Electroacoustics - Hearing aids - Part 4: Induction-loop systems for hearing aid purposes - System performance requirements

IEC 60118-4:2014 is applicable to audio-frequency induction-loop systems producing an alternating magnetic field at audio frequencies and intended to provide an input signal for hearing aids operating with an induction pick-up coil (telecoil). Throughout this standard, it is assumed that the hearing aids used with it conform to all relevant parts of IEC 60118. This standard specifies requirements for the field strength in audio-frequency induction loops for hearing aid purposes, which will give adequate signal-to-noise ratio without overloading the hearing aid. The standard also specifies the minimum frequency response requirements for acceptable intelligibility. Methods for measuring the magnetic field strength are specified, and information is given on appropriate measuring equipment (see Annex B), information that should be provided to the operator and users of the system (see Annex C), and other important considerations. This standard does not specify requirements for loop driver amplifiers or associated microphone or audio signal sources, which are dealt with in IEC 62489-1, or for the field strength produced by equipment, such as telephone handsets, within the scope of ITU-T P.370. This third edition cancels and replaces the second edition published in 2006. This edition constitutes a technical revision which includes the following significant technical changes with respect to the previous edition: Addition of Annexes G, H and I where more information is provided about practical considerations and methods of measurement.

Akustik - Hörgeräte - Teil 4: Induktionsschleifen für Hörgeräte - Leistungsanforderungen

Électroacoustique - Appareils de correction auditive - Partie 4: Systèmes de boucles d'induction utilisées à des fins de correction auditive - Exigences de performances système

L'IEC 60118-4:2014 s'applique aux systèmes de boucles d'induction audiofréquences qui produisent un champ magnétique alternatif à fréquence acoustique et destinés à produire un signal d'entrée aux appareils de correction auditive en exploitation avec une bobine d'induction captrice. Dans toute la présente norme, on suppose que les appareils de correction auditive utilisés sont conformes à toutes les parties appropriées de l'IEC 60118. Cette norme spécifie des exigences concernant l'intensité du champ dans les boucles d'induction utilisées à des fins de correction auditive, qui correspond à un rapport signal sur bruit convenable sans surcharge de l'appareil de correction auditive. La norme spécifie également des exigences concernant la réponse en fréquence minimale pour une intelligibilité acceptable. Les méthodes de mesure de l'intensité du champ magnétique et des renseignements sur l'équipement de mesure approprié (voir Annexe B), ainsi que les renseignements qu'il convient de fournir aux opérateurs et aux utilisateurs du système (voir Annexe C) sont donnés, avec d'autres considérations importantes. Par contre, la présente norme ne spécifie pas d'exigences concernant les amplificateurs qui alimentent la boucle, le microphone associé, les sources du signal acoustique, qui sont décrits dans l'IEC 62489-1, ou l'intensité du champ produit par des appareillages tels que les appareils téléphoniques, dans le domaine d'application de la norme UIT-T P370. Cette troisième édition annule et remplace la deuxième édition parue en 2006. L'édition de cette partie constitue une révision technique qui inclut les modifications techniques majeures suivantes par rapport à l'édition précédente: ajout des Annexes G, H et I qui apportent plus d'information concernant des considérations pratiques et les méthodes de mesure.

Elektroakustika - Slušni pripomočki - 4. del: Sistemi z indukcijsko zanko za slušne pripomočke - Zahteve sistema

Ta del standarda IEC 60118 velja za sisteme z avdio-frekvenčno indukcijsko zanko, ki proizvajajo izmenično magnetno polje pri zvočnih frekvencah in ki so namenjeni proizvajanju vhodnih signalov za slušne pripomočke s tuljavo, ki sprejema indukcijske signale (teletuljava). Standard predvideva, da so slušni aparati, s katerimi se uporablja, v skladu z vsemi ustreznimi deli standarda IEC 60118. Ta standard določa zahteve za poljsko jakost v avdio-frekvenčnih indukcijskih zankah za namene slušnih pripomočkov, ki bo omogočala ustrezno razmerje med signalom in šumom, ne da bi prišlo do preobremenitve slušnega pripomočka. Standard prav tako določa minimalne zahteve za frekvenčni odziv za sprejemljivo razumljivost. Določene so metode za merjenje jakosti magnetnega polja in podane so informacije o primerni merilni opremi (glejte Dodatek B), informacije, namenjene operaterju in uporabnikom sistema (glejte Dodatek C) ter drugi pomembni vidiki. Ta standard ne določa zahtev za ojačevalnike z zančnim gonilnikom ali sorodne mikrofone ali vire zvočnih signalov, ki so zajeti v standardu IEC 62489-1, ali za jakost polja, ki jo ustvari oprema, kot na primer telefonska slušalka, zajeta v standardu ITU-T P.370.

