IEC TR 63079:2017
(Main)Code of practice for hearing-loop systems (HLS)
Code of practice for hearing-loop systems (HLS)
IEC TR 63079:2017(E) gives recommendations for and guidance on the design, planning, installation, testing, operation and maintenance of a hearing-loop system (HLS) intended for communicating speech, music and/or other signals. It is mainly concerned with HLS for hearing enhancement, in which the signals are communicated to users of hearing aids equipped with magnetic pick-up coils.
This document does not apply to induction-loop systems which use a carrier frequency, nor to other systems for hearing enhancement purposes which do not use magnetic induction.
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IEC TR 63079 ®
Edition 1.1 2018-09
CONSOLIDATED VERSION
TECHNICAL
REPORT
colour
inside
Code of practice for hearing-loop systems (HLS)
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IEC TR 63079 ®
Edition 1.1 2018-09
CONSOLIDATED VERSION
TECHNICAL
REPORT
colour
inside
Code of practice for hearing-loop systems (HLS)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.140.50 ISBN 978-2-8322-6041-8
IEC TR 63079 ®
Edition 1.1 2018-09
CONSOLIDATED VERSION
REDLINE VERSION
colour
inside
Code of practice for hearing-loop systems (HLS)
– 2 – IEC TR 63079:2017+AMD1:2018 CSV
IEC 2018
CONTENTS
FOREWORD . 7
INTRODUCTION . 9
1 Scope . 10
2 Normative references. 10
3 Terms, definitions, signs and symbols . 10
3.1 Terms and definitions . 10
3.2 Signs and symbols . 12
3.2.1 Symbol for an induction-loop . 12
3.2.2 Symbol for multiple loops . 13
3.2.3 Sign for display in premises where an HLS is installed and for HLS
equipment identification . 13
3.2.4 Identification of areas where reception of the HLS is not satisfactory . 14
4 General . 14
4.1 How to use this document . 14
4.1.1 Persons addressed . 14
4.1.2 Objectives . 14
4.2 Specialist advice . 15
4.3 Safety aspects . 15
4.4 Conforming to existing performance documents . 15
5 Technical advice . 15
5.1 Complying with this document . 15
5.2 Seeking technical advice . 16
5.3 The nature of the advice . 16
5.4 Professional (consultancy) advice . 16
6 Purpose of the system . 16
7 Choosing the system supplier . 17
7.1 General . 17
7.2 Approaching a contractor . 17
7.3 Approaching a manufacturer of HLS equipment. 17
7.4 Approaching a specialist consultant . 17
8 Contractual provisions . 18
8.1 Performance specification . 18
8.2 Verifying that the completed system delivers its intended performance . 18
8.3 Arbitration . 18
9 Classification of systems . 19
10 Design . 20
10.1 General . 20
10.2 Symbols . 21
10.3 Basic theory . 21
10.3.1 Production of a magnetic field . 21
10.3.2 Directional pattern of the magnetic field . 23
10.3.3 Uniformity of the magnetic field strength . 27
10.3.4 Reference points for magnetic field strength . 27
10.3.5 Relationship between the requirements of IEC 60118-4 and the
characteristics of hearing aids and speech signals . 28
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10.3.6 Impedance of the loop . 29
10.3.7 Selection of the method of driving the loop . 30
10.3.8 Current-driven loop . 30
10.3.9 Voltage-driven loop . 31
10.3.10 Voltage-driven loop with high-level equalization . 32
10.3.11 Voltage-driven loop with low-level equalization . 34
10.3.12 Use of transformers. 34
10.3.13 Effects of building construction . 35
10.3.14 Electromagnetic compatibility (EMC) . 36
10.3.15 Tone signals . 38
10.3.16 Equalization, other than for compensating loop impedance
characteristics. 38
10.3.17 Multiple loops. 38
10.3.18 Protection of loop conductors . 41
10.3.19 Automatic gain control (AGC), compression, limiting, noise gating and
voice control . 42
10.3.20 Signal-to-noise ratio . 42
10.3.21 HLS for purposes other than assisted hearing . 43
10.4 System components . 44
10.4.1 Final amplifiers . 44
10.4.2 Preamplifiers and mixers . 44
10.4.3 Signal sources . 45
10.5 Objective measurement of intelligibility . 52
10.6 Safety and reliability considerations . 53
10.7 Designing for monitoring and maintenance. 53
10.8 External factors . 53
10.8.1 Magnetic noise interference . 53
10.8.2 Effect of metal in the building . 54
10.9 Magnetic field overspill . 54
10.10 The role of the system designer in commissioning . 54
11 Responsibility of the installer . 54
12 Installation practices and workmanship . 56
13 Inspection and testing of wiring . 57
14 Commissioning . 58
15 Documentation . 60
16 Certification . 60
17 Acceptance . 61
18 Verification . 62
19 Owner responsibilities . 63
19.1 Signage . 63
19.2 User feedback . 63
19.3 Staff training . 63
20 Operation and maintenance . 63
20.1 General . 63
20.2 Routine testing . 63
20.3 Inspection and servicing . 64
20.4 Non-routine attention . 65
20.5 Special inspection on appointment of a new maintenance organization . 65
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20.6 Arrangements for repair of faults or damage . 65
20.7 Modifications to the system . 66
21 User responsibilities . 66
21.1 Responsible person . 66
21.2 Logbook . 67
Annex A (informative) HLS monitoring receivers . 68
A.1 General . 68
A.2 Recommendations for fixed receivers . 68
A.3 Recommendations for portable receivers . 69
Annex B (informative) What is a hearing-loop system (HLS) and how does it work? . 70
B.1 General . 70
B.2 Benefits of HLS (for hearing enhancement) . 70
B.3 Limitations of HLS . 71
Annex C (informative) Explanations of the basis of the design formulas . 72
C.1 Magnetic field strength . 72
C.1.1 Magnetic field strength produced by an element of conductor . 72
C.1.2 Field strength produced by a circular loop at a point on its axis . 73
C.2 Magnetic field strength at the centre of a rectangular loop . 74
C.3 Magnetic field strength at an arbitrary point . 76
C.4 Loop conductor sizes and resistances . 80
C.4.1 General . 80
C.4.2 Resistance of the loop conductor and relation between conductor size
and cut-off frequency for a voltage-driven loop . 80
C.4.3 Quick reference tables (derived from IEC 60228) . 82
C.4.4 Inductance of two parallel wires, leading to the inductance of a
rectangular loop . 82
C.4.5 Variation of the impedance of a loop of fixed dimensions with conductor
resistance . 84
C.4.6 High-level equalizer . 85
Annex D (informative) Explanation of the specification and measurement of magnetic
field strength of induction-loop systems . 88
Annex E (normative) Specification of the PPM-based field strength meter . 90
E.1 General . 90
E.2 Checking magnetic field strength meters . 91
Annex F (informative) Magnetic field direction near the loop conductor . 92
Annex G (informative) Direct-to-reverberant sound pressure ratio . 95
Annex H (informative) Model certificates . 96
H.1 Design certificate . 96
H.2 Installation certificate . 97
H.3 Commissioning certificate . 98
H.4 Acceptance certificate . 100
H.5 Verification certificate (optional) . 101
H.6 Inspection and servicing certificate . 102
H.7 Modification certificate . 104
Annex I (informative) Measuring STI with hearing loop systems (HLS) . 106
I.1 Causes of loss of intelligibility . 106
I.2 Measurement method . 106
I.2.1 General . 106
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I.2.2 Limitations of the method . 106
I.2.3 Measurement equipment . 106
I.2.4 Procedure . 107
Annex J (informative) Example of an architect's specification for an HLS . 108
Bibliography . 109
Figure 1 – Symbol for use on diagrams . 13
Figure 2 – Symbol for multiple loops for use on diagrams . 13
Figure 3 – Sign for display in premises to indicate that an HLS is installed and for HLS
equipment identification . 14
Figure 4 – Sign to show seating areas where HLS reception is not satisfactory . 14
Figure 5 – Pictorial view of the magnetic field (lines of force) of a rectangular loop . 22
Figure 6 – Directional response of a hearing aid telecoil . 24
Figure 7 – Variation of the strength of the perpendicular component of the magnetic
field across an axis of a rectangular loop, with listening height as parameter . 25
