IEC 62489-1:2010

IEC 62489-1 ®
Edition 1.1 2014-12
CONSOLIDATED
VERSION
VERSION
CONSOLIDÉE
colour
inside
Electroacoustics – Audio-frequency induction loop systems for assisted
hearing –
Part 1: Methods of measuring and specifying the performance of system
components
Électroacoustique – Systèmes de boucles d’induction audiofréquences pour
améliorer l'audition –
Partie 1: Méthodes de mesure et de spécification des performances des
composants de systèmes
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IEC 62489-1 ®
Edition 1.1 2014-12
CONSOLIDATED
VERSION
VERSION
CONSOLIDÉE
colour
inside
Electroacoustics – Audio-frequency induction loop systems for assisted

hearing –
Part 1: Methods of measuring and specifying the performance of system

components
Électroacoustique – Systèmes de boucles d’induction audiofréquences pour

améliorer l'audition –
Partie 1: Méthodes de mesure et de spécification des performances des

composants de systèmes
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.140.50 ISBN 978-2-8322-2107-5

IEC 62489-1 ®
Edition 1.1 2014-12
REDLINE VERSION
VERSION REDLINE
colour
inside
Electroacoustics – Audio-frequency induction loop systems for assisted
hearing –
Part 1: Methods of measuring and specifying the performance of system
components
Électroacoustique – Systèmes de boucles d’induction audiofréquences pour
améliorer l'audition –
Partie 1: Méthodes de mesure et de spécification des performances des
composants de systèmes
– 2 – IEC 62489-1:2010
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CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Rated values . 7
5 Amplifiers . 8
5.1 General . 8
5.2 Rated conditions and standard measuring conditions . 8
5.2.1 Rated conditions . 8
5.2.2 Standard measurement conditions . 9
5.3 Pre-conditioning . 9
5.4 Characteristics to be specified, methods of measurement and presentation of
results . 9
5.4.1 Rated source impedance . 9
5.4.2 Rated source e.m.f. . 9
5.4.3 Rated load . 10
5.4.4 Rated temperature-limited output current . 10
5.4.5 Rated time for delivery of rated distortion-limited output current . 10
5.4.6 Rated total harmonic distortion of the output current . 10
5.4.7 Maximum (distortion-limited) output current . 11
5.4.8 Compliance voltage . 11
5.4.9 Noise . 12
5.4.10 Magnetic field strength . 13
5.4.11 Loop dimensions . 13
5.4.12 Frequency response . 14
5.4.13 Automatic gain control and compression . 14
5.4.14 Phase error of quadrature networks for phased loop arrays . 15
5.4.15 Installation . 15
5.4.16 Physical characteristics . 15
6 Microphones . 16
6.1 General . 16
6.2 Characteristics to be specified, methods of measurement and presentation of
results . 16
9 Neck loops . 16
9.1 Input voltage . 16
9.1.1 Characteristic to be specified. 16
9.1.2 Method of measurement . 16
9.2 Input impedance . 16
9.2.1 Characteristic to be specified. 16
9.2.2 Method of measurement . 16
9.2.3 Recommended values . 16
9.3 Frequency response . 17
9.3.1 Characteristic to be specified. 17
9.3.2 Method of measurement . 17
9.3.3 Presentation of results . 17
9.3.4 Connector . 17

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10 Monitoring devices . 17
10.1 General . 17
10.2 Recommendations for fixed devices . 17
10.3 Recommendations for portable devices . 19
7 Other components . 19
7.1 General . 19
7.2 Characteristics to be specified, methods of measurement and presentation of
results . 19
8 Measurement uncertainty . 19
Annex A (informative) Automatic gain control and compression . 20
Annex B (informative) Loop conductors and some typical impedance values . 21
Annex C (informative) Microphones . 24
Annex D (informative) Neck loops . 25
Annex E (normative) Test jig for measuring the performance of neck loops . 26
Annex F (normative) Specifications for loop listeners and assistive listening devices
(ALDs) . 27
Bibliography . 29

Figure A.1 – Typical steady-state output/input characteristics . 20
Figure E.1 – Side view . 20
Figure E.2 – Front view . 20
Figure E.3 – Back view . 20
Figure E.4 – Arrangement of the neck loop in the jig . 20
Figure F.1 – Target frequency response . 20

Table 1 – Types of fixed monitoring device . 22
Table B.1 – Typical loop characteristics . 22
Table B.2 – Cable characteristics . 23

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS –
AUDIO-FREQUENCY INDUCTION LOOP
SYSTEMS FOR ASSISTED HEARING –

Part 1: Methods of measuring and specifying
the performance of system components

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-
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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
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Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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services carried out by independent certification bodies.
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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 IEC 62489-1 bears the edition number 1.1. It consists of
the first edition (2010-01) [documents 29/667/CDV and 29/668/RVC] and its amendment 1
(2014-12) [documents 29/853/FDIS and 29/860/RVD]. The technical content is identical to
the base edition and its amendment.
In this Redline version, a vertical line in the margin shows where the technical content
is modified by amendment 1. Additions and deletions are displayed in red, with
deletions being struck through. A separate Final version with all changes accepted is
available in this publication.
This publication has been prepared for user convenience.

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International Standard IEC 62489-1 has been prepared by IEC technical committee 29:
Electroacoustics.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
This standard is to be used in conjunction with IEC 60118-4:2006, Electroacoustics – Hearing
aids – Part 4: Induction loop systems for hearing aid purposes – Magnetic field strength.
A list of all the parts in the IEC 62489 series, 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 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.
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.
– 6 – IEC 62489-1:2010
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ELECTROACOUSTICS –
AUDIO-FREQUENCY INDUCTION LOOP
SYSTEMS FOR ASSISTED HEARING –