General Information

Status
Published
Publication Date
12-Feb-2015
Technical Committee
Drafting Committee
Current Stage
6060 - Document made available
Due Date
13-Feb-2015
Completion Date
13-Feb-2015

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Akustik - Hörgeräte - Teil 4: Induktionsschleifen für Hörgeräte - LeistungsanforderungenElectroacoustique - Appareils de correction auditive - Partie 4: Systèmes de boucles d'induction utilisées à des fins - Exigences de performances systèmeElectroacoustics - Hearing aids - Part 4: Induction loop systems for hearing aid purposes - System performance requirements17.140.50ElektroakustikaElectroacoustics11.180.15Aids for deaf and hearing impaired peopleICS:Ta slovenski standard je istoveten z:EN 60118-4:2015SIST EN 60118-4:2015en01-september-2015SIST EN 60118-4:2015SLOVENSKI

STANDARDSIST EN 60118-4:20081DGRPHãþD
SIST EN 60118-4:2015
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 60118-4
February 2015 ICS 17.140.50
Supersedes
EN 60118-4:2006
English Version
Electroacoustics - Hearing aids -

Part 4: Induction-loop systems for hearing aid purposes - System performance requirements (IEC 60118-4:2014)

Électroacoustique - Appareils de correction auditive -

Partie 4: Systèmes de boucles d'induction utilisées à des fins de correction auditive - Exigences de performances système (IEC 60118-4:2014)

Akustik - Hörgeräte -

Teil 4: Induktionsschleifen für Hörgeräte - Leistungsanforderungen (IEC 60118-4:2014) This European Standard was approved by CENELEC on 2015-01-15. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions. CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom. European Committee for Electrotechnical Standardization

Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung CEN-CENELEC Management Centre: Avenue Marnix 17,

B-1000 Brussels © 2015 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.

Ref. No. EN 60118-4:2015 E SIST EN 60118-4:2015
EN 60118-4:2015 - 2 -

Foreword The text of document 29/855/FDIS, future edition 3 of IEC 60118-4, prepared by IEC TC 29, Electroacoustics, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as EN 60118-4:2015. The following dates are fixed: • latest date by which the document has to be implemented at national level by publication of an identical national standard or by endorsement (dop) 2015-10-15 • latest date by which the national standards conflicting with the document have to be withdrawn (dow) 2018-01-15

This document supersedes EN 60118-4:2006.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent rights.

Endorsement notice The text of the International Standard IEC 60118-4:2014 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following notes have to be added for the standards indicated:

IEC 61938 NOTE Harmonised as
EN 61938. IEC 61260-1 NOTE Harmonised as
EN 61260-1.
SIST EN 60118-4:2015
- 3 - EN 60118-4:2015 Annex ZA
(normative)

Normative references to international publications with their corresponding European publications

The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

NOTE 1 When an International Publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.

NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available here: www.cenelec.eu.

Publication Year Title EN/HD Year IEC 60268-3 2013
Sound system equipment --
Part 3: Amplifiers EN 60268-3 2013
IEC 60268-10 1991
Sound system equipment --
Part 10: Peak programme level meters HD 483.10 S1 1993
IEC 61672-1 2013
Electroacoustics - Sound level meters --
Part 1: Specifications EN 61672-1 2013
IEC 62489-1 2010

Electroacoustics - Audio frequency induction loop systems for assisted hearing --

Part 1: Methods of measuring and specifying the performance of system components EN 62489-1 2010

SIST EN 60118-4:2015
SIST EN 60118-4:2015

IEC 60118-4 Edition 3.0 2014-12 INTERNATIONAL STANDARD NORME INTERNATIONALE Electroacoustics – Hearing aids –

Part 4: Induction-loop systems for hearing aid purposes – System performance requirements

Électroacoustique – Appareils de correction auditive –

Partie 4: Systèmes de boucles d'induction utilisées à des fins de correction auditive – Exigences de performances système