Figure 8 – Variation across the median of a square loop of the perpendicular and
coplanar components of the magnetic field at a height ratio h = 1 . 26
n
Figure 9 – Variation across the diagonal of a square loop of the perpendicular and
coplanar components of the magnetic field at a height ratio h = 1 . 26
n
Figure 10 – Variation with frequency of the logarithm of the impedance of loops of
constant inductance L, with the loop resistance R as parameter . 30
Figure 11 – Circuit diagram of a "high-level" equalizer (for insertion between the
amplifier and the loop) . 34
Figure 12 – Preferred loop layout in a steel-framed building . 35
Figure 13 – Methods of breaking eddy current paths in metal structures . 36
Figure 14 – Filter for attenuating VHF signals picked up by the loop. 36
Figure 15 – Methods of "breaking" loops into which interference voltages could be
induced by an HLS. 37
Figure 16 – Multiple loop layouts . 39
Figure 17 – Variation across an axis of the system, at a height ratio of 0,32, of the
component of the field strength, for a loop system similar to that shown in Figure 16 c),
compared with the corresponding variations for a conventional single loop . 41
Figure 18 – Horizontal directional response of an omni-directional microphone with its
axis vertical: decibel scale . 48
Figure 19 – Directional response of a cardioid microphone: decibel scale . 48
Figure 20 – Directional response of a supercardioid microphone: decibel scale . 49
Figure 21 – Directional response of a hypercardioid microphone: decibel scale . 49
Figure 22 – Directional response of a highly directional shotgun (rifle) microphone
where f = 250 Hz and barrel length is 275 mm . 49
Figure 23 – Directional response of the same microphone where f = 2,5 kHz . 49
Figure 24 – Directional response of the same microphone where f = 10 kHz . 49
Figure 25 – Relative operating distances of directional microphones for equal direct-to-
reverberant signal ratios . 50
Figure 26 – Two methods of positioning microphones on a conference table . 51
Figure 27 – The 3-to-1 ratio for microphone positioning (normal) . 51
Figure 28 – A reduced 3-to-1 ratio using angled microphones . 51
Figure 29 – Illustrations of good and bad microphone placements . 52
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Figure 30 – Optimization of the height of the microphone above the table . 52
Figure 31 – Measuring field strength of type 2 HLS – Plan views. 59
Figure C.1 – Generation of magnetic fields . 73
Figure C.2 – Diagram for calculating magnetic field strength at the centre of a
rectangular loop . 76
Figure C.3 – Diagram for calculating magnetic field strength at an arbitrary point . 77
Figure C.4 – Diagram for calculating cosΦ and sinΦ . 78
Figure C.5 – Diagram for calculating magnetic field strength at point (x, y, z) . 80
Figure C.6 – Circuit diagram of a “high-level” or Poperwell equalizer (for insertion
between the amplifier and the loop) . 86
Figure C.7 – Frequency response obtained with a high-level equalizer . 87
Figure E.1 – "EQ" or "wideband" frequency response: target curve and tolerances on
response . 90
Figure F.1 – Magnetic field patterns . 92
Figure F.2 – Magnetic field directions for a floor-level loop . 93
Figure F.3 – Magnetic field directions for a ceiling-level loop . 94
Table 1 – Classes of loop system . 20
Table 2 – Relative operating distances of directional microphones for equal direct-to-
reverberant signal ratios . 50
Table C.1 – Factor by which the loop current has to be increased, compared with that
required for a given magnetic field strength at the centre of a square loop, to obtain
the same field strength for a rectangular loop at a point at height ratio of h above or
n
below the centre of the loop . 74
Table C.2 – Class 5 flexible annealed copper conductors for standard single-core and
multi-core cables . 81
Table C.3 – Class 1 solid annealed copper conductors for single-core and multi-core
cables . 82
Table C.4 – Ratio of approximate to exact inductance . 84
Table C.5 – Values of L . 85
|z|
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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CODE OF PRACTICE FOR HEARING-LOOP SYSTEMS (HLS)
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
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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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
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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.
This consolidated version of the official IEC Standard and its amendment has been prepared
for user convenience.
IEC TR 63079 edition 1.1 contains the first edition (2017-04) [documents 29/917/DTR and
29/923/RVC] and its amendment 1 (2018-09) [documents 29/983/DTR and 29/992/RVDTR].
In this Redline version, a vertical line in the margin shows where the technical content is
modified by amendment 1. Additions are in green text, deletions are in strikethrough red
text. A separate Final version with all changes accepted is available in this
publication.
– 8 – IEC TR 63079:2017+AMD1:2018 CSV
IEC 2018
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a Technical Report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 63079, which is a Technical Report, has been prepared by IEC technical
committee 29: Electroacoustics.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of the base publication and its amendment will
remain unchanged until the stability 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.
A bilingual version of this publication may be issued at a later date.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
IEC 2018
INTRODUCTION
The performance of induction-loop systems is specified in IEC 60118-4, whereas
IEC TR 63079 gives recommendations and guidance for their design, planning, installation,
testing, operation and maintenance. Provisions for components of a system are given in
IEC 62489-1. Methods of calculation and measurement of the magnetic field, in the context of
human exposure, are given in IEC 62489-2.
This document takes the form of guidance and recommendations. It should not be quoted as if
it were a specification and particular care should be taken to ensure that claims of compliance
are not misleading.
Any user claiming compliance with this document is expected to be able to justify any course
of action that deviates from its recommendations.
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IEC 2018
CODE OF PRACTICE FOR HEARING-LOOP SYSTEMS (HLS)
1 Scope
This document, which is a Technical Report, gives recommendations for and guidance on the
design, planning, installation, testing, operation and maintenance of a hearing-loop system
(HLS) intended for communicating speech, music and/or other signals. It is mainly concerned
with HLS for hearing enhancement, in which the signals are communicated to users of hearing
aids equipped with magnetic pick-up coils.
This document does not apply to induction-loop systems which use a carrier frequency, nor to
other systems for hearing enhancement purposes which do not use magnetic induction.
2 Normative references
There are no normative references in this document.
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements 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 60118-4:2014, Electroacoustics – Hearing aids – Part 4: Induction-loop systems for
hearing aid purposes – System performance requirements
IEC 60268-16, Sound system equipment – Part 16: Objective rating of speech intelligibility by
speech transmission index
3 Terms, definitions, signs and symbols
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
hearing-loop system
HLS
system including amplifier(s), microphones and/or other signal sources, in which magnetic
fields are created by the flow of audio-frequency current in a conductor arranged in the form
of one or more loops or a coil or solenoid
Note 1 to entry: The technical term for a hearing-loop system is "audio-frequency induction-lop system" (AFILS).
