude étendue réelle de mesure du laboratoire d'essai, sont entièrement compris dans les tolérances spécifiées dans la présente Norme Internationale étendues aux valeurs Umax.

Elektroakustika – Slušni pripomočki – 8. del: Metode za merjenje zmogljivostnih karakteristik slušnih pripomočkov v pogojih simuliranega delovanja na kraju samem (IEC 60118-8:2005)

General Information

Status
Published
Publication Date
15-Dec-2005
Technical Committee
Drafting Committee
Parallel Committee
Current Stage
6060 - Document made available
Due Date
16-Dec-2005
Completion Date
16-Dec-2005

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SLOVENSKI SIST EN 60118-8:2006
STANDARD
marec 2006
Elektroakustika – Slušni pripomočki – 8. del: Metode za merjenje
zmogljivostnih karakteristik slušnih pripomočkov v pogojih simuliranega
delovanja na kraju samem (IEC 60118-8:2005)
Electroacoustics - Hearing aids - Part 8: Methods of measurement of performance
characteristics of hearing aids under simulated in situ working conditions (IEC
60118-8:2005)
ICS 11.180.15 Referenčna številka
SIST EN 60118-8:2006(en)

© Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

---------------------- Page: 1 ----------------------
EUROPEAN STANDARD EN 60118-8
NORME EUROPÉENNE
EUROPÄISCHE NORM December 2005
ICS 17.140.50
English version
Electroacoustics –
Hearing aids
Part 8: Methods of measurement of performance characteristics
of hearing aids under simulated in situ working conditions
(IEC 60118-8:2005)
Electroacoustique – Akustik –
Appareils de correction auditive Hörgeräte
Partie 8: Méthodes de mesure Teil 8: Verfahren zur Messung der
des caractéristiques fonctionnelles Übertragungseigenschaften von
des appareils de correction auditive Hörgeräten unter simulierten
In-Situ-Bedingungen
dans des conditions simulées
de fonctionnement in situ (IEC 60118-8:2005)
(CEI 60118-8:2005)

This European Standard was approved by CENELEC on 2005-12-01. CENELEC members are bound to

comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European

Standard the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on

application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other

language made by translation under the responsibility of a CENELEC member into its own language and

notified to the Central Secretariat has the same status as the official versions.

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

Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,

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

Switzerland and United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels

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

Ref. No. EN 60118-8:2005 E
---------------------- Page: 2 ----------------------
EN 60118-8:2005 - 2 -
Foreword

The text of document 29/584/FDIS, future edition 2 of IEC 60118-8, prepared by IEC TC 29,

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

as EN 60118-8 on 2005-12-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2006-09-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2008-12-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice

The text of the International Standard IEC 60118-8:2005 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 60068 NOTE Harmonized in the EN 60068 series (not modified).
IEC 60118-7 NOTE Harmonized as EN 60118-7:2005 (not modified).
__________
---------------------- Page: 3 ----------------------
- 3 - EN 60118-8:2005
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application of this document. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenced

document (including any amendments) applies.

NOTE Where an international publication has been modified by common modifications, indicated by (mod), the relevant

EN/HD applies.
Publication Year Title EN/HD Year
IEC 60118-0 1983 Hearing aids EN 60118-0 1993
Part 0: Measurement of electroacoustical
characteristics
IEC 60263 - Scales and sizes for plotting frequency - -
characteristics and polar diagrams
1) 2)
IEC 60711 - Occluded-ear simulator for the HD 443 S1 1983
measurement of earphones coupled to
the ear by ear inserts
IEC 60959 - Provisional head and torso simulator for - -
acoustic measurements on air conduction
hearing aids
Undated reference.
Valid edition at date of issue.
---------------------- Page: 4 ----------------------
NORME CEI
INTERNATIONALE
IEC
60118-8
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
2005-10
Electroacoustique –
Appareils de correction auditive –
Partie 8:
Méthodes de mesure des caractéristiques
fonctionnelles des appareils de correction
auditive dans des conditions simulées de
fonctionnement in situ
Electroacoustics –
Hearing aids –
Part 8:
Methods of measurement of performance
characteristics of hearing aids under
simulated in situ working conditions
 IEC 2005 Droits de reproduction réservés  Copyright - all rights reserved

Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in any

utilisée sous quelque forme que ce soit et par aucun procédé, form or by any means, electronic or mechanical, including

électronique ou mécanique, y compris la photocopie et les photocopying and microfilm, without permission in writing from

microfilms, sans l'accord écrit de l'éditeur. the publisher.