Part 1: Methods of measuring and specifying
the performance of system components

1 Scope
This part of the IEC 62489 series applies to the components of 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. This standard is intended to encourage accurate and
uniform presentation of manufacturers' specifications, which can be verified by standardized
methods of measurement. It is intended for type testing.
The components considered are the following:
– amplifiers;
– microphones;
– other components, such as playback equipment.
This standard does not deal with safety, for which IEC 60065 applies. It also does not deal
with EMC (Electromagnetic compatibility) and EMF (Electromagnetic fields, in the context of
human exposure).
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 60118-4:2006, Electroacoustics – Hearing aids – Part 4: Induction loop systems for
hearing aid purposes – Magnetic field strength
IEC 60268-1:1985, Sound system equipment – Part 1: General
IEC 60268-2, Sound system equipment – Part 2: Explanation of general terms and calculation
methods
IEC 60268-3:2000 2013, Sound system equipment – Part 3: Amplifiers
IEC 60268-4:2004, Sound system equipment – Part 4: Microphones
IEC 60417, Graphical symbols for use on equipment (available at http://www.graphical-
symbols.info/equipment)
IEC 60603-11, Connectors for frequencies below 3 MHz for use with printed boards – Part 11:
Detail specification for concentric connectors (dimensions for free connectors and fixed
connectors)
IEC 61938, Audio, video and audiovisual systems – Interconnections and matching values –
Preferred matching values of analogue signals

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ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995 )
3 Terms and definitions
For the purposes of this document, the following definition applies.
3.1
useful magnetic field volume
volume within which the system provides hearing-aid users with a signal of acceptable quality
(see 6.7 of IEC 60118-4:2006)
3.2
phased loop array
system of neighbouring loops in which the currents are not in phase with each other
3.3
neck loop
small induction loop intended to be worn around the neck
NOTE See Annex D.
3.4
telecoil
magnetic pickup coil intended to receive signals from an induction-loop system in accordance
with IEC 60118-4
NOTE A telecoil can be part of a hearing aid or of any other device for receiving signals for an induction-loop
system in accordance with IEC 60118-4.
3.5
loop listener
system consisting of a portable amplifier incorporating a telecoil and headphones or
earphones, intended to receive signals from an induction-loop system in accordance with
IEC 60118-4
NOTE The functions of loop listener and assistive listening device (see 3.6) can be combined.
3.6
assistive listening device
ALD
system consisting of a microphone, a portable amplifier and headphones, earphone or a neck
loop
NOTE 1 This device is not the portable receiver described in Annex E of IEC 60118-4:2006, which includes
measurement of magnetic field strength but no compensation for frequency-dependent hearing loss and AGC.
NOTE 2 This note applies to the French language only.
4 Rated values
The term “rated” means the value stated by the manufacturer. Rated values are of two kinds;
rated conditions, fundamental values that can be determined only by the manufacturer, and
others that can be measured. See 5.2.1. For a full explanation, see IEC 60268-2.

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5 Amplifiers
5.1 General
In most systems, the amplifier(s) accepts input from microphone(s) and delivers current to the
induction loop(s). If there are separate preamplifier or mixer and final amplifier components,
the methods described can be used with the interpretation that input signals are applied to the
preamplifier or mixer and measurements of output characteristics are made at the output of
the final amplifier.
The characteristics to be specified are consistent with the lists in Annex C of
IEC 60118-4:2006.
NOTE For characteristics not mentioned in this standard, the provisions of IEC 60268-3 can be applied, with the
provisions of 5.2 of this standard replacing those of 3.1 of IEC 60268-3:2000 2013.
5.2 Rated conditions and standard measuring conditions
5.2.1 Rated conditions
The rated conditions for amplifiers are the following:
• rated power supply voltage;
• rated source impedance;
• rated source e.m.f. (electromotive force);
• rated load;
• rated temperature-limited output current;
• rated maximum time for delivery of rated distortion-limited output current (see below);
• rated total harmonic distortion of the output current;
• rated mechanical and climatic conditions.
NOTE 1 Total harmonic distortion and (distortion-limited) output current are interdependent. Both cannot be taken
as rated conditions simultaneously because normally a given sample amplifier produces less than rated total
harmonic distortion at rated output current.
NOTE 2 If the power supply frequency is critical, it is also a rated condition.
NOTE 3 The rated temperature-limited output current and the rated maximum time for delivery of rated distortion-
limited output current need not be included in published specifications, provided they are supplied on request.
To obtain the correct conditions for measurements, the values for the above-mentioned rated
conditions shall be taken from the manufacturer's specification. These values themselves are
not subject to verification, but they constitute the basis for measuring the other
characteristics.
Methods of measurement for these other characteristics are given in this standard and the
manufacturer is required or permitted to state rated values for these characteristics in the
specification of the equipment. These include
• rated maximum distortion limited output current;
• rated equivalent noise source e.m.f., or a measure of noise performance related to it.
An amplifier, considered as a four-terminal network with regard to a specified pair of input
terminals and a specified pair of output terminals, shall be understood to be working under
rated conditions when the following conditions are fulfilled:
a) the amplifier is connected to its rated power supply;
b) the source e.m.f. is connected in series with the rated source impedance to the input
terminals;
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c) the output terminals are terminated with the rated load;
d) the terminals which are not used during the measurement are terminated, if necessary, as
specified by the manufacturer;
e) the source e.m.f. is a sinusoidal voltage equal to the rated source e.m.f. at 1 000 Hz
according to IEC 60268-1;
f) the volume control, if any, is set to such a position that the rated distortion-limited output
current appears at the output terminals;
g) the tone controls, if any, are set to a specified position to give as flat a frequency
response as possible;
h) other controls, if any, are set to their normal positions as specified by the manufacturer;
i) the climatic conditions given in Clause 8 of IEC 60268-1:1985 are complied with.
Amplifiers for which the rated distortion-limited output current exceeds the rated temperature-
limited output current are likely to be subject to unacceptable effects when operated under
rated conditions for an extended period of time. For these amplifiers, rated conditions shall be
maintained for no longer than can be tolerated by the amplifier.
5.2.2 Standard measurement conditions
Standard measurement conditions are obtained by bringing the amplifier under rated
conditions (see 5.2.1) and then reducing the output current to a level of –10 dB referred to the
rated output current.
NOTE 1 Since testing for temperature rise is carried out according to IEC 60065 under conditions normally
requiring an input source e.m.f. greater than that specified above, it is not expected that amplifier would overheat
under standard measuring conditions.
NOTE 2 Many amplifiers have an output current control that can be adjusted for the above purpose. For an
amplifier without such a control, the source e.m.f. can be adjusted instead.
5.3 Pre-conditioning
Immediately before any measurements are made, the amplifier shall be operated under
standard measuring conditions for at least 10 min. The measurements may be made in any
order.
5.4 Characteristics to be specified, methods of measurement and presentation of
results
NOTE 5.4.1 to 5.4.5 describe rated conditions, which can be determined only by the manufacturer. See 5.2.1.
5.4.1 Rated source impedance
5.4.1.1 Characteristic to be specified
For each input, the internal impedance of the signal source, or range of acceptable
impedances, stated by the manufacturer. This is a rated condition.
NOTE The relevant provisions of IEC 61938 should be applied.
5.4.1.2 Presentation of results
The value or range shall be stated in ohms or a suitable SI multiple, and is assumed to be a
pure resistance unless otherwise stated.
5.4.2 Rated source e.m.f.
5.4.2.1 Characteristic to be specified
For each input, the e.m.f. of the signal source, stated by the manufacturer, which, when
connected through the rated source impedance to the input, produces rated output current

