IEC 62489-2:2011

IEC 62489-2
®
Edition 1.0 2011-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Audio-frequency induction loop systems for assisted
hearing –
Part 2: Methods of calculating and measuring the low-frequency magnetic field
emissions from the loop for assessing conformity with guidelines on limits for
human exposure

Electroacoustique – Systèmes de boucles d’induction audiofréquences pour
améliorer l'audition –
Partie 2: Méthodes de calcul et de mesure des émissions de champ magnétique
basse fréquence à partir de la boucle pour l’évaluation de la conformité aux
instructions sur les limites d’exposition humaine

IEC 62489-2:2011

---------------------- Page: 1 ----------------------
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IEC 62489-2
®

Edition 1.0 2011-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE


Electroacoustics – Audio-frequency induction loop systems for assisted
hearing –
Part 2: Methods of calculating and measuring the low-frequency magnetic field
emissions from the loop for assessing conformity with guidelines on limits for
human exposure

Electroacoustique – Systèmes de boucles d’induction audiofréquences pour
améliorer l'audition –
Partie 2: Méthodes de calcul et de mesure des émissions de champ magnétique
basse fréquence à partir de la boucle pour l’évaluation de la conformité aux
instructions sur les limites d’exposition humaine

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX Q
ICS 17.140.50 ISBN 978-2-88912-318-6
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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– 2 – 62489-2  IEC:2011

CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Rated values . 5
4 Situation regarding current standards . 6
5 Configurations of loops . 6
5.1 Main types of configuration . 6
5.2 General considerations . 6
5.3 Large-area loops . 6
5.4 Medium-area loops . 7
5.5 Small-area loops . 7
5.6 Solenoid antennas . 7
6 Calculations . 8
6.1 General . 8
6.2 Solenoid antennas . 8
7 Measurements . 8
7.1 General . 8
7.2 Input signal . 8
7.3 Measuring instrument . 9
8 Comparison of calculated or measured results with guidelines or limits . 9
9 Meeting limits or guidelines . 10
10 Measurement uncertainty . 10
Annex A (informative) Rationale for this product-family magnetic field emission
standard for audio-frequency induction-loop systems (AFILS) in the context of human
exposure to non-ionizing radiation . 11
Annex B (informative) Example calculation using the coupling factor K . 15
Bibliography . 16

Figure 1 – An earhook induction transducer, with a BTE (Behind The Ear) hearing aid
body for scale . 7
Figure 2 – K factor as a function of least distance between wire and disc . 10
Figure A.1 – ICNIRP magnetic field reference levels . 11

Table A.1 – Extract from IEC 62311, Table A.1 – Characteristics and parameters of the
equipment to be considered, with the application to AFILS added . 13

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62489-2  IEC:2011 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

ELECTROACOUSTICS –
AUDIO-FREQUENCY INDUCTION LOOP SYSTEMS
FOR ASSISTED HEARING –

Part 2: Methods of calculating and measuring the low-frequency
magnetic field emissions from the loop for assessing conformity
with guidelines on limits for human exposure


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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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 62489-2 has been prepared by IEC technical committee 29:
Electroacoustics.
The text of this standard is based on the following documents:
FDIS Report on voting
29/728/FDIS 29/736/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.

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– 4 – 62489-2  IEC:2011
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 62489 series, published under the general title Electroacoustics –
Audio-frequency induction loop systems for assisted hearing, can be found on the IEC
website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC 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.