3.1.2
HLS for hearing enhancement
HLS in which the intended receivers are hearing aids or specially designed listening devices
equipped with coils acting as magnetic antennas
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3.1.3
direct-to-reverberant ratio
ratio, at a given point in the sound field, of the sound pressure due to the wanted sound
source to the sound pressure due to reverberation
3.1.4
hearing aid
personal amplification system, worn entirely on the listener, which is designed to enable a
person with impaired hearing to hear more easily
3.1.5
loop listener
listening device which is designed to give an audible output in response to signals transmitted
by an HLS
3.1.6
HLS monitoring receiver
equipment designed to verify the performance of an HLS by audio and visual means
a) providing visible indication that it is powered and when the strength of the magnetic field
produced by the loop falls within a specified range,
b) providing an audio-frequency output by which the sound quality of the HLS transmissions
can be assessed, and
c) providing other, optional facilities (see Annex A)
3.1.7
magnetic field strength
magnitude of the magnetic field, at a stated point in space and in a stated direction,
generated by the flow of alternating current in an HLS
3.1.8
listening plane
plane perpendicular to the axis of telecoils in hearing aids (see Annex B)
3.1.9
useful magnetic field volume
volume within which the system provides hearing-aid users with a signal of acceptable quality
[SOURCE: IEC 60118-4:2014, 3.2, modified — The brackets have been deleted from the
definition, as well as the notes.]
3.1.10
system designer
technically competent person who takes responsibility for the technical specification, design
and performance of the system
3.1.11
simple system
HLS that is neither large nor complex and does not require specialist skills in order to achieve
a satisfactory result
3.1.12
counter system
ticket office system
small area HLS designed to assist communication between (usually) two persons, sometimes
through a transparent screen
Note 1 to entry: For example at a ticket office or bank.
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3.1.13
complex installation
HLS installation in which any of the following apply:
• HLS that has an approximate coverage area greater than 400 m ;
• it is required to operate close to another HLS, which could lead to co-interference;
• it is required that a certain area is not covered by the HLS, in case it might interfere with
sensitive electronic equipment, for example electric guitars or dynamic microphones;
• it is required that the system operates on a number of different listening planes;
• an unconventional layout of loop conductors is indicated;
• there is metal in the building’s structure and this causes irregularity in the field strength
when using a perimeter loop
Note 1 to entry: Such installations are likely to require specialized design knowledge in order to obtain a
satisfactory result.
3.1.14
direction of the magnetic field
resultant direction of the magnetic field at a point in three dimensional space, arising from the
phasor sum of components of the field derived by integration over all elements of the
induction-loop
3.1.15
HLS reference plane
the plane to which the magnetic characteristics of the HLS are referred when stating specified
values
Note 1 to entry The reference plane specification includes its inclination to the horizontal.
3.1.16
reference magnetic field strength level
0 dB reference for magnetic field strength levels, which is 400 mA/m
[SOURCE: IEC 60118-4:2014, 3.1, modified – The definition has been rephrased, and the
note has been deleted.]
3.1.17
overspill
magnetic field of usable strength that is present outside the volume in which is it required
Note 1 to entry This magnetic field extends outside the useful magnetic field volume because a field of lower
strength than the minimum specified in IEC 60118-4 can still interfere with other nearby HLS or be received with
suitable equipment.
3.2 Signs and symbols
3.2.1 Symbol for an induction-loop
The symbol shown in Figure 1 should be used on circuit diagrams to indicate an HLS.
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IEC
Figure 1 – Symbol for use on diagrams
3.2.2 Symbol for multiple loops
The symbol shown in Figure 2 should be used on circuit diagrams to indicate a complex or
low-spill HLS.
IEC
NOTE n is the number or letter corresponding to the classification of loop. See Clause 11.
Figure 2 – Symbol for multiple loops for use on diagrams
3.2.3 Sign for display in premises where an HLS is installed and for HLS equipment
identification
Signs for display should be produced on a durable material.
Areas where the reception of the HLS is satisfactory should be clearly indicated at visible
positions by means of the sign shown in Figure 3, which is based on the one originally
adopted by the World Federation of the Deaf. The sign should indicate T as shown. The
background colour should be Pantone reference 661 or 662 (blue) and the printing should be
in white.
NOTE Some organizations that support people with hearing loss do not approve HLS unless signs are displayed.
Signs of other colours are found in use.
Normally the dimensions of the display signs should be a minimum of 100 mm × 100 mm;
when used on equipment, they can be of any convenient size.
The sign should appear at a position visible in normal use on HLS equipment, on loop cable
junction boxes and adjacent to the loop cable itself where it would be helpful to maintenance
personnel or help to prevent accidental disturbance.
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IEC
Figure 3 – Sign for display in premises to indicate
that an HLS is installed and for HLS
equipment identification
3.2.4 Identification of areas where reception of the HLS is not satisfactory
Areas where the reception of the HLS is not satisfactory should be clearly indicated on a plan
of the space by means of the sign shown in Figure 4. The plan should be placed where it can
be seen by both hearing-aid users and staff. This sign should also be placed visibly in the
unsatisfactory area and/or on all seats in that area.
IEC
Figure 4 – Sign to show seating areas where
HLS reception is not satisfactory
4 General
4.1 How to use this document
4.1.1 Persons addressed
This document is divided into clauses, with recommendations intended primarily for
• purchasers of HLS (Clauses 1 to 9, 20 and Annex B),
• designers of HLS and HLS equipment (Clauses 9 and 10, 20, Annex A, Annex C to
Annex G and Annex J),
• installers of HLS (Clauses 11 to 17, Annex A and Annex H),
• those responsible for commissioning, testing, operation and maintenance (Clauses 14 to
18, 20 and Annex H), and
• those responsible for day-to-day operations, ensuring that the facilities are accessible to
as many people as is reasonably possible (Clauses 19 to 21, Annex A and Annex B).
4.1.2 Objectives
The recommendations of this document are intended to lead to the following.
IEC 2018
a) Prospective purchasers clearly understand the functions and limitations of an HLS (see
Annex B).
b) The design, installation and setting-up of the HLS allows compliance with the appropriate
requirements for safety and electromagnetic compatibility and all relevant regulations
relating to electrical installations.
c) Users wearing
...