International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland

Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch Web: www.iec.ch

CODE PRIX
PRICE CODE
Commission Electrotechnique Internationale
International Electrotechnical Commission
МеждународнаяЭлектротехническаяКомиссия
Pour prix, voir catalogue en vigueur
For price, see current catalogue
---------------------- Page: 5 ----------------------
60118-8  IEC:2005 – 3 –
CONTENTS

FOREWORD...........................................................................................................................7

INTRODUCTION...................................................................................................................11

1 Scope.............................................................................................................................13

2 Normative references .....................................................................................................13

3 Terms and definitions .....................................................................................................13

4 Limitations......................................................................................................................21

5 Test equipment...............................................................................................................23

5.1 Acoustical requirements for the test space ............................................................23

5.2 Sound source ........................................................................................................23

5.3 Manikin .................................................................................................................25

5.4 Ear simulator.........................................................................................................25

5.5 Ear insert simulator ...............................................................................................25

5.6 Equipment for the measurement of occluded-ear simulator sound pressure

level ......................................................................................................................25

5.7 Equipment for automatic sweep frequency recording .............................................27

5.8 Equipment for calibration of free field sound pressure level ...................................27

6 Test conditions ...............................................................................................................27

6.1 Choice of test point ...............................................................................................27

6.2 Ambient conditions ................................................................................................27

6.3 Manikin .................................................................................................................27

6.4 Location of the hearing aid ....................................................................................29

6.5 Normal operating conditions for the hearing aid.....................................................29

7 Measurements................................................................................................................31

7.1 General .................................................................................................................31

7.2 Adjustment of the reference input sound pressure level.........................................31

7.3 Manikin frequency response (MFR) .......................................................................31

7.4 Full-on simulated insertion gain measured by the constant reference input

SPL method ..........................................................................................................33

7.5 Full-on simulated insertion gain measured by the constant ear simulator SPL

method..................................................................................................................33

7.6 Directional characteristics .....................................................................................35

7.7 Simulated in situ OSPL90 measurements ..............................................................39

7.8 Simplified method to measure simulated in situ and insertion gain response .........41

8 Frequency response recording charts .............................................................................41

9 Maximum permitted expanded uncertainty of measurements ..........................................41

Annex A (normative) Free-field to hearing-aid-microphone transformation ...........................45

Annex B (normative) Manikin unoccluded-ear gain (open ear response) .............................53

Annex C (informative) General requirements for a manikin ..................................................57

Bibliography..........................................................................................................................63

---------------------- Page: 6 ----------------------
60118-8  IEC:2005 – 5 –
Figure A.1 – Microphone location and corresponding free-field to hearing-aid-

microphone transformation for behind-the-ear instruments....................................................47

Figure A.2 – Microphone location and corresponding free-field to hearing-aid-

microphone transformation for full-concha instruments .........................................................47

Figure A.3 – Microphone location and corresponding free-field to hearing-aid-

microphone transformation for canal size instruments...........................................................49

Figure A.4 – Microphone location and corresponding free-field to hearing-aid-

microphone transformation for completely-in-the-ear canal instruments ...............................49

Figure B.1 – Manikin unoccluded-ear gain frequency response.............................................53

Figure C.1 – Manikin geometrical references ........................................................................59

Figure C.2 – Co-ordinates for angles of azimuth and elevation..............................................61

Table 1 – Values of U for basic measurements ................................................................43

max
Table A.1 – Numerical data for the various free-field to hearing-aid-microphone

transformation responses......................................................................................................51

Table B.1 – Numerical data of manikin unoccluded-ear gain frequency response..................55

---------------------- Page: 7 ----------------------
60118-8  IEC:2005 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS –
HEARING AIDS –
Part 8: Methods of measurement of performance characteristics
of hearing aids under simulated in situ working conditions
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

international co-operation on all questions concerning standardization in the electrical and electronic fields. To

this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,

Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC

Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested

in the subject dealt with may participate in this preparatory work. International, governmental and non-

governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely

with the International Organization for Standardization (ISO) in accordance with conditions determined by

agreement between the two organizations.