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62489-2  IEC:2011 – 5 –
ELECTROACOUSTICS –
AUDIO-FREQUENCY INDUCTION LOOP SYSTEMS
FOR ASSISTED HEARING –

Part 2: Methods of calculating and measuring the low-frequency
magnetic field emissions from the loop for assessing conformity
with guidelines on limits for human exposure



1 Scope
This part of IEC 62489 applies to audio-frequency induction-loop systems for assisted
hearing. It may also be applied to such systems used for other purposes, as far as it is
applicable. The standard is intended for assessment of human exposure to low-frequency
magnetic fields produced by the system, by calculation and by in-situ testing.
This standard does not deal with other aspects of safety, for which IEC 60065 applies, or with
EMC.
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 60118-4, Electroacoustics – Hearing aids – Part 4: Induction loop systems for hearing aid
purposes – Magnetic field strength
IEC 60268-1, Sound system equipment – Part 1: General
IEC 60268-2, Sound system equipment – Part 2: Explanation of general terms and calculation
methods
IEC 60268-10:1991, Sound system equipment – Part 10: Peak programme level meters
IEC 61786, Measurement of low-frequency magnetic and electric fields with regard to
exposure of human beings – Special requirements for instruments and guidance for
measurements
IEC 62226-2-1:2004, Exposure to electric or magnetic fields in the low and intermediate
frequency range – Methods for calculating the current density and internal electric field
induced in the human body – Part 2-1: Exposure to magnetic fields – 2D models
IEC 62311:2007, Assessment of electronic and electrical equipment related to human
exposure restrictions for electromagnetic fields (0 Hz – 300 GHz)
3 Rated values
The term rated means 'the value stated by the manufacturer'. Rated values are of two kinds:
rated conditions, fundamental values that cannot be verified by measurement, and others that
can be so verified. For a full explanation, see IEC 60268-2.

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– 6 – 62489-2  IEC:2011
4 Situation regarding current standards
Current published and draft IEC standards on EMF exposure do not give unambiguous
guidance on the approach that should be taken by product committees. The differences
between the signals that we are concerned with and those considered in depth in EMF
exposure standards are the following:
• wide relative bandwidth (ratio of highest to lowest frequency present, 100 Hz to 5 kHz);
• no predominant frequency within the band;
• rapidly-varying amplitude;
• high ratio of peak amplitude to average r.m.s. amplitude (at least 4).
5 Configurations of loops
5.1 Main types of configuration
There are four main types of configuration:
a) large area loops, with the smallest dimension larger than 1 m, usually installed at floor
level in a room;
b) medium-area loops, with dimensions of the order of 1 m, often oriented in a vertical plane,
installed at service desks and similar positions;
c) small area loops, with the largest dimension less than 1 m;
d) solenoid antennas, including the ear-hook.
NOTE Examples of small-area loops are portable systems, clipboards, neck loops, cushion loops (including those
for use in vehicles) and chair loops.
5.2 General considerations
All loops produce strong fields close to the loop conductor(s). This is shown by the
relationship between current I in a long, straight wire and the magnetic field strength H
produced at a distance R from the centre of the wire, where R is greater than the radius r of
the wire:
H = I/2πR (1)

NOTE 1 Within the wire, the field strength decreases linearly from I/2πr at the surface to zero at the centre.
NOTE 2 For n parallel conductors very close together (i.e. a multi-turn loop), the magnetic field strength is n times
that produced by a single conductor.
For calculations of field strengths in the high field strength regions, very close to the
conductor(s), the 'long, straight wire' approximation is almost always sufficiently accurate,
except for solenoids, which need a completely different treatment (see 6.2).
5.3 Large-area loops
The occupants of the room are likely to come close to the loop conductor only by stepping on
the floor at a point below which the conductor is installed. Such proximity is normally
transient. However, in places of worship, devotional postures may bring parts of the body
other than the feet into proximity. This may also apply in hospitals, treatment rooms and
gymnasia.
Maintenance staff might come into closer contact and for longer periods, but it is unlikely that
the system would then be operating.