IEC TR 63079 ®
Edition 1.0 2017-04
TECHNICAL
REPORT
colour
inside
Code of practice for hearing-loop systems (HLS)
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IEC TR 63079 ®
Edition 1.0 2017-04
TECHNICAL
REPORT
colour
inside
Code of practice for hearing-loop systems (HLS)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.140.50 ISBN 978-2-8322-4154-7
– 2 – IEC TR 63079:2017 IEC 2017
CONTENTS
FOREWORD . 7
INTRODUCTION . 9
1 Scope . 10
2 Normative references . 10
3 Terms, definitions, signs and symbols . 10
3.1 Terms and definitions . 10
3.2 Signs and symbols . 12
3.2.1 Symbol for an induction-loop . 12
3.2.2 Symbol for multiple loops . 12
3.2.3 Sign for display in premises where an HLS is installed and for HLS
equipment identification . 13
3.2.4 Identification of areas where reception of the HLS is not satisfactory . 13
4 General . 14
4.1 How to use this document . 14
4.1.1 Persons addressed . 14
4.1.2 Objectives . 14
4.2 Specialist advice . 15
4.3 Safety aspects . 15
4.4 Conforming to existing performance documents . 15
5 Technical advice . 15
5.1 Complying with this document . 15
5.2 Seeking technical advice . 15
5.3 The nature of the advice . 16
5.4 Professional (consultancy) advice . 16
6 Purpose of the system . 16
7 Choosing the system supplier . 16
7.1 General . 16
7.2 Approaching a contractor . 17
7.3 Approaching a manufacturer of HLS equipment . 17
7.4 Approaching a specialist consultant . 17
8 Contractual provisions . 17
8.1 Performance specification . 17
8.2 Verifying that the completed system delivers its intended performance . 17
8.3 Arbitration . 18
9 Classification of systems . 18
10 Design . 20
10.1 General . 20
10.2 Symbols . 20
10.3 Basic theory . 21
10.3.1 Production of a magnetic field . 21
10.3.2 Directional pattern of the magnetic field . 22
10.3.3 Uniformity of the magnetic field strength . 26
10.3.4 Reference points for magnetic field strength . 26
10.3.5 Relationship between the requirements of IEC 60118-4 and the
characteristics of hearing aids and speech signals . 27
10.3.6 Impedance of the loop . 28
10.3.7 Selection of the method of driving the loop . 29
10.3.8 Current-driven loop . 29
10.3.9 Voltage-driven loop . 30
10.3.10 Voltage-driven loop with high-level equalization . 31
10.3.11 Voltage-driven loop with low-level equalization . 33
10.3.12 Use of transformers . 33
10.3.13 Effects of building construction . 34
10.3.14 Electromagnetic compatibility (EMC) . 35
10.3.15 Tone signals . 37
10.3.16 Equalization, other than for compensating loop impedance
characteristics . 37
10.3.17 Multiple loops . 37
10.3.18 Protection of loop conductors . 40
10.3.19 Automatic gain control (AGC), compression, limiting, noise gating and
voice control . 40
10.3.20 Signal-to-noise ratio . 41
10.3.21 HLS for purposes other than assisted hearing . 42
10.4 System components . 42
10.4.1 Final amplifiers . 42
10.4.2 Preamplifiers and mixers . 42
10.4.3 Signal sources . 43
10.5 Objective measurement of intelligibility . 50
10.6 Safety and reliability considerations . 51
10.7 Designing for monitoring and maintenance . 51
10.8 External factors . 51
10.8.1 Magnetic noise interference . 51
10.8.2 Effect of metal in the building . 52
10.9 Magnetic field overspill . 52
10.10 The role of the system designer in commissioning . 52
11 Responsibility of the installer . 52
12 Installation practices and workmanship . 54
13 Inspection and testing of wiring . 55
14 Commissioning . 56
15 Documentation . 57
16 Certification . 58
17 Acceptance. 59
18 Verification . 60
19 Owner responsibilities . 60
19.1 Signage . 60
19.2 User feedback . 61
19.3 Staff training . 61
20 Operation and maintenance . 61
20.1 General . 61
20.2 Routine testing . 61
20.3 Inspection and servicing . 62
20.4 Non-routine attention . 63
20.5 Special inspection on appointment of a new maintenance organization . 63
20.6 Arrangements for repair of faults or damage . 63
– 4 – IEC TR 63079:2017 IEC 2017
20.7 Modifications to the system . 63
21 User responsibilities . 64
21.1 Responsible person . 64
21.2 Logbook . 65
Annex A (informative) HLS monitoring receivers . 66
A.1 General . 66
A.2 Recommendations for fixed receivers. 66
A.3 Recommendations for portable receivers . 67
Annex B (informative) What is a hearing-loop system (HLS) and how does it work? . 68
B.1 General . 68
B.2 Benefits of HLS (for hearing enhancement) . 68
B.3 Limitations of HLS . 69
Annex C (informative) Explanations of the basis of the design formulas . 70
C.1 Magnetic field strength . 70
C.1.1 Magnetic field strength produced by an element of conductor . 70
C.1.2 Field strength produced by a circular loop at a point on its axis . 70
C.2 Magnetic field strength at the centre of a rectangular loop . 72
C.3 Magnetic field strength at an arbitrary point . 73
C.4 Loop conductor sizes and resistances . 78
C.4.1 General . 78
C.4.2 Resistance of the loop conductor and relation between conductor size
and cut-off frequency for a voltage-driven loop . 78
C.4.3 Quick reference tables (derived from IEC 60228) . 79
C.4.4 Inductance of two parallel wires, leading to the inductance of a
rectangular loop . 79
C.4.5 Variation of the impedance of a loop of fixed dimensions with conductor
resistance . 82
C.4.6 High-level equalizer . 82
Annex D (informative) Explanation of the specification and measurement of magnetic
field strength of induction-loop systems . 85
Annex E (normative) Specification of the PPM-based field strength meter. 87
E.1 General . 87
E.2 Checking magnetic field strength meters . 88
Annex F (informative) Magnetic field direction near the loop conductor . 89
Annex G (informative) Direct-to-reverberant sound pressure ratio . 91
Annex H (informative) Model certificates . 92
H.1 Design certificate . 92
H.2 Installation certificate . 93
H.3 Commissioning certificate . 94
H.4 Acceptance certificate . 96
H.5 Verification certificate (optional) . 97
H.6 Inspection and servicing certificate . 98
H.7 Modification certificate . 100
Bibliography . 102
Figure 1 – Symbol for use on diagrams . 12
Figure 2 – Symbol for multiple loops for use on diagrams . 13
Figure 3 – Sign for display in premises to indicate that an HLS is installed and for HLS
equipment identification . 13
Figure 4 – Sign to show seating areas where HLS reception is not satisfactory . 14
Figure 5 – Pictorial view of the magnetic field (lines of force) of a rectangular loop . 21
Figure 6 – Directional response of a hearing aid telecoil . 23
Figure 7 – Variation of the strength of the perpendicular component of the magnetic
field across an axis of a rectangular loop, with listening height as parameter . 24
Figure 8 – Variation across the median of a square loop of the perpendicular and
coplanar components of the magnetic field at a height ratio h = 1 . 25
n
Figure 9 – Variation across the diagonal of a square loop of the perpendicular and
coplanar components of the magnetic field at a height ratio h = 1 . 25
n
Figure 10 – Variation with frequency of the logarithm of the impedance of loops of
constant inductance L, with the loop resistance R as parameter . 29
Figure 11 – Circuit diagram of a "high-level" equalizer (for insertion between the
amplifier and the loop) . 33
Figure 12 – Preferred loop layout in a steel-framed building . 34
Figure 13 – Methods of breaking eddy current paths in metal structures . 35
Figure 14 – Filter for attenuating VHF signals picked up by the loop . 35
Figure 15 – Methods of "breaking" loops into which interference voltages could be
induced by an HLS . 36
Figure 16 – Multiple loop layouts . 38
Figure 17 – Variation across an axis of the system, at a height ratio of 0,32, of the
component of the field strength, for a loop system similar to that shown in Figure 16 c),
compared with the corresponding variations for a conventional single loop . 39
Figure 18 – Horizontal directional response of an omni-directional microphone with its
axis vertical: decibel scale . 46
Figure 19 – Directional response of a cardioid microphone: decibel scale . 46
Figure 20 – Directional response of a supercardioid microphone: decibel scale . 47
Figure 21 – Directional response of a hypercardioid microphone: decibel scale . 47
Figure 22 – Directional response of a highly directional shotgun (rifle) microphone
where f = 250 Hz and barrel length is 275 mm . 47
Figure 23 – Directional response of the same microphone where f = 2,5 kHz . 47
Figure 24 – Directional response of the same microphone where f = 10 kHz . 47
Figure 25 – Relative operating distances of directional microphones for equal direct-to-
reverberant signal ratios . 48
Figure 26 – Two methods of positioning microphones on a conference table . 49
Figure 27 – The 3-to-1 ratio for microphone positioning (normal) . 49
Figure 28 – A reduced 3-to-1 ratio using angled microphones . 49
Figure 29 – Illustrations of good and bad microphone placements . 50
Figure 30 – Optimization of the height of the microphone above the table . 50
Figure 31 – Measuring field strength of type 2 HLS – Plan views . 57
Figure C.1 – Generation of magnetic fields . 71
Figure C.2 – Diagram for calculating magnetic field strength at the centre of a
rectangular loop . 73
Figure C.3 – Diagram for calculating magnetic field strength at an arbitrary point . 75
Figure C.4 – Diagram for calculating cosΦ and sinΦ . 75
Figure C.5 – Diagram for calculating magnetic field strength at point (x, y, z) . 77
– 6 – IEC TR 63079:2017 IEC 2017
Figure C.6 – Circuit diagram of a “high-level” or Poperwell equalizer (for insertion
between the amplifier and the loop) . 84
Figure C.7 – Frequency response obtained with a high-level equalizer . 84
Figure E.1 – "EQ" or "wideband" frequency response: target curve and tolerances on
response . 87
Figure F.1 – Magnetic field patterns . 89
Figure F.2 – Magnetic field directions for a floor-level loop . 90
Figure F.3 – Magnetic field directions for a ceiling-level loop . 90
Table 1 – Classes of loop system. 19
Table 2 – Relative operating distances of directional microphones for equal direct-to-
reverberant signal ratios . 48
Table C.1 – Factor by which the loop current has to be increased, compared with that
required for a given magnetic field strength at the centre of a square loop, to obtain
the same field strength for a rectangular loop at a point at height ratio of h above or
n
below the centre of the loop . 72
Table C.2 – Class 5 flexible annealed copper conductors for standard single-core and
multi-core cables . 79
Table C.3 – Class 1 solid annealed copper conductors for single-core and multi-core
cables . 79
Table C.4 – Ratio of approximate to exact inductance . 82
Table C.5 – Values of L . 82
|z|
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CODE OF PRACTICE FOR HEARING-LOOP SYSTEMS (HLS)
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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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
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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.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a Technical Report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 63079, which is a Technical Report, has been prepared by IEC technical
committee 29: Electroacoustics.