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into the rated load impedance, with all controls set as specified in 5.2.1. This is a rated
condition.
NOTE The signal levels and impedance values given in IEC 61938 should be applied.
5.4.2.2 Presentation of results
The value for each input shall be stated in volts or millivolts, or as a level in decibels referred
to 1 V or 0,775 V. See IEC 60268-1.
5.4.3 Rated load
5.4.3.1 Characteristic to be specified
The load, stated by the manufacturer, to which the amplifier output shall be connected for
measurement purposes. This is a rated condition.
NOTE 1 Example of values for typical loops are given in Annex B.
NOTE 2 It is essential that both the resistor and the inductor do not introduce amplitude distortion, and are able to
carry the output current without overheating or changing in value. The inductor can usually be made as an air-
cored component of acceptable size for use in measurements.
5.4.3.2 Presentation of results
The load shall be stated as a series combination of resistance and inductance.
5.4.4 Rated temperature-limited output current
5.4.4.1 Characteristic to be specified
The maximum output current, stated by the manufacturer, that can be delivered for an
indefinite period to the rated load without unacceptable effects. This is a rated condition.
5.4.4.2 Presentation of results
The value is expressed in amperes.
5.4.5 Rated time for delivery of rated distortion-limited output current
5.4.5.1 Characteristic to be specified
The time, stated by the manufacturer, for which rated distortion-limited output current can be
delivered to the rated load without unacceptable effects.
5.4.5.2 Presentation of results
The value is expressed in seconds or minutes.
NOTE A value of less than 30 s is likely to make some measurements difficult.
5.4.6 Rated total harmonic distortion of the output current
5.4.6.1 Characteristic to be specified
The value of total harmonic distortion, stated by the manufacturer, which is not exceeded
when delivering the rated maximum (distortion-limited) current to the rated load. This is a
rated condition.
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5.4.6.2 Presentation of results
The value shall be stated as a percentage of the total output current or as a level in decibels
referred to that current.
5.4.7 Maximum (distortion-limited) output current
5.4.7.1 Characteristic to be specified
The maximum current, produced by a sinusoidal input signal at 1 kHz, deliverable for at least
10 s into the rated load without exceeding the rated total harmonic distortion (THD).
NOTE The value of 10 s is chosen as long enough to make the measurement, while not so long as to cause
damage through unacceptable effects. See 5.4.4 and 5.4.5.
5.4.7.2 Method of measurement
With the amplifier initially working under standard measuring conditions, the load current is
increased (by adjusting the loop current control, if fitted, or the source e.m.f. if not) until the
total harmonic distortion measured, as specified in IEC 60268-3, across the resistive part of
the load, is equal to the rated value. The current is then calculated from the total voltage
across the resistive part of the load and the resistance value.
NOTE It is usual for neither end of the load resistor to be at signal common potential, so it is necessary to make a
balanced floating connection of the THD meter to the resistor. Alternatively, a current transformer or clip-on current
probe may be used, provided its own harmonic distortion is negligible.
5.4.7.3 Presentation of results
The result shall be stated in amperes.
5.4.8 Compliance voltage
5.4.8.1 Characteristic to be specified
The average, divided by √2, of the maximum positive-going and negative-going peak voltages
that the amplifier can deliver to the rated load when the signal is a specified pink-noise signal.
5.4.8.2 Method of measurement
a) With the amplifier working under standard measuring conditions, the loop current is
increased to achieve the maximum output current as defined in 5.4.5. The value of this
current is noted.
NOTE 1 The measurement should be made as quickly as possible. See 5.4.4 and 5.4.5.
b) An input signal is applied, in accordance with the following specification.
The signal shall be bandwidth limited, with a peak-to-peak voltage (as measured with an
oscilloscope) to true RMS voltage ratio of 18 dB ± 2 dB (crest factor = 4), with a third-
octave-band spectrum flat within ± 1 dB from 100 Hz to 5 kHz.
Bandwidth limitation shall be carried out by means of at least third-order Butterworth high
pass and low pass filters giving −3 dB responses at 75 Hz and 6,5 kHz.
NOTE 2 This specification is given to ensure that the test signal stimulates the system in a manner similar to
normal speech.
NOTE 3 The tolerance of ±1 dB is necessary because the theoretical responses of the specified 3rd order
Butterworth filters are −0,8 dB at 100 Hz and −0,7 dB at 5 kHz, and component tolerances affect the exact
values.
– 12 – IEC 62489-1:2010
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c) The input signal level is adjusted so that automatic gain control (AGC), if fitted, is fully in
operation, or until the RMS output current is 6 dB below the maximum output current,
measured in step a) above. The positive and negative peak voltages across the load are
then measured, over a period of at least 60 s, using a digital oscilloscope (more than
50 000 samples per second or with a bandwidth of more than 25 kHz). The magnitudes of
the positive and negative voltages are then arithmetically averaged.
NOTE 4 If possible, a digital oscilloscope with peak capture should be used. The oscilloscope display should
be examined to detect any undesirable effect, such as a blocking' on peaks of signal.
d) The averaged peak voltage is divided by √2 to derive an equivalent sine-wave r.m.s.
voltage, which is recorded as the result.
NOTE 5 For amplifiers with a low-side current-sense resistor of less than 100 mΩ, the measurement can be
made from the non-earthy output terminal to functional earth, because the magnitude and phase of the
current-sensing signal across the sensing resistor has a negligible effect on the result.
5.4.9 Noise
5.4.9.1 Characteristic to be specified
The equivalent input noise voltage, that is, the 1 kHz sinusoidal input voltage that would
produce the same output current as the amplifier noise produces under the conditions
specified below.
NOTE This characteristic is chosen because it is independent of the gain (transconductance) of the amplifier. To
determine the output current due to noise under a particular condition, multiply the equivalent input noise by the
gain under that condition.
Alternatively, the signal-to-noise ratio, expressed as the ratio in decibels of the rated output
current to the output current due to noise, measured as specified below, may be specified.
5.4.9.2 Method of measurement
With the amplifier working under standard measuring conditions, the source e.m.f. of a 1 kHz
sinusoidal signal is reduced until no AGC action is taking place. If the amplifier has noise
gating or low-level volume expansion, the source e.m.f. is adjusted to a value at which a
change of input level of 2 dB produces a change of level of output current as close to 2 dB as
possible. The source e.m.f., U, and the output current, I, are then measured. The source e.m.f.
I , due to noise measured with A-weighting
is then reduced to zero and the output current,
n
(see IEC 60268-1, for example). The equivalent input noise voltage, U , is then calculated as
n
U = UI /I
n n
The signal-to-noise ratio S in dB is calculated as
2 2
S = 10 lg(I /I ) dB
r n
where I is the rated output current.
r
5.4.9.3 Presentation of results
The equivalent input noise voltage shall be stated in microvolts or as a level in decibels
referred to 1 V or 0,775 V. See IEC 60268-1.
The signal-to-noise ratio shall be stated in decibels, calculated as specified above.