INTERNATIONAL ELECTROTECHNICAL COMMISSION COMMISSION ELECTROTECHNIQUE INTERNATIONALE XA ICS 17.140.50 PRICE CODE CODE PRIX ISBN 978-2-8322-1984-3

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale ®

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé. SIST EN 60118-4:2015 colourinside

– 2 – IEC 60118-4:2014 © IEC 2014 CONTENTS FOREWORD ........................................................................................................................... 6 INTRODUCTION ..................................................................................................................... 8 1 Scope .............................................................................................................................. 9 2 Normative references ...................................................................................................... 9 3 Terms and definitions ...................................................................................................... 9 4 General ......................................................................................................................... 10 4.1 Procedure for setting up and commissioning an audio-frequency induction loop system .......................................................................................................... 10 4.2 Suitability of the site for the installation of an audio-frequency induction-loop system .................................................................................................................. 10 4.3 Relation of the magnetic field strength level at the telecoil to the sound pressure level at the microphone. ......................................................................... 11 5 Using components of a sound system in an induction-loop system ................................ 11 5.1 General ................................................................................................................. 11 5.2 Microphones ......................................................................................................... 11 5.3 Mixer .................................................................................................................... 11 5.4 Power amplifier ..................................................................................................... 11 6 Meters and test signals .................................................................................................. 11 6.1 Meters .................................................................................................................. 11 6.1.1 Meters in general ........................................................................................... 11 6.1.2 Requirements common to both types ............................................................. 11 6.1.3 True-r.m.s. meter ........................................................................................... 12 6.1.4 Peak programme meter (PPM) ....................................................................... 12 6.2 Test signals in general .......................................................................................... 12 6.3 Speech signals ..................................................................................................... 13 6.3.1 Live speech signals ....................................................................................... 13 6.3.2 Recorded speech material ............................................................................. 13 6.3.3 Simulated speech material ............................................................................. 13 6.4 Pink noise signal ................................................................................................... 13 6.5 Sinusoidal signal ................................................................................................... 13 6.6 Combi signal ......................................................................................................... 14 7 Magnetic background noise level of the installation site ................................................. 14 7.1 Method of measurement ....................................................................................... 14 7.2 Recommended maximum magnetic noise levels .................................................... 15 8 Characteristics to be specified, methods of measurement and requirements .................. 15 8.1 General ................................................................................................................. 15 8.2 Magnetic field strength .......................................................................................... 16 8.2.1 Characteristic to be specified ......................................................................... 16 8.2.2 Method of measurement with a simulated speech signal ................................ 16 8.2.3 Method of measurement with pink noise ........................................................ 17 8.2.4 Method of measurement with a sinusoidal signal ........................................... 17 8.2.5 Method of measurement with a combi signal .................................................. 17 8.2.6 Method of measurement – Other .................................................................... 17 8.2.7 Requirements ................................................................................................ 17 8.3 Frequency response of the magnetic field ............................................................. 18 8.3.1 Characteristic to be specified ......................................................................... 18 SIST EN 60118-4:2015

IEC 60118-4:2014 © IEC 2014 – 3 – 8.3.2 Method of measurement with a simulated speech signal ................................ 18 8.3.3 Method of measurement with pink noise ........................................................ 18 8.3.4 Method of measurement with a sinusoidal signal ........................................... 18 8.3.5 Method of measurement with combi signal ..................................................... 19 8.3.6 Method of measurement – Other .................................................................... 19 8.3.7 Requirements ................................................................................................ 19 8.4 Useful magnetic field volume ................................................................................ 19 8.4.1 Characteristic to be specified ......................................................................... 19 8.4.2 Methods of measurement .............................................................................. 19 8.4.3 Requirements ................................................................................................ 19 9 Small-volume systems ................................................................................................... 19 9.1 Inapplicability of the 'useful magnetic field volume' concept .................................. 19 9.2 Disabled refuge and similar call-points .................................................................. 20 9.3 Requirements for disabled refuge and similar call-points ....................................... 22 9.4 Counter systems ................................................................................................... 22 9.5 Requirements for counter systems ........................................................................ 24 10 Setting up (commissioning) the system .......................................................................... 24 10.1 Procedure ............................................................................................................. 24 10.2 Magnetic noise level due to the system ................................................................. 24 10.2.1 Explanation of term ........................................................................................ 24 10.2.2 Method of measurement with a speech signal ................................................ 24 10.2.3 Method of measurement with pink noise ........................................................ 25 10.2.4 Method of measurement with a sinusoidal signal ........................................... 25 10.2.5 Method of measurement with a combi signal .................................................. 25 10.2.6 Method of measurement – Other (no input signal) .......................................... 25 10.2.7 Requirements ................................................................................................ 25 10.3 Amplifier overload at 1,6 kHz ................................................................................ 25 10.3.1 Explanation of term ........................................................................................ 25 10.3.2 Methods of test .............................................................................................. 25 10.4 Requirements ....................................................................................................... 25 Annex A (informative)