The text of this Technical Report is based on the following documents:
Enquiry draft Report on voting
29/917/DTR 29/923/RVC
Full information on the voting for the approval of this Technical Report can be found in the
report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
– 8 – IEC TR 63079:2017 IEC 2017
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://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.
A bilingual version of this publication may be issued at a later date.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
INTRODUCTION
The performance of induction-loop systems is specified in IEC 60118-4, whereas
IEC TR 63079 gives recommendations and guidance for their design, planning, installation,
testing, operation and maintenance. Provisions for components of a system are given in
IEC 62489-1. Methods of calculation and measurement of the magnetic field, in the context of
human exposure, are given in IEC 62489-2.
This document takes the form of guidance and recommendations. It should not be quoted as if
it were a specification and particular care should be taken to ensure that claims of compliance
are not misleading.
Any user claiming compliance with this document is expected to be able to justify any course
of action that deviates from its recommendations.
– 10 – IEC TR 63079:2017 IEC 2017
CODE OF PRACTICE FOR HEARING-LOOP SYSTEMS (HLS)
1 Scope
This document, which is a Technical Report, gives recommendations for and guidance on the
design, planning, installation, testing, operation and maintenance of a hearing-loop system
(HLS) intended for communicating speech, music and/or other signals. It is mainly concerned
with HLS for hearing enhancement, in which the signals are communicated to users of hearing
aids equipped with magnetic pick-up coils.
This document does not apply to induction-loop systems which use a carrier frequency, nor to
other systems for hearing enhancement purposes which do not use magnetic induction.
2 Normative references
There are no normative references in this document.
3 Terms, definitions, signs and symbols
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
hearing-loop system
HLS
system including amplifier(s), microphones and/or other signal sources, in which magnetic
fields are created by the flow of audio-frequency current in a conductor arranged in the form
of one or more loops or a coil or solenoid
Note 1 to entry: The technical term for a hearing-loop system is "audio-frequency induction-lop system" (AFILS).
3.1.2
HLS for hearing enhancement
HLS in which the intended receivers are hearing aids or specially designed listening devices
equipped with coils acting as magnetic antennas
3.1.3
direct-to-reverberant ratio
ratio, at a given point in the sound field, of the sound pressure due to the wanted sound
source to the sound pressure due to reverberation
3.1.4
hearing aid
personal amplification system, worn entirely on the listener, which is designed to enable a
person with impaired hearing to hear more easily
3.1.5
loop listener
listening device which is designed to give an audible output in response to signals transmitted
by an HLS
3.1.6
HLS monitoring receiver
equipment designed to verify the performance of an HLS by audio and visual means
a) providing visible indication that it is powered and when the strength of the magnetic field
produced by the loop falls within a specified range,
b) providing an audio-frequency output by which the sound quality of the HLS transmissions
can be assessed, and
c) providing other, optional facilities (see Annex A)
3.1.7
magnetic field strength
magnitude of the magnetic field, at a stated point in space and in a stated direction,
generated by the flow of alternating current in an HLS
3.1.8
listening plane
plane perpendicular to the axis of telecoils in hearing aids (see Annex B)
3.1.9
useful magnetic field volume
volume within which the system provides hearing-aid users with a signal of acceptable quality
[SOURCE: IEC 60118-4:2014, 3.2, modified — The brackets have been deleted from the
definition, as well as the notes.]
3.1.10
system designer
technically competent person who takes responsibility for the technical specification, design
and performance of the system
3.1.11
simple system
HLS that is neither large nor complex and does not require specialist skills in order to achieve
a satisfactory result
3.1.12
counter system
ticket office system
small area HLS designed to assist communication between (usually) two persons, sometimes
through a transparent screen
Note 1 to entry: For example at a ticket office or bank.
3.1.13
complex installation
HLS installation in which any of the following apply:
• HLS that has an approximate coverage area greater than 400 m ;
• it is required to operate close to another HLS, which could lead to co-interference;
• it is required that a certain area is not covered by the HLS, in case it might interfere with
sensitive electronic equipment, for example electric guitars or dynamic microphones;
– 12 – IEC TR 63079:2017 IEC 2017
• it is required that the system operates on a number of different listening planes;
• an unconventional layout of loop conductors is indicated;
• there is metal in the building’s structure and this causes irregularity in the field strength
when using a perimeter loop
Note 1 to entry: Such installations are likely to require specialized design knowledge in order to obtain a
satisfactory result.
3.1.14
direction of the magnetic field
resultant direction of the magnetic field at a point in three dimensional space, arising from the
phasor sum of components of the field derived by integration over all elements of the
induction-loop
3.1.15
HLS reference plane
the plane to which the magnetic characteristics of the HLS are referred when stating specified
values
Note 1 to entry The reference plane specification includes its inclination to the horizontal.
3.1.16
reference magnetic field strength level
0 dB reference for magnetic field strength levels, which is 400 mA/m
[SOURCE: IEC 60118-4:2014, 3.1, modified – The definition has been rephrased, and the
note has been deleted.]
3.1.17
overspill
magnetic field of usable strength that is present outside the volume in which is it required
Note 1 to entry This magnetic field extends outside the useful magnetic field volume because a field of lower
strength than the minimum specified in IEC 60118-4 can still interfere with other nearby HLS or be received with
suitable equipment.
3.2 Signs and symbols
3.2.1 Symbol for an induction-loop
The symbol shown in Figure 1 should be used on circuit diagrams to indicate an HLS.
IEC
Figure 1 – Symbol for use on diagrams
3.2.2 Symbol for multiple loops
The symbol shown in Figure 2 should be used on circuit diagrams to indicate a complex or
low-spill HLS.
IEC
NOTE n is the number or letter corresponding to the classification of loop. See Clause 11.
Figure 2 – Symbol for multiple loops for use on diagrams
3.2.3 Sign for display in premises where an HLS is installed and for HLS equipment
identification
Signs for display should be produced on a durable material.
Areas where the reception of the HLS is satisfactory should be clearly indicated at visible
positions by means of the sign shown in Figure 3, which is based on the one originally
adopted by the World Federation of the Deaf. The sign should indicate T as shown. The
background colour should be Pantone reference 661 or 662 (blue) and the printing should be
in white.
NOTE Some organizations that support people with hearing loss do not approve HLS unless signs are displayed.
Signs of other colours are found in use.
Normally the dimensions of the display signs should be a minimum of 100 mm × 100 mm;
when used on equipment, they can be of any convenient size.