+AMD1:2014 CSV  IEC 2014
5.4.10 Magnetic field strength
5.4.10.1 Characteristic to be specified
The short-time maximum average (averaging time 0,125 s) of the RMS value of the magnetic
field strength, under the following conditions:
• measured with a pick-up coil whose magnetic axis is vertical,
• produced by the system at one point, at least, within the useful magnetic field volume
(see 3.1).
NOTE The methods of measurement given below are based on the use of an amplifier which has a 'loop drive'
gain control following an AGC stage, and are intended only to show that the amplifier is capable of producing the
required magnetic field strength. If such a control is not provided, the manufacturer's instructions apply. In order to
determine that the whole system is capable of producing the required magnetic field strength from the
microphone(s) and any other signal source(s), the procedure described in 6.4 of IEC 60118-4:2006 is necessary.
5.4.10.2 Method of measurement with a sinusoidal signal
Connect the amplifier to a horizontal square loop whose dimensions and conductor size give
the same resistance and inductance as the rated load. Apply a 1 kHz sinusoidal signal to the
amplifier and adjust its controls in accordance with the manufacturer's instructions. Measure
the loop current (using a clamp ammeter or other suitable method) to ensure that it does not
exceed the rated maximum loop current. For the purposes of this standard, the magnetic field
strength shall be measured at a height of 1,4 m above the centre of the loop plane.
NOTE 1 It is necessary to specify a single value for the height above the loop plane, and 1,4 m is seen as a
suitable median value.
NOTE 2 It is essential that the results are not affected by electrically-conducting (not only magnetic) material in
the neighbourhood of the loop.
NOTE 3 The manufacturer is free to specify a maximum duration of the test, which is long enough to perform the
measurement but does not result in excessive temperature rise in the amplifier.
5.4.10.3 Measurement with other test signals
The manufacturer shall specify the value obtained:
• under the test conditions specified in 5.4.10.1;
• with the specified test signal, fully activating the AGC circuit when the amplifier is set
up to produce, from a sinusoidal test signal also fully activating the AGC circuit, a field
strength of 400 mA/m, measured with a true RMS meter with 0,125 s averaging time.
The procedure described in 5.4.10.2 applies, but using the specified test signal.
5.4.10.4 Presentation of results
The field strength shall be stated in milliamperes per metre.
5.4.11 Loop dimensions
5.4.11.1 Characteristics to be specified
The linear dimensions of a square loop, and a loop of aspect ratio 3:1, for which the magnetic
field strength, measured as specified in 5.4.10.1, is 400 mA/m.
NOTE 1 It is undesirable to specify an area, because the shape of the area is important. See Annex A.
NOTE 2 The electrical characteristics of typical loops of various dimensions are given in Annex B.

– 14 – IEC 62489-1:2010
+AMD1:2014 CSV  IEC 2014
5.4.11.2 Method of measurement
Proceed as in 5.4.10.2, using a loop whose linear dimensions are those specified by the
manufacturer. Measure the magnetic field strength.
5.4.11.3 Presentation of results
The results shall be stated as linear dimensions in metres.
5.4.12 Frequency response
5.4.12.1 Characteristic to be specified
The variation of output current into the rated load with frequency.
5.4.12.2 Method of measurement
For an amplifier without AGC or compression, or if AGC and compression can be, and is,
disabled, set it up under standard measuring conditions. Then change the signal frequency in
turn to one-third-octave band centre frequencies over the range of at least 50 Hz to 8 kHz,
and measure the load current at each frequency.
For an amplifier with AGC or compression, follow the manufacturer's instructions on the input
source e.m.f. to be used.
5.4.12.3 Presentation of results
The results shall be presented as a graph, with the response at 1 kHz taken as 0 dB.
5.4.13 Automatic gain control and compression
5.4.13.1 Characteristic to be specified
The steady-state relationship between output current and input source e.m.f. See Annex A.
5.4.13.2 Method of measurement
Set up the amplifier under standard measuring conditions (see 5.2.2), and then reduce the
input source e.m.f. to zero. Activate (if necessary) the automatic gain control or compression
feature and then increase the input source
...