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62489-2  IEC:2011 – 7 –
5.4 Medium-area loops
For these, there are three considerations:
a) The hearing-aid user is normally at a distance from the loop comparable to its dimensions.
Thus the loop current required to produce a maximum r.m.s. field strength of 400 mA/m (in
compliance with IEC 60118-4) at the hearing-aid is much larger than the current required
to produce it at the centre of the plane of the loop.
b) Nevertheless, the separation ensures that the hearing-aid user is not exposed to the high
fields strengths near the loop conductor.
c) However, staff may come into close proximity of the loop conductor while the system is
working unless steps are taken to maintain a minimum separation.
NOTE These loops often have more than one turn, so that the loop current can be kept reasonably small.
5.5 Small-area loops
For these, again, there are three considerations:
a) The separation for portable loops is very much greater than the loop dimensions, but for
other types, the separation distance may be small or very small unless steps are taken to
maintain a minimum separation.
b) The current apparently required is quite large, because of the large separation.
c) Both users and staff may come into close proximity of the loop, even that of a portable
system.
NOTE These loops usually have many turns, so that the actual current is not so large.
5.6 Solenoid antennas
One example that is commercially available is the ear-hook. This device is typically as shown
in Figure 1. A very small solenoid is incorporated in the stem of the device.


IEC  050/11

Figure 1 – An earhook induction transducer, with a BTE (behind the ear)
hearing aid body for scale

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– 8 – 62489-2  IEC:2011
6 Calculations
6.1 General
Calculation of the field strength can be reliably made using Equation (1) in almost all cases,
except where the loop is very small or is a solenoid of length which is not very small
compared with its plan dimensions, such as for the ear-hook device. It is necessary to
calculate the current required in the loop to produce a field strength of 400 mA/m at the
hearing-aid position, taking into account the orientation of the pick-up coil in the hearing-aid
relative to the plane of the loop. In general, this calculation is not easy, but simple
approximate methods give sufficiently accurate results when used with insight. Proprietary
calculation software, based on published mathematical analyses, exists. General-purpose
mathematics software can also be used.
Translating the calculated field strengths into a form comparable with exposure guidelines or
limits is again, not simple. See Clause 8.
6.2 Solenoid antennas
There is no simple expression for the field strength at a point outside a solenoid. A solenoid
may be treated as a stack of loops, or as a magnetic dipole, or the field strength can be
calculated by means of a rather complex equation (See Bibliography).
7 Measurements
7.1 General
In the audio-frequency range, exposure time is irrelevant, because the predominant
physiological effect, if it occurs, is nerve stimulation, which operates over a time-scale of a
few milliseconds. It is therefore appropriate to use a quasi-peak measurement of field
strength. Furthermore, exposure limits and guidelines are given in r.m.s. values, so the quasi-
peak meter should be scaled to read r.m.s. values with a sinusoidal signal. This type of meter,
the peak programme meter (PPM), is further described in IEC 60118-4 and IEC 60268-10
(type II).
It is also necessary to consider the type of magnetic field pick-up coil or sensor. Sensors may
be single-axis, with just one coil, or three-axis, with three orthogonal coils. For use with a
PPM, the single-axis sensor is most convenient, and if it is properly constructed, it is not
difficult to orient it for maximum reading, especially as the likely direction of the field can
usually be predicted from text-book field patterns.
The first measurement to be made shall determine that the field strength is correct at the
point or points where it is intended to be 400 mA/m (or the agreed lower value if adjusted to
reduce loudness, as specified in IEC 60118-4).
NOTE IEC 60118-4 specifies the use of either a PPM or an r.m.s. meter with a 125 ms integration time for the
measurement of magnetic field strength. However, for the purpose of this standard, the 125 ms integration time is
incompatible with the requirement to measure field strengths over times of the order of a few milliseconds.
The instrument specified for measurements on other equipment and systems, such as in IEC 62233, has an
averaging time specified only as an upper limit of 1 s, which is also too slow for the assessment of fields due to
audio-frequency signals.
7.2 Input signal
The input signal for the amplifier shall be the simulated programme signal described in
IEC 60268-1, with additional filtering, -3 dB at 100 Hz and 5 kHz relative to the 1 kHz level,
with ultimate attenuation slopes of at least 12 dB/octave.