The sign should appear at a position visible in normal use on HLS equipment, on loop cable
junction boxes and adjacent to the loop cable itself where it would be helpful to maintenance
personnel or help to prevent accidental disturbance.
IEC
Figure 3 – Sign for display in premises to indicate
that an HLS is installed and for HLS
equipment identification
3.2.4 Identification of areas where reception of the HLS is not satisfactory
Areas where the reception of the HLS is not satisfactory should be clearly indicated on a plan
of the space by means of the sign shown in Figure 4. The plan should be placed where it can
be seen by both hearing-aid users and staff. This sign should also be placed visibly in the
unsatisfactory area and/or on all seats in that area.
– 14 – IEC TR 63079:2017 IEC 2017
IEC
Figure 4 – Sign to show seating areas where
HLS reception is not satisfactory
4 General
4.1 How to use this document
4.1.1 Persons addressed
This document is divided into clauses, with recommendations intended primarily for
• purchasers of HLS (Clauses 1 to 9, 20 and Annex B),
• designers of HLS and HLS equipment (Clauses 9 and 10, 20, Annex A, Annex C to
Annex G),
• installers of HLS (Clauses 11 to 17, Annex A and Annex H),
• those responsible for commissioning, testing, operation and maintenance (Clauses 14 to
18, 20 and Annex H), and
• those responsible for day-to-day operations, ensuring that the facilities are accessible to
as many people as is reasonably possible (Clauses 19 to 21, Annex A and Annex B).
4.1.2 Objectives
The recommendations of this document are intended to lead to the following.
a) Prospective purchasers clearly understand the functions and limitations of an HLS (see
Annex B).
b) The design, installation and setting-up of the HLS allows compliance with the appropriate
requirements for safety and electromagnetic compatibility and all relevant regulations
relating to electrical installations.
c) Users wearing hearing aids equipped with a magnetic pick-up coil receive signals
transmitted by the HLS that are intelligible and free from undue distortion and interference
even when all non-HLS equipment normally used in the environment is in operation.
d) The reception of such signals is possible in any part of the space; where, by force of
circumstance, the reception in some parts is limited, this is clearly defined and identified
(see 3.2.4).
e) Visual indication is given to prospective users that an HLS is installed.
f) Means are provided for the building management to easily verify that an HLS is working
satisfactorily (see Annex A).
g) The HLS can be satisfactorily operated and maintained by properly trained and competent
personnel.
h) Where necessary, the HLS is arranged for minimum extraneous coverage. It should be
understood, however, that even if an HLS is designed to limit extraneous coverage, it
might be possible for a determined person outside the intended area of coverage to
receive what might be confidential information.
Guidance is given on means to reduce possible effects of the magnetic field produced by the
loop on other nearby equipment.
4.2 Specialist advice
Except for home systems sold as a complete kit, the engagement of a skilled system designer
is advisable and is considered to be essential whenever large or complex installations are
involved. Professional bodies can be approached for lists of such specialists.
4.3 Safety aspects
It is essential that an HLS, even if temporary, is installed in a safe manner and in accordance
with safe work practices (see 10.7).
4.4 Conforming to existing performance documents
The performance document for HLS is IEC 60118-4, the requirements of which have been
incorporated (where applicable) as recommendations in this document.
5 Technical advice
5.1 Complying with this document
Significant problems can arise with poorly designed or installed HLS. Purchasers should
stipulate that the design and installation are to comply with this code of practice and that the
...
IEC TR 63079 ®
Edition 1.2 2020-04
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TECHNICAL
REPORT
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Code of practice for hearing-loop systems (HLS)
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IEC TR 63079 ®
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TECHNICAL
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Code of practice for hearing-loop systems (HLS)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.140.50 ISBN 978-2-8322-8141-3
IEC TR 63079 ®
Edition 1.2 2020-04
CONSOLIDATED VERSION
REDLINE VERSION
colour
inside
Code of practice for hearing-loop systems (HLS)
– 2 – IEC TR 63079:2017+AMD1:2018
+AMD2:2020 CSV IEC 2020
CONTENTS
FOREWORD . 7
INTRODUCTION . 9
1 Scope . 10
2 Normative references . 10
3 Terms, definitions, signs and symbols . 10
3.1 Terms and definitions . 10
3.2 Signs and symbols . 12
3.2.1 Symbol for an induction-loop . 12
3.2.2 Symbol for multiple loops . 13
3.2.3 Sign for display in premises where an HLS is installed and for HLS
equipment identification . 13
3.2.4 Identification of areas where reception of the HLS is not satisfactory . 14
4 General . 14
4.1 How to use this document . 14
4.1.1 Persons addressed . 14
4.1.2 Objectives . 14
4.2 Specialist advice . 15
4.3 Safety aspects . 15
4.4 Conforming to existing performance documents . 15
5 Technical advice . 15
5.1 Complying with this document . 15
5.2 Seeking technical advice . 16
5.3 The nature of the advice . 16
5.4 Professional (consultancy) advice . 16
6 Purpose of the system . 16
7 Choosing the system supplier . 17
7.1 General . 17
7.2 Approaching a contractor . 17
7.3 Approaching a manufacturer of HLS equipment . 17
7.4 Approaching a specialist consultant . 17
8 Contractual provisions . 18
8.1 Performance specification . 18
8.2 Verifying that the completed system delivers its intended performance . 18
8.3 Arbitration . 18
9 Classification of systems . 19
10 Design . 20
10.1 General . 20
10.2 Symbols . 21
10.3 Basic theory . 21
10.3.1 Production of a magnetic field . 21
10.3.2 Directional pattern of the magnetic field . 23
10.3.3 Uniformity of the magnetic field strength . 27
10.3.4 Reference points for magnetic field strength . 27
10.3.5 Relationship between the requirements of IEC 60118-4 and the
characteristics of hearing aids and speech signals . 28
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10.3.6 Impedance of the loop . 29
10.3.7 Selection of the method of driving the loop . 30
10.3.8 Current-driven loop . 30
10.3.9 Voltage-driven loop . 31
10.3.10 Voltage-driven loop with high-level equalization . 32
10.3.11 Voltage-driven loop with low-level equalization . 34
10.3.12 Use of transformers . 34
10.3.13 Effects of building construction . 35
10.3.14 Electromagnetic compatibility (EMC) . 36
10.3.15 Tone signals . 38
10.3.16 Equalization, other than for compensating loop impedance
characteristics . 38
10.3.17 Multiple loops . 38
10.3.18 Protection of loop conductors . 41
10.3.19 Automatic gain control (AGC), compression, limiting, noise gating and
voice control . 42
10.3.20 Signal-to-noise ratio . 42
10.3.21 HLS for purposes other than assisted hearing . 43
10.3.22 Time-differences between related information streams . 44
10.4 System components . 44
10.4.1 Final amplifiers . 44
10.4.2 Preamplifiers and mixers . 44
10.4.3 Signal sources . 45
10.5 Objective measurement of intelligibility . 52
10.6 Safety and reliability considerations . 53
10.7 Designing for monitoring and maintenance . 53
10.8 External factors . 53
10.8.1 Magnetic noise interference . 53
10.8.2 Effect of metal in the building . 54
10.9 Magnetic field overspill . 54
10.10 The role of the system designer in commissioning . 54
11 Responsibility of the installer . 54
12 Installation practices and workmanship . 56
13 Inspection and testing of wiring . 57
14 Commissioning . 58
15 Documentation . 60
16 Certification . 60
17 Acceptance. 61
18 Verification . 62
19 Owner responsibilities . 63
19.1 Signage . 63
19.2 User feedback . 63
19.3 Staff training . 63
20 Operation and maintenance . 63
20.1 General . 63
20.2 Routine testing . 63
20.3 Inspection and servicing . 64