IEC 62489-1 ®
Edition 1.2 2017-11
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Electroacoustics – Audio-frequency induction loop systems for assisted
hearing –
Part 1: Methods of measuring and specifying the performance of system
components
Électroacoustique – Systèmes de boucles d’induction audiofréquences pour
améliorer l'audition –
Partie 1: Méthodes de mesure et de spécification des performances des
composants de systèmes
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IEC 62489-1 ®
Edition 1.2 2017-11
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Electroacoustics – Audio-frequency induction loop systems for assisted

hearing –
Part 1: Methods of measuring and specifying the performance of system

components
Électroacoustique – Systèmes de boucles d’induction audiofréquences pour

améliorer l'audition –
Partie 1: Méthodes de mesure et de spécification des performances des

composants de systèmes
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.140.50 ISBN 978-2-8322-5103-4

IEC 62489-1 ®
Edition 1.2 2017-11
CONSOLIDATED VERSION
REDLINE VERSION
VERSION REDLINE
colour
inside
Electroacoustics – Audio-frequency induction loop systems for assisted
hearing –
Part 1: Methods of measuring and specifying the performance of system
components
Électroacoustique – Systèmes de boucles d’induction audiofréquences pour
améliorer l'audition –
Partie 1: Méthodes de mesure et de spécification des performances des
composants de systèmes
– 2 – IEC 62489-1:2010+AMD1:2014
+AMD2:2017 CSV  IEC 2017
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Rated values . 7
5 Amplifiers . 8
5.1 General . 8
5.2 Rated conditions and standard measuring conditions . 8
5.2.1 Rated conditions . 8
5.2.2 Standard measurement conditions . 9
5.3 Pre-conditioning . 9
5.4 Characteristics to be specified, methods of measurement and presentation of
results . 9
5.4.1 Rated source impedance . 9
5.4.2 Rated source e.m.f. . 9
5.4.3 Rated load . 10
5.4.4 Rated temperature-limited output current . 10
5.4.5 Rated time for delivery of rated distortion-limited output current . 10
5.4.6 Rated total harmonic distortion of the output current . 10
5.4.7 Maximum (distortion-limited) output current . 11
5.4.8 Compliance voltage . 11
5.4.9 Noise . 12
5.4.10 Magnetic field strength . 13
5.4.11 Loop dimensions . 13
5.4.12 Frequency response . 14
5.4.13 Automatic gain control and compression . 14
5.4.14 Phase error of quadrature networks for phased loop arrays . 15
5.4.15 Installation . 15
5.4.16 Physical characteristics . 15
6 Microphones . 16
6.1 General . 16
6.2 Characteristics to be specified, methods of measurement and presentation of
results . 16
7 Other components . 19
7.1 General . 19
7.2 Characteristics to be specified, methods of measurement and presentation of
results . 19
8 Measurement uncertainty . 19
9 Neck loops . 16
9.1 Input voltage . 16
9.1.1 Characteristic to be specified. 16
9.1.2 Method of measurement . 16
9.2 Input impedance . 16
9.2.1 Characteristic to be specified. 16
9.2.2 Method of measurement . 16
9.2.3 Recommended values . 16

+AMD2:2017 CSV  IEC 2017
9.3 Frequency response . 17
9.3.1 Characteristic to be specified. 17
9.3.2 Method of measurement . 17
9.3.3 Presentation of results . 17
9.3.4 Connector . 17
10 Monitoring devices . 17
10.1 General . 17
10.2 Recommendations for fixed devices . 17
10.3 Recommendations for portable devices . 19
Annex A (informative) Automatic gain control and compression . 20
Annex B (informative) Loop conductors and some typical impedance values . 21
Annex C (informative) Microphones . 24
Annex D (normative) Neck loops . 25
Annex E (normative) Test jig for measuring the performance of neck loops . 27
Annex F (normative) Specifications for loop listeners and assistive listening
devices (ALDs) . 28
Bibliography . 30

Figure A.1 – Typical steady-state output/input characteristics . 20
Figure E.1 – Side view . 27
Figure E.2 – Front view . 27
Figure E.3 – Back view . 27
Figure E.4 – Arrangement of the neck loop on the jig . 27
Figure F.1 – Target frequency response . 28

Table 1 – Types of fixed monitoring device . 18
Table B.1 – Typical loop characteristics . 22
Table B.2 – Cable characteristics . 23
Table D.1 – Examples of performance of type 1 neck loop design . 25

– 4 – IEC 62489-1:2010+AMD1:2014
+AMD2:2017 CSV  IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS –
AUDIO-FREQUENCY INDUCTION LOOP
SYSTEMS FOR ASSISTED HEARING –
Part 1: Methods of measuring and specifying
the performance of system components
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-
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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 62489-1 edition 1.2 contains the first edition (2010-01) [documents 29/667/CDV and 29/668/
RVC], its amendment 1 (2014-12) [documents 29/853/FDIS and 29/860/RVD] and its amendment
2 (2017-11) [documents 29/955/CDV and 29/963/RVC].
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.

+AMD2:2017 CSV  IEC 2017
International Standard IEC 62489-1 has been prepared by IEC technical committee 29:
Electroacoustics.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
This standard is to be used in conjunction with IEC 60118-4:2006, Electroacoustics – Hearing
aids – Part 4: Induction loop systems for hearing aid purposes – Magnetic field strength.
A list of all the parts in the IEC 62489 series, 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 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.
– 6 – IEC 62489-1:2010+AMD1:2014
+AMD2:2017 CSV  IEC 2017
ELECTROACOUSTICS –
AUDIO-FREQUENCY INDUCTION LOOP
SYSTEMS FOR ASSISTED HEARING –