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62489-2  IEC:2011 – 9 –
7.3 Measuring instrument
It is unlikely that a suitable complete instrument is commercially available at present, since
the application is extremely specialized. However, the design of an adapter for use with
widely-available audio test equipment, or that itself provides the PPM function, is not very
difficult. The elements are the following:
• the pick-up coil, which, because the field strengths of interest are high, needs few turns
and no magnetic core material. Because the fields are highly inhomogeneous, the coil
should be of small dimensions, to minimise averaging. A coil covering four faces of a 1 cm
cube of insulating material is convenient;
• a frequency-response correction circuit, which produces a constant output from a
magnetic field that varies with frequency in the same way as the guidelines or limits, with
bandwidth control so as to discard out-of band interference signals;
• amplification of the signal such that the maximum permissible field strength produces an
output voltage of 0,775 V for connection to the audio test equipment;
• optionally, a quasi-peak detector substantially as specified in IEC 60268-10 (type II) and
means to display its output with a resolution of 1 dB.
8 Comparison of calculated or measured results with guidelines or limits
IEC 62226-2-1 uses the approach specified in IEC 62226-1, whereby a coupling factor, K, is
used to determine the allowance to be made for non-uniformity of the induced current density
in a thin disc, caused by an inhomogeneous magnetic field:
K =J /J (2)

n u
where J is the actual maximum current density and J is the maximum current density due to
n
u
a homogeneous field:
J = σπRfB (3)
u
where
σ is the conductivity of the disc;
R is the radius of the disc;
f is the frequency; and
B is the magnetic induction (flux density) at the edge of the disc nearest to the source of the
magnetic field.
For most purposes, σ may be taken as 0,2 S/m, but for particular cases, the values in Table
C.1 of IEC 62311:2007 should be used. For the majority of cases, Equation (1) applies and
thus Annex B of IEC 62226-2-1:2004 is relevant. Since the values of K are larger for the 100
mm radius disc, this radius is adopted for this standard.
Using the data from Tables B.1 and B.2 of IEC 62226-2-1:2004, a convenient presentation of
the value of K as a function of the distance between the source and the nearest point of the
disc is given in Figure A.1.

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– 10 – 62489-2  IEC:2011

1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
0 1 2 3 4
1×10 1×10 1×10 1×10 1×10
Distance  (mm)
IEC  051/11

NOTE The dotted line represents the equation K = 0,2ln(r) – 0,24, where r is the distance. An example of a
calculation using K is given in Annex B.
Figure 2 – K factor as a function of least distance between wire and disc
9 Meeting limits or guidelines
The field strength near the loop conductor is fixed by the current, which in turn is fixed by the
field strength required at the hearing-aid position. It is clearly not possible to meet exposure
requirements by reducing the current. It is also obvious that any form of shielding is unlikely
to be practicable in most cases.
However, what can be done is to insert a physical barrier between the loop conductor and the
person who might otherwise come too close to it. This barrier can be of any non-magnetic,
non-electrically conducting material.
10 Measurement uncertainty
The total measurement uncertainty includes sensor position and orientation, operating
conditions and, for in-situ measurements, magnetic background noise (although if the system
complies with IEC 60118-4, the effect of noise is negligible). Guidance on uncertainty is
provided in IEC 61786.
K factor

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62489-2  IEC:2011 – 11 –
Annex A
(informative)

Rationale for this product-family magnetic field emission standard for
audio-frequency induction-loop systems (AFILS) in the context of human
exposure to non-ionizing radiation

A.1 General
Why do we need such a standard? Because the European Low Voltage Directive includes
'radiation' in its Essential Requirements, and although that was probably originally thought of
as 'ionizing radiation' from cathode-ray tubes, it is now interpreted to include non-ionizing
radiation as well. There are also exposure limits in place in the USA and probably other
countries, or there will be in the foreseeable future. Consequently, the manufacturer of any
product that emits electric or magnetic fields or electromagnetic energy shall be able to
demonstrate that its emissions are not hazardous, and in practice, if there is any doubt this is
v
...