20.4 Non-routine attention . 65
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20.5 Special inspection on appointment of a new maintenance organization . 65
20.6 Arrangements for repair of faults or damage . 65
20.7 Modifications to the system . 66
21 User responsibilities . 66
21.1 Responsible person . 66
21.2 Logbook . 67
Annex A (informative) HLS monitoring receivers . 68
A.1 General . 68
A.2 Recommendations for fixed receivers. 68
A.3 Recommendations for portable receivers . 69
Annex B (informative) What is a hearing-loop system (HLS) and how does it work? . 70
B.1 General . 70
B.2 Benefits of HLS (for hearing enhancement) . 70
B.3 Limitations of HLS . 71
Annex C (informative) Explanations of the basis of the design formulas . 72
C.1 Magnetic field strength . 72
C.1.1 Magnetic field strength produced by an element of conductor . 72
C.1.2 Field strength produced by a circular loop at a point on its axis . 73
C.2 Magnetic field strength at the centre of a rectangular loop . 74
C.3 Magnetic field strength at an arbitrary point . 76
C.4 Loop conductor sizes and resistances . 80
C.4.1 General . 80
C.4.2 Resistance of the loop conductor and relation between conductor size
and cut-off frequency for a voltage-driven loop . 80
C.4.3 Quick reference tables (derived from IEC 60228) . 82
C.4.4 Inductance of two parallel wires, leading to the inductance of a
rectangular loop . 82
C.4.5 Variation of the impedance of a loop of fixed dimensions with conductor
resistance . 84
C.4.6 High-level equalizer . 85
Annex D (informative) Explanation of the specification and measurement of magnetic
field strength of induction-loop systems . 88
Annex E (normative) Specification of the PPM-based field strength meter. 90
E.1 General . 90
E.2 Checking magnetic field strength meters . 91
Annex F (informative) Magnetic field direction near the loop conductor . 92
Annex G (informative) Direct-to-reverberant sound pressure ratio . 95
Annex H (informative) Model certificates . 96
H.1 Design certificate . 96
H.2 Installation certificate . 97
H.3 Commissioning certificate . 98
H.4 Acceptance certificate . 100
H.5 Verification certificate (optional) . 101
H.6 Inspection and servicing certificate . 102
H.7 Modification certificate . 104
Annex I (informative) Measuring STI with hearing loop systems (HLS) . 106
I.1 Causes of loss of intelligibility . 106
I.2 Measurement method . 106
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I.2.1 General . 106
I.2.2 Limitations of the method . 106
I.2.3 Measurement equipment . 106
I.2.4 Procedure . 107
Annex J (informative) Example of an architect's specification for an HLS . 108
Annex K (informative) Control of time-differences between information presented to a
recipient via two transmission channels or media . 109
K.1 General . 109
K.2 Audible speech signals . 109
K.3 Audible speech signal and video display of the talker . 109
K.4 Priority . 109
Bibliography . 110
Figure 1 – Symbol for use on diagrams . 13
Figure 2 – Symbol for multiple loops for use on diagrams . 13
Figure 3 – Sign for display in premises to indicate that an HLS is installed and for HLS
equipment identification . 14
Figure 4 – Sign to show seating areas where HLS reception is not satisfactory . 14
Figure 5 – Pictorial view of the magnetic field (lines of force) of a rectangular loop . 22
Figure 6 – Directional response of a hearing aid telecoil . 24
Figure 7 – Variation of the strength of the perpendicular component of the magnetic
field across an axis of a rectangular loop, with listening height as parameter . 25
Figure 8 – Variation across the median of a square loop of the perpendicular and
coplanar components of the magnetic field at a height ratio h = 1 . 26
n
Figure 9 – Variation across the diagonal of a square loop of the perpendicular and
coplanar components of the magnetic field at a height ratio h = 1 . 26
n
Figure 10 – Variation with frequency of the logarithm of the impedance of loops of
constant inductance L, with the loop resistance R as parameter . 30
Figure 11 – Circuit diagram of a "high-level" equalizer (for insertion between the
amplifier and the loop) . 34
Figure 12 – Preferred loop layout in a steel-framed building . 35
Figure 13 – Methods of breaking eddy current paths in metal structures . 36
Figure 14 – Filter for attenuating VHF signals picked up by the loop . 36
Figure 15 – Methods of "breaking" loops into which interference voltages could be
induced by an HLS . 37
Figure 16 – Multiple loop layouts . 39
Figure 17 – Variation across an axis of the system, at a height ratio of 0,32, of the
component of the field strength, for a loop system similar to that shown in Figure 16 c),
compared with the corresponding variations for a conventional single loop . 41
Figure 18 – Horizontal directional response of an omni-directional microphone with its
axis vertical: decibel scale . 48
Figure 19 – Directional response of a cardioid microphone: decibel scale . 48
Figure 20 – Directional response of a supercardioid microphone: decibel scale . 49
Figure 21 – Directional response of a hypercardioid microphone: decibel scale . 49
Figure 22 – Directional response of a highly directional shotgun (rifle) microphone
where f = 250 Hz and barrel length is 275 mm . 49
Figure 23 – Directional response of the same microphone where f = 2,5 kHz . 49
Figure 24 – Directional response of the same microphone where f = 10 kHz . 49
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Figure 25 – Relative operating distances of directional microphones for equal direct-to-
reverberant signal ratios . 50
Figure 26 – Two methods of positioning microphones on a conference table . 51
Figure 27 – The 3-to-1 ratio for microphone positioning (normal) . 51
Figure 28 – A reduced 3-to-1 ratio using angled microphones . 51
Figure 29 – Illustrations of good and bad microphone placements . 52
Figure 30 – Optimization of the height of the microphone above the table . 52
Figure 31 – Measuring field strength of type 2 HLS – Plan views . 59
Figure C.1 – Generation of magnetic fields . 73
Figure C.2 – Diagram for calculating magnetic field strength at the centre of a
rectangular loop . 76
Figure C.3 – Diagram for calculating magnetic field strength at an arbitrary point . 77
Figure C.4 – Diagram for calculating cosΦ and sinΦ . 78
Figure C.5 – Diagram for calculating magnetic field strength at point (x, y, z) . 80
Figure C.6 – Circuit diagram of a “high-level” or Poperwell equalizer (for insertion
between the amplifier and the loop) . 86
Figure C.7 – Frequency response obtained with a high-level equalizer . 87
Figure E.1 – "EQ" or "wideband" frequency response: target curve and tolerances on
response . 90
Figure F.1 – Magnetic field patterns . 92
Figure F.2 – Magnetic field directions for a floor-level loop . 93
Figure F.3 – Magnetic field directions for a ceiling-level loop . 94
Table 1 – Classes of loop system. 20
Table 2 – Relative operating distances of directional microphones for equal direct-to-
reverberant signal ratios . 50
Table C.1 – Factor by which the loop current has to be increased, compared with that
required for a given magnetic field strength at the centre of a square loop, to obtain
the same field strength for a rectangular loop at a point at height ratio of h above or
n
below the centre of the loop . 74
Table C.2 – Class 5 flexible annealed copper conductors for standard single-core and
multi-core cables . 81
Table C.3 – Class 1 solid annealed copper conductors for single-core and multi-core
cables . 82
Table C.4 – Ratio of approximate to exact inductance . 84
Table C.5 – Values of L . 85
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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CODE OF PRACTICE FOR HEARING-LOOP SYSTEMS (HLS)
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.
This consolidated version of the official IEC Standard and its amendments has been
prepared for user convenience.
IEC TR 63079 edition 1.1 contains the first edition (2017-04) [documents 29/917/DTR and
29/923/RVC], its amendment 1 (2018-09) [documents 29/983/DTR and 29/992/RVDTR] and
its amendment 2 (2020-04) [documents 29/1037/DTR and 29/1046/RVDTR].