Part 1: Methods of measuring and specifying
the performance of system components

1 Scope
This part of the IEC 62489 series applies to the components of 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. This standard is intended to encourage accurate and
uniform presentation of manufacturers' specifications, which can be verified by standardized
methods of measurement. It is intended for type testing.
The components considered are the following:
– amplifiers;
– microphones;
– other components, such as playback equipment.
This standard does not deal with safety, for which IEC 60065 applies. It also does not deal
with EMC (Electromagnetic compatibility) and EMF (Electromagnetic fields, in the context of
human exposure).
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 60118-4:2006 2014, Electroacoustics – Hearing aids – Part 4: Induction loop systems for
hearing aid purposes – Magnetic field strength System performance requirements
IEC 60268-1:1985, Sound system equipment – Part 1: General
IEC 60268-2:1987, Sound system equipment – Part 2: Explanation of general terms and
calculation methods
IEC 60268-3:2000 2013, Sound system equipment – Part 3: Amplifiers
IEC 60268-4:2004 2014, Sound system equipment – Part 4: Microphones
IEC 60417, Graphical symbols for use on equipment (available at http://www.graphical-
symbols.info/equipment)
IEC 60603-11:1992, Connectors for frequencies below 3 MHz for use with printed boards –
Part 11: Detail specification for concentric connectors (dimensions for free connectors and
fixed connectors)
+AMD2:2017 CSV  IEC 2017
IEC 61938:2013, Audio, video and audiovisual Multimedia systems – Interconnections and
matching values – Preferred matching values of analogue signals Guide to the recommended
characteristics of analogue interfaces to achieve interoperability
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995 )
3 Terms and definitions
For the purposes of this document, the following definition applies.
3.1
useful magnetic field volume
volume within which the system provides hearing-aid users with a signal of acceptable quality
(see 6.7 of IEC 60118-4:2006 2014)
3.2
phased loop array
system of neighbouring loops in which the currents are not in phase with each other
3.3
neck loop
small induction loop intended to be worn around the neck
NOTE See Annex D.
3.4
telecoil
magnetic pickup coil intended to receive signals from an induction-loop system in accordance
with IEC 60118-4
NOTE A telecoil can be part of a hearing aid or of any other device for receiving signals for an induction-loop
system in accordance with IEC 60118-4.
3.5
loop listener
system consisting of a portable amplifier incorporating a telecoil and headphones or
earphones, intended to receive signals from an induction-loop system in accordance with
IEC 60118-4
NOTE The functions of loop listener and assistive listening device (see 3.6) can be combined.
3.6
assistive listening device
ALD
system consisting of a microphone, a portable amplifier and headphones, earphone or a neck
loop
NOTE 1 This device is not the portable receiver described in Annex E of IEC 60118-4:2014, which includes
measurement of magnetic field strength but no compensation for frequency-dependent hearing loss and AGC.
NOTE 2 This note applies to the French language only.
4 Rated values
The term “rated” means the value stated by the manufacturer. Rated values are of two kinds;
rated conditions, fundamental values that can be determined only by the manufacturer, and
others that can be measured. See 5.2.1. For a full explanation, see IEC 60268-2.

– 8 – IEC 62489-1:2010+AMD1:2014
+AMD2:2017 CSV  IEC 2017
5 Amplifiers
5.1 General
In most systems, the amplifier(s) accepts input from microphone(s) and delivers current to the
induction loop(s). If there are separate preamplifier or mixer and final amplifier components,
the methods described can be used with the interpretation that input signals are applied to the
preamplifier or mixer and measurements of output characteristics are made at the output of
the final amplifier.
The characteristics to be specified are consistent with the lists in Annex C of
IEC 60118-4:2006 2014.
NOTE For characteristics not mentioned in this standard, the provisions of IEC 60268-3 can be applied, with the
provisions of 5.2 of this standard replacing those of 3.1 of IEC 60268-3:2000 2013.
5.2 Rated conditions and standard measuring conditions
5.2.1 Rated conditions
The rated conditions for amplifiers are the following:
• rated power supply voltage;
• rated source impedance;
• rated source e.m.f. (electromotive force);
• rated load;
• rated temperature-limited output current;
• rated maximum time for delivery of rated distortion-limited output current (see below);
• rated total harmonic distortion of the output current;
• rated mechanical and climatic conditions.
NOTE 1 Total harmonic distortion and (distortion-limited) output current are interdependent. Both cannot be taken
as rated conditions simultaneously because normally a given sample amplifier produces less than rated total
harmonic distortion at rated output current.
NOTE 2 If the power supply frequency is critical, it is also a rated condition.
NOTE 3 The rated temperature-limited output current and the rated maximum time for delivery of rated distortion-
limited output current need not be included in published specifications, provided they are supplied on request.
To obtain the correct conditions for measurements, the values for the above-mentioned rated
conditions shall be taken from the manufacturer's specification. These values themselves are
not subject to verification, but they constitute the basis for measuring the other
characteristics.
Methods of measurement for these other characteristics are given in this standard and the
manufacturer is required or permitted to state rated values for these characteristics in the
specification of the equipment. These include
• rated maximum distortion limited output current;
• rated equivalent noise source e.m.f., or a measure of noise performance related to it.
An amplifier, considered as a four-terminal network with regard to a specified pair of input
terminals and a specified pair of output terminals, shall be understood to be working under
rated conditions when the following conditions are fulfilled:
a) the amplifier is connected to its rated power supply;
b) the source e.m.f. is connected in series with the rated source impedance to the input
terminals;
+AMD2:2017 CSV  IEC 2017
c) the output terminals are terminated with the rated load;
d) the terminals which are not used during the measurement are terminated, if necessary, as
specified by the manufacturer;
e) the source e.m.f. is a sinusoidal voltage equal to the rated source e.m.f. at 1 000 Hz
according to IEC 60268-1;
f) the volume control, if any, is set to such a position that the rated distortion-limited output
current appears at the output terminals;
g) the tone controls, if any, are set to a specified position to give as flat a frequency
response as possible;
h) other controls, if any, are set to their normal positions as specified by the manufacturer;
i) the climatic conditions given in Clause 8 of IEC 60268-1:1985 are complied with.
Amplifiers for which the rated distortion-limited output current exceeds the rated temperature-
limited output current are likely to be subject to unacceptable effects when operated under
rated conditions for an extended period of time. For these amplifiers, rated conditions shall be
maintained for no longer than can be tolerated by the amplifier.
5.2.2 Standard measurement conditions
Standard measurement conditions are obtained by bringing the amplifier under rated
conditions (see 5.2.1) and then reducing the output current to a level of –10 dB referred to the
rated output current.
NOTE 1 Since testing for temperature rise is carried out according to IEC 60065 under conditions normally
requiring an input source e.m.f. greater than that specified above, it is not expected that amplifier would overheat
under standard measuring conditions.
NOTE 2 Many amplifiers have an output current control that can be adjusted for the above purpose. For an
amplifier without such a control, the source e.m.f. can be adjusted instead.
5.3 Pre-conditioning
Immediately before any measurements are made, the amplifier shall be operated under
standard measuring conditions for at least 10 min. The measurements may be made in any
order.
5.4 Characteristics to be specified, methods of measurement and presentation of
results
NOTE 5.4.1 to 5.4.5 describe rated conditions, which can be determined only by the manufacturer. See 5.2.1.
5.4.1 Rated source impedance
5.4.1.1 Characteristic to be specified
For each input, the internal impedance of the signal source, or range of acceptable
impedances, stated by the manufacturer. This is a rated condition.
NOTE The relevant provisions of IEC 61938:2013 should be applied.
5.4.1.2 Presentation of results
The value or range shall be stated in ohms or a suitable SI multiple, and is assumed to be a
pure resistance unless otherwise stated.
5.4.2 Rated source e.m.f.
5.4.2.1 Characteristic to be specified
For each input, the e.m.f. of the signal source, stated by the manufacturer, which, when
connected through the rated source impedance to the input, produces rated output current