2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international

consensus of opinion on the relevant subjects since each technical committee has representation from all

interested IEC National Committees.

3) IEC Publications have the form of recommendations for international use and are accepted by IEC National

Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC

Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any

misinterpretation by any end user.

4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications

transparently to the maximum extent possible in their national and regional publications. Any divergence

between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in

the latter.

5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any

equipment declared to be in conformity with an IEC Publication.

6) All users should ensure that they have the latest edition of this publication.

7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and

members of its technical committees and IEC National Committees for any personal injury, property damage or

other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and

expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC

Publications.

8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of

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

International Standard IEC 60118-8 has been prepared by IEC technical committee 29:

Electroacoustics.

This second edition cancels and replaces the first edition published in 1983, and constitutes a

technical revision. The purpose of this revision is to update the publication with a simplified

method for simulated in situ measurements of hearing aids and a description for

determination of the directivity index (DI) of directional microphones in hearing aids in the

horizontal plane. Other parts of the standard are basically left unchanged.
---------------------- Page: 8 ----------------------
60118-8  IEC:2005 – 9 –
The text of this standard is based on the following documents:
FDIS Report on voting
29/584/FDIS 29/589/RVD

Full information on the voting for the approval of this standard can be found in the report on

voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

IEC 60118 consists of the following parts, under the general title Electroacoustics – Hearing

aids
Part 0: Measurement of electroacoustical characteristics
Part 1: Hearing aids with induction pick-up coil input
Part 2: Hearing aids with automatic gain control circuits
Part 3: Hearing aid equipment not entirely worn on the listener

Part 4: Magnetic field strength in audio-frequency induction loops for hearing aid

purposes
Part 5: Nipples for insert earphones
Part 6: Characteristics of electrical input circuits for hearing aids

Part 7: Measurement of the performance characteristics of hearing aids for production,

supply and delivery quality assurance purposes

Part 8: Methods of measurement of performance characteristics of hearing aids under

simulated in situ working conditions

Part 9: Methods of measurement of characteristics of hearing aids with bone vibrator

output
Part 11: Symbols and other markings on hearing aids and related equipment
Part 12: Dimensions of electrical connector systems
Part 13: Electromagnetic compatibility (EMC)
Part 14: Specification of a digital interface device.

The committee has decided that the contents of this publication will remain unchanged until

the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in

the data related to the specific publication. At this date, the publication will be

• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
—————————

Various parts of the series were published under the general title Hearing aids. Future editions of these parts

will appear under the new general title.
---------------------- Page: 9 ----------------------
60118-8  IEC:2005 – 11 –
INTRODUCTION

Measurement methods that take into account the acoustical influence of the wearer on the

performance of hearing aids are important, particularly when the results are to be used to

assist in the fitting of hearing aids. The information obtained using this standard is likely to be

more relevant to the fitting of hearing aids than that provided by publications concerned with

type approval and quality control such as IEC 60118-0, and IEC 60118-7.

The methods specified in this standard require a device such as a manikin to simulate the

presence of the wearer. It has been found necessary to establish certain guidelines for

simulated in situ measurements of hearing aids. The recommended methods are described in

this standard.
---------------------- Page: 10 ----------------------
60118-8  IEC:2005 – 13 –
ELECTROACOUSTICS –
HEARING AIDS –
Part 8: Methods of measurement of performance characteristics
of hearing aids under simulated in situ working conditions
1 Scope

The purpose of this part of IEC 60118 is to describe methods for a test which simulates the

acoustical effects of a median adult wearer on the performance of a hearing aid.