In this Redline version, a vertical line in the margin shows where the technical content
is modified by amendments 1 and 2. Additions are in green text, deletions are in
strikethrough red text. A separate Final version with all changes accepted is available
in this publication.
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a Technical Report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 63079, which is a Technical Report, has been prepared by IEC technical
committee 29: Electroacoustics.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of the base publication and its amendments will
remain unchanged until the stability 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
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INTRODUCTION
The performance of induction-loop systems is specified in IEC 60118-4, whereas
IEC TR 63079 gives recommendations and guidance for their design, planning, installation,
testing, operation and maintenance. Provisions for components of a system are given in
IEC 62489-1. Methods of calculation and measurement of the magnetic field, in the context of
human exposure, are given in IEC 62489-2.
This document takes the form of guidance and recommendations. It should not be quoted as if
it were a specification and particular care should be taken to ensure that claims of compliance
are not misleading.
Any user claiming compliance with this document is expected to be able to justify any course
of action that deviates from its recommendations.
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CODE OF PRACTICE FOR HEARING-LOOP SYSTEMS (HLS)
1 Scope
This document, which is a Technical Report, gives recommendations for and guidance on the
design, planning, installation, testing, operation and maintenance of a hearing-loop system
(HLS) intended for communicating speech, music and/or other signals. It is mainly concerned
with HLS for hearing enhancement, in which the signals are communicated to users of hearing
aids equipped with magnetic pick-up coils.
This document does not apply to induction-loop systems which use a carrier frequency, nor to
other systems for hearing enhancement purposes which do not use magnetic induction.
2 Normative references
There are no normative references in this document.
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements 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 60118-4:2014, Electroacoustics – Hearing aids – Part 4: Induction-loop systems for
hearing aid purposes – System performance requirements
IEC 60268-16, Sound system equipment – Part 16: Objective rating of speech intelligibility by
speech transmission index
3 Terms, definitions, signs and symbols
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
hearing-loop system
HLS
system including amplifier(s), microphones and/or other signal sources, in which magnetic
fields are created by the flow of audio-frequency current in a conductor arranged in the form
of one or more loops or a coil or solenoid
Note 1 to entry: The technical term for a hearing-loop system is "audio-frequency induction-lop system" (AFILS).
3.1.2
HLS for hearing enhancement
HLS in which the intended receivers are hearing aids or specially designed listening devices
equipped with coils acting as magnetic antennas
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3.1.3
direct-to-reverberant ratio
ratio, at a given point in the sound field, of the sound pressure due to the wanted sound
source to the sound pressure due to reverberation
3.1.4
hearing aid
personal amplification system, worn entirely on the listener, which is designed to enable a
person with impaired hearing to hear more easily
3.1.5
loop listener
listening device which is designed to give an audible output in response to signals transmitted
by an HLS
3.1.6
HLS monitoring receiver
equipment designed to verify the performance of an HLS by audio and visual means
a) providing visible indication that it is powered and when the strength of the magnetic field
produced by the loop falls within a specified range,
b) providing an audio-frequency output by which the sound quality of the HLS transmissions
can be assessed, and
c) providing other, optional facilities (see Annex A)
3.1.7
magnetic field strength
magnitude of the magnetic field, at a stated point in space and in a stated direction,
generated by the flow of alternating current in an HLS
3.1.8
listening plane
plane perpendicular to the axis of telecoils in hearing aids (see Annex B)
3.1.9
useful magnetic field volume
volume within which the system provides hearing-aid users with a signal of acceptable quality
[SOURCE: IEC 60118-4:2014, 3.2, modified — The brackets have been deleted from the
definition, as well as the notes.]
3.1.10
system designer
technically competent person who takes responsibility for the technical specification, design
and performance of the system
3.1.11
simple system
HLS that is neither large nor complex and does not require specialist skills in order to achieve
a satisfactory result
3.1.12
counter system
ticket office system
small area HLS designed to assist communication between (usually) two persons, sometimes
through a transparent screen
Note 1 to entry: For example at a ticket office or bank.
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3.1.13
complex installation
HLS installation in which any of the following apply:
• HLS that has an approximate coverage area greater than 400 m ;
• it is required to operate close to another HLS, which could lead to co-interference;
• it is required that a certain area is not covered by the HLS, in case it might interfere with
sensitive electronic equipment, for example electric guitars or dynamic microphones;
• it is required that the system operates on a number of different listening planes;
• an unconventional layout of loop conductors is indicated;
• there is metal in the building’s structure and this causes irregularity in the field strength
when using a perimeter loop
Note 1 to entry: Such installations are likely to require specialized design knowledge in order to obtain a
satisfactory result.
3.1.14
direction of the magnetic field
resultant direction of the magnetic field at a point in three dimensional space, arising from the
phasor sum of components of the field derived by integration over all elements of the
induction-loop
3.1.15
HLS reference plane
the plane to which the magnetic characteristics of the HLS are referred when stating specified
values
Note 1 to entry The reference plane specification includes its inclination to the horizontal.
3.1.16
reference magnetic field strength level
0 dB reference for magnetic field strength levels, which is 400 mA/m
[SOURCE: IEC 60118-4:2014, 3.1, modified – The definition has been rephrased, and the
note has been deleted.]
3.1.17
overspill
magnetic field of usable strength that is present outside the volume in which is it required
Note 1 to entry This magnetic field extends outside the useful magnetic field volume because a field of lower
strength than the minimum specified in IEC 60118-4 can still interfere with other nearby HLS or be received with
suitable equipment.
3.2 Signs and symbols
3.2.1 Symbol for an induction-loop
The symbol shown in Figure 1 should be used on circuit diagrams to indicate an HLS.
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IEC
Figure 1 – Symbol for use on diagrams
3.2.2 Symbol for multiple loops
The symbol shown in Figure 2 should be used on circuit diagrams to indicate a complex or
low-spill HLS.
IEC
NOTE n is the number or letter corresponding to the classification of loop. See Clause 11.
Figure 2 – Symbol for multiple loops for use on diagrams
3.2.3 Sign for display in premises where an HLS is installed and for HLS equipment
identification
Signs for display should be produced on a durable material.
Areas where the reception of the HLS is satisfactory should be clearly indicated at visible
positions by means of the sign shown in Figure 3, which is based on the one originally
adopted by the World Federation of the Deaf. The sign should indicate T as shown. The
background colour should be Pantone reference 661 or 662 (blue) and the printing should be
in white.
NOTE Some organizations that support people with hearing loss do not approve HLS unless signs are displayed.
Signs of other colours are found in use.
Normally the dimensions of the display signs should be a minimum of 100 mm × 100 mm;
when used on equipment, they can be of any convenient size.
The sign should appear at a position visible in normal use on HLS equipment, on loop cable
junction boxes and adjacent to the loop cable itself where it would be helpful to maintenance
personnel or help to prevent accidental disturbance.
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IEC
Figure 3 – Sign for display in premises to indicate
that an HLS is installed and for HLS
equipment identification
3.2.4 Identification of areas where reception of the HLS is not satisfactory
Areas where the reception of the HLS is not satisfactory should be clearly indicated on a plan
of the space by means of the sign shown in Figure 4. The plan should be placed where it can
be seen by both hearing-aid users and staff. This sign should also be placed visibly in the
unsatisfactory area and/or on all seats in that area.
IEC
Figure 4 – Sign to show seating areas where
HLS reception is not satisfactory
4 General
4.1 How to use this document
4.1.1 Persons addressed
This document is divided into clauses, with recommendations intended primarily for
• purchasers of HLS (Clauses 1 to 9, 20 and Annex B),
• designers of HLS and HLS equipment (Clauses 9 an
...
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