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into the rated load impedance, with all controls set as specified in 5.2.1. This is a rated
condition.
NOTE The signal levels and impedance values given in IEC 61938:2013 should be applied.
5.4.2.2 Presentation of results
The value for each input shall be stated in volts or millivolts, or as a level in decibels referred
to 1 V or 0,775 V. See IEC 60268-1.
5.4.3 Rated load
5.4.3.1 Characteristic to be specified
The load, stated by the manufacturer, to which the amplifier output shall be connected for
measurement purposes. This is a rated condition.
NOTE 1 Example of values for typical loops are given in Annex B.
NOTE 2 It is essential that both the resistor and the inductor do not introduce amplitude distortion, and are able to
carry the output current without overheating or changing in value. The inductor can usually be made as an air-
cored component of acceptable size for use in measurements.
5.4.3.2 Presentation of results
The load shall be stated as a series combination of resistance and inductance.
5.4.4 Rated temperature-limited output current
5.4.4.1 Characteristic to be specified
The maximum output current, stated by the manufacturer, that can be delivered for an
indefinite period to the rated load without unacceptable effects. This is a rated condition.
5.4.4.2 Presentation of results
The value is expressed in amperes.
5.4.5 Rated time for delivery of rated distortion-limited output current
5.4.5.1 Characteristic to be specified
The time, stated by the manufacturer, for which rated distortion-limited output current can be
delivered to the rated load without unacceptable effects.
5.4.5.2 Presentation of results
The value is expressed in seconds or minutes.
NOTE A value of less than 30 s is likely to make some measurements difficult.
5.4.6 Rated total harmonic distortion of the output current
5.4.6.1 Characteristic to be specified
The value of total harmonic distortion, stated by the manufacturer, which is not exceeded
when delivering the rated maximum (distortion-limited) current to the rated load. This is a
rated condition.
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5.4.6.2 Presentation of results
The value shall be stated as a percentage of the total output current or as a level in decibels
referred to that current.
5.4.7 Maximum (distortion-limited) output current
5.4.7.1 Characteristic to be specified
The maximum current, produced by a sinusoidal input signal at 1 kHz, deliverable for at least
10 s into the rated load without exceeding the rated total harmonic distortion (THD).
NOTE The value of 10 s is chosen as long enough to make the measurement, while not so long as to cause
damage through unacceptable effects. See 5.4.4 and 5.4.5.
5.4.7.2 Method of measurement
With the amplifier initially working under standard measuring conditions, the load current is
increased (by adjusting the loop current control, if fitted, or the source e.m.f. if not) until the
total harmonic distortion measured, as specified in IEC 60268-3, across the resistive part of
the load, is equal to the rated value. The current is then calculated from the total voltage
across the resistive part of the load and the resistance value.
NOTE It is usual for neither end of the load resistor to be at signal common potential, so it is necessary to make a
balanced floating connection of the THD meter to the resistor. Alternatively, a current transformer or clip-on current
probe may be used, provided its own harmonic distortion is negligible.
5.4.7.3 Presentation of results
The result shall be stated in amperes.
5.4.8 Compliance voltage
5.4.8.1 Characteristic to be specified
The average, divided by √2, of the maximum positive-going and negative-going peak voltages
that the amplifier can deliver to the rated load when the signal is a specified pink-noise signal.
5.4.8.2 Method of measurement
a) With the amplifier working under standard measuring conditions, the loop current is
increased to achieve the maximum output current as defined in 5.4.5. The value of this
current is noted.
NOTE 1 The measurement should be made as quickly as possible. See 5.4.4 and 5.4.5.
b) An input signal is applied, in accordance with the following specification.
The signal shall be bandwidth limited, with a peak-to-peak voltage (as measured with an
oscilloscope) to true RMS voltage ratio of 18 dB ± 2 dB (crest factor = 4), with a third-
octave-band spectrum flat within ± 1 dB from 100 Hz to 5 kHz.
Bandwidth limitation shall be carried out by means of at least third-order Butterworth high
pass and low pass filters giving −3 dB responses at 75 Hz and 6,5 kHz.
NOTE 2 This specification is given to ensure that the test signal stimulates the system in a manner similar to
normal speech.
NOTE 3 The tolerance of ±1 dB is necessary because the theoretical responses of the specified 3rd order
Butterworth filters are −0,8 dB at 100 Hz and −0,7 dB at 5 kHz, and component tolerances affect the exact
values.
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c) The input signal level is adjusted so that automatic gain control (AGC), if fitted, is fully in
operation, or until the RMS output current is 6 dB below the maximum output current,
measured in step a) above. The positive and negative peak voltages across the load are
then measured, over a period of at least 60 s, using a digital oscilloscope (more than
50 000 samples per second or with a bandwidth of more than 25 kHz). The magnitudes of
the positive and negative voltages are then arithmetically averaged.
NOTE 4 If possible, a digital oscilloscope with peak capture should be used. The oscilloscope display should
be examined to detect any undesirable effect, such as a blocking' on peaks of signal.
d) The averaged peak voltage is divided by √2 to derive an equivalent sine-wave r.m.s.
voltage, which is recorded as the result.
NOTE 5 For amplifiers with a low-side current-sense resistor of less than 100 mΩ, the measurement can be
made from the non-earthy output terminal to functional earth, because the magnitude and phase of the
current-sensing signal across the sensing resistor has a negligible effect on the result.
5.4.9 Noise
5.4.9.1 Characteristic to be specified
The equivalent input noise voltage, that is, the 1 kHz sinusoidal input voltage that would
produce the same output current as the amplifier noise produces under the conditions
specified below.
NOTE This characteristic is chosen because it is independent of the gain (transconductance) of the amplifier. To
determine the output current due to noise under a particular condition, multiply the equivalent input noise by the
gain under that condition.
Alternatively, the signal-to-noise ratio, expressed as the ratio in decibels of the rated output
current to the output current due to noise, measured as specified below, may be specified.
5.4.9.2 Method of measurement
With the amplifier working under standard measuring conditions, the source e.m.f. of a 1 kHz
sinusoidal signal is reduced until no AGC action is taking place. If the amplifier has noise
gating or low-level volume expansion, the source e.m.f. is adjusted to a value at which a
change of input level of 2 dB produces a change of level of output current as close to 2 dB as
possible. The source e.m.f., U, and the output current, I, are then measured. The source e.m.f.
I , due to noise measured with A-weighting
is then reduced to zero and the output current,
n
(see IEC 60268-1, for example). The equivalent input noise voltage, U , is then calculated as
n
U = UI /I
n n
The signal-to-noise ratio S in dB is calculated as
2 2
S = 10 lg(I /I ) dB
r n
where I is the rated output current.
r
5.4.9.3 Presentation of results
The equivalent input noise voltage shall be stated in microvolts or as a level in decibels
referred to 1 V or 0,775 V. See IEC 60268-1.
The signal-to-noise ratio shall be stated in decibels, calculated as specified above.