It establishes certain guidelines for simulated in situ measurements of hearing aids; it

describes a simplified method for simulated in situ measurements of hearing aids and a

description for determination of the directivity index (DI) of directional microphones in hearing

aids in the horizontal plane.

In addition this second edition now specifies tolerances. Conformance to the specifications in

this International Standard is demonstrated only when the result of a measurement, extended

by the actual expanded uncertainty of measurement of the testing laboratory, lies fully within

the tolerances specified in this International Standard extended by the values for U

max
2 Normative references

The following referenced documents are indispensable for the application of this document.

For dated references, only the edition cited applies. For undated references, the latest edition

of the referenced document (including any amendments) applies.

IEC 60118-0:1983, Hearing aids – Part 0: Measurement of electroacoustical characteristics

IEC 60263, Scales and sizes for plotting frequency characteristics and polar diagrams

IEC 60711, Occluded-ear simulator for the measurement of earphones coupled to the ear by

ear inserts

IEC 60959, Provisional head and torso simulator for acoustic measurements of air conduction

hearing aids
3 Terms and definitions
For the purpose of this document, the following definitions apply:
---------------------- Page: 11 ----------------------
60118-8  IEC:2005 – 15 –
3.1
sound pressure level

throughout this standard all sound pressure levels (abbreviated SPL) are referred to 20 µPa

3.2
pinna simulator

device which has the approximate shape and dimensions of a median adult human pinna

3.3
ear simulator

device for measuring the output sound pressure level of an earphone under well defined

loading conditions in a specified frequency range. It consists essentially of a principal cavity,

acoustic load networks and a calibrated microphone. The location of the microphone is

chosen so that the sound pressure at the microphone corresponds approximately to the sound

pressure existing at the human eardrum
3.4
occluded-ear simulator

ear simulator which simulates the inner part of the ear canal, from the tip of an ear insert to

the eardrum
3.5
ear canal extension

device which connects the concha portion of the pinna simulator with the outer (reference

plane) face of the occluded-ear simulator, simulating the outer part of the ear canal excluding

the pinna
3.6
ear insert simulator

device used to represent the acoustic coupling between an earphone and the ear canal (e.g.

an earmould or a similar device without a connecting tube)
3.7
manikin (head and torso simulator)

head and torso simulator extending downward from the top of the head to the waist and

designed to simulate the acoustic diffraction produced by a median adult human head and

torso. The head includes two pinna simulators, and contains at least one occluded-ear

simulator
3.8
reference point of a subject or manikin

point bisecting the line joining the centres of the openings of the ear canals (at the junction

between concha and ear canal) (see Figure C.1)
3.9
plane of symmetry of the manikin

plane passing through the reference point of the manikin that divides the left and right

portions of the manikin into symmetrical halves (see Figure C.1)
3.10
axis of rotation of the manikin

straight line passing through the reference point of the manikin and lying in the plane of

symmetry of the manikin, and having a direction that would be vertical if the manikin were

mounted in a position corresponding to that of a standing person (and about which the

manikin can be rotated) (see Figure C.1)
---------------------- Page: 12 ----------------------
60118-8  IEC:2005 – 17 –
3.11
reference plane of the manikin

plane perpendicular to the axis of rotation containing the reference point of the manikin (see

Figure C.1)
3.12
test point

reproducible position in the test space at which the sound pressure level is measured with the

manikin absent and at which the reference point of the manikin is to be located for test

purposes
3.13
reference input sound pressure level
free field sound pressure level at the test point in the absence of the manikin
3.14
test axis
line joining the test point and the centre of the sound source (see Figure C.2)
3.15
test plane (for measurement of the uniformity of the free field wavefront)
plane perpendicular to the test axis and containing the test point
3.16
azimuth angle of sound incidence

angle between the plane of symmetry of the manikin and the plane defined by the axis of

rotation and the test axis (see Figure C.2). When the manikin faces the sound source the

azimuth angle of sound incidence is defined as 0°. When the right ear of the manikin faces

the sound source, the azimuth angle is defined as 90°. When the left ear faces the sound

source, the angle is defined as 270°
3.17
elevation angle of sound incidence

angle between the reference plane of the manikin and the test axis (see Figure C.2). When

the top of the manikin points towards the sound source the elevation angle is defined as 90°.