Systems for small useful magnetic field volumes ............................... 27 A.1 Overview............................................................................................................... 27 A.2 Body-worn audio systems ..................................................................................... 27 A.3 Small volume, defined seating, mainly in households ............................................ 27 A.4 Specific locations such as help and information points, ticket and bank counters, etc. ........................................................................................................ 27 Annex B (informative)

Measuring equipment ........................................................................ 30 B.1 Overview............................................................................................................... 30 B.2 Signal sources ...................................................................................................... 30 B.2.1 Real speech .................................................................................................. 30 B.2.2 Simulated speech .......................................................................................... 30 B.2.3 Pink noise ...................................................................................................... 30 B.2.4 Sine wave ...................................................................................................... 30 B.3 Magnetic field strength level meter ........................................................................ 31 B.3.1 General recommendations ............................................................................. 31 B.3.2 Peak-programme meter (PPM) type ............................................................... 31 B.3.3 True r.m.s. meter type ................................................................................... 31 B.4 Field strength level meter calibrator ...................................................................... 32 B.5 Spectrum analyzer ................................................................................................ 32 SIST EN 60118-4:2015

– 4 – IEC 60118-4:2014 © IEC 2014 Annex C (informative)

Provision of information..................................................................... 33 C.1 General ................................................................................................................. 33 C.2 Information to be provided to the hearing aid user ................................................ 33 C.3 Information to be provided to system installers and by them to users .................... 34 C.4 Information to be provided by the manufacturer of the amplifying equipment ......... 34 Annex D (informative)

Measuring speech signals ................................................................. 35 Annex E (informative)

Basic theory and practice of audio-frequency induction-loop systems ................................................................................................................................ 36 E.1 Properties of the loop and its magnetic field .......................................................... 36 E.2 Directional response of the telecoil of a hearing aid .............................................. 37 E.3 Supplying the loop current .................................................................................... 42 E.4 Signal sources and cables .................................................................................... 43 E.4.1 Microphones .................................................................................................. 43 E.4.2 Other signal sources ...................................................................................... 44 E.4.3 Cables ........................................................................................................... 44 E.5 Care of the system ................................................................................................ 44 E.6 Magnetic units ...................................................................................................... 44 Annex F (informative)

Effects of metal in the building structure on the magnetic field ........... 45 Annex G (informative)

Calibration of field-strength meters ................................................... 47 Annex H (informative)

Effect of the aspect ratio of the loop on the magnetic field strength ................................................................................................................................ 49 H.1 Overview............................................................................................................... 49 H.2 Effect of aspect ratio on field patterns ................................................................... 49 Annex I (informative)

Overspill of magnetic field from an induction-loop system ................... 51 I.1 General ................................................................................................................. 51 I.2 Examples of overspill issues ................................................................................. 51 I.3 Addressing overspill issues ................................................................................... 51 Bibliography .......................................................................................................................... 53

Figure 1 – Flow chart for the operations in this standard ....................................................... 10 Figure 2 – Measurement points for disabled refuge and similar call-points ............................ 21 Figure 3 – Measurement points for a counter system ............................................................ 23 Figure A.1 – Field pattern of a vertical loop ........................................................................... 28 Figure A.2 – Contour plot of field strength of vertical loop ..................................................... 29 Figure C.1 – Graphical symbol: inductive coupling ................................................................ 33 Figure E.1 – Perspective view of a loop, showing the magnetic field vector paths ................. 37 Figure E.2 – Strengths of the components of the magnetic field due to current in a horizontal rectangular loop at points in a plane above or below the loop plane ...................