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5.4.10 Magnetic field strength
5.4.10.1 Characteristic to be specified
The short-time maximum average (averaging time 0,125 s) of the RMS value of the magnetic
field strength, under the following conditions:
• measured with a pick-up coil whose magnetic axis is vertical,
• produced by the system at one point, at least, within the useful magnetic field volume
(see 3.1).
NOTE The methods of measurement given below are based on the use of an amplifier which has a 'loop drive'
gain control following an AGC stage, and are intended only to show that the amplifier is capable of producing the
required magnetic field strength. If such a control is not provided, the manufacturer's instructions apply. In order to
determine that the whole system is capable of producing the required magnetic field strength from the
microphone(s) and any other signal source(s), the procedure described in 6.4 Clause 10 of IEC 60118-4:2006 2014
is necessary.
5.4.10.2 Method of measurement with a sinusoidal signal
Connect the amplifier to a horizontal square loop whose dimensions and conductor size give
the same resistance and inductance as the rated load. Apply a 1 kHz sinusoidal signal to the
amplifier and adjust its controls in accordance with the manufacturer's instructions. Measure
the loop current (using a clamp ammeter or other suitable method) to ensure that it does not
exceed the rated maximum loop current. For the purposes of this standard, the magnetic field
strength shall be measured at a height of 1,4 m above the centre of the loop plane.
NOTE 1 It is necessary to specify a single value for the height above the loop plane, and 1,4 m is seen as a
suitable median value.
NOTE 2 It is essential that the results are not affected by electrically-conducting (not only magnetic) material in
the neighbourhood of the loop.
NOTE 3 The manufacturer is free to specify a maximum duration of the test, which is long enough to perform the
measurement but does not result in excessive temperature rise in the amplifier.
5.4.10.3 Measurement with other test signals
The manufacturer shall specify the value obtained:
• under the test conditions specified in 5.4.10.1;
• with the specified test signal, fully activating the AGC circuit when the amplifier is set
up to produce, from a sinusoidal test signal also fully activating the AGC circuit, a field
strength of 400 mA/m, measured with a true RMS meter with 0,125 s averaging time.
The procedure described in 5.4.10.2 applies, but using the specified test signal.
5.4.10.4 Presentation of results
The field strength shall be stated in milliamperes per metre.
5.4.11 Loop dimensions
5.4.11.1 Characteristics to be specified
The linear dimensions of a square loop, and a loop of aspect ratio 3:1, for which the magnetic
field strength, measured as specified in 5.4.10.1, is 400 mA/m.
NOTE 1 It is undesirable to specify an area, because the shape of the area is important. See Annex A.
NOTE 2 The electrical characteristics of typical loops of various dimensions are given in Annex B.

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5.4.11.2 Method of measurement
Proceed as in 5.4.10.2, using a loop whose linear dimensions are those specified by the
manufacturer. Measure the magnetic field strength.
5.4.11.3 Presentation of results
The results shall be stated as linear dimensions in metres.
5.4.12 Frequency response
5.4.12.1 Characteristic to be specified
The variation of output current into the rated load with frequency.
5.4.12.2 Method of measurement
For an amplifier without AGC or compression, or if AGC and compression can be, and is,
disabled, set it up under standard measuring conditions. Then change the signal frequency in
turn to one-third-octave band centre frequencies over the range of at least 50 Hz to 8 kHz,
and measure the load current at each frequency.
For an amplifier with AGC or compression, follow the manufacturer's instructions on the input
source e.m.f. to be used.
5.4.12.3 Presentation of results
The results shall be presented as a graph, with the response at 1 kHz taken as 0 dB.
5.4.13 Automatic gain contro
...