When the test axis lies in the reference plane, the elevation angle is defined as 0°

3.18
reference position of the manikin in the test space
position of the manikin in the test space that meets the following conditions:
– the reference point coincides with the test point, and
– the angles of azimuth and elevation are both equal to zero.
3.19
manikin unoccluded-ear gain
MUEG

difference between the sound pressure level in the unoccluded-ear simulator and the

reference input sound pressure level. This will be a function of manikin position

---------------------- Page: 13 ----------------------
60118-8  IEC:2005 – 19 –
3.20
manikin unoccluded-ear gain frequency response
MUEGFR

manikin unoccluded-ear gain expressed as a function of frequency, MFR (see 7.3) being a

function of manikin position
3.21
simulated in situ gain
SISG

difference between the SPL in the ear simulator produced by the hearing aid and the

reference input SPL. This will be a function of manikin position
3.22
simulated in situ gain frequency response
SISGFR
SISG expressed as a function of frequency
3.23
simulated insertion gain
SIG

difference between the SPL in the ear simulator produced by the hearing aid and the SPL in

the ear simulator with the hearing aid absent. This gain is equal to SISG-MUEG. This will be a

function of manikin position
3.24
full-on simulated insertion gain

SIG obtainable from a hearing aid with the gain control at maximum (full-on) and at stated

settings of the other hearing aid controls
3.25
simulated insertion gain frequency response
SIGFR
SIG expressed as a function of frequency
3.26
manikin unoccluded-ear directional response
MDR

sound pressure level in the ear simulator at a stated frequency as a function of azimuth

and/or elevation angle with the hearing aid absent
3.27
simulated in situ directional response
SISDR

sound pressure level in the ear simulator produced by the hearing aid as a function of azimuth

and/or elevation angle at a stated frequency, gain value and input level
---------------------- Page: 14 ----------------------
60118-8  IEC:2005 – 21 –
3.28
directivity index DI

for the purpose of this standard DI as a function of frequency is calculated from the SISDR

as the difference between the sound intensity level for azimuth and elevation angle equal to

0° and the average sound intensity level for all azimuth angles and elevation angles,

assuming rotational symmetry about an axis defined by the intersection of a vertical plane

with zero azimuth angle and the reference plane.
3.29
SII weighted directivity index
SIIDI

index calculated from the DI by applying a band importance function representing the

relative importance of the different frequencies for speech perception and as such obtaining a

frequency independent index. The weighting factors used in the calculation are according to

ANSI S3.5:1997 (see 7.6.4.2)
3.30
simulated insertion directional response D
SIDR
difference between SISDR and MDR
3.31
simulated in situ OSPL90 (output sound pressure level for 90 dB input SPL)

output sound pressure level in the ear simulator produced by the hearing aid at a specified

frequency with the hearing aid gain control at maximum (full-on) and a reference input SPL of

90 dB
3.32
simulated in situ OSPL90 frequency response
simulated in situ OSPL90 expressed as a function of frequency
4 Limitations

4.1 The results obtained under simulated in situ conditions may differ substantially from

results obtained on an individual person, due to anatomical variation of head, torso, pinna,

ear canal, and eardrum. Care should therefore be taken when interpreting the results.

4.2 The methods recommended in this standard give information on the measurement of the

following parameters that are considered important for the evaluation of the performance of a

hearing aid as normally worn, and for which simulated in situ conditions are considered

essential:
– full-on insertion gain;
– insertion frequency response;
– directional characteristics;
– simulated in situ OSPL90.

NOTE The accuracy and repeatability of results obtained under simulated in situ conditions cannot generally be

expected to be as good as when using the free-field technique laid down in IEC 60118-0:1983. The use of

simulated in situ conditions for the measurements of hearing aid parameters other than those listed above is

therefor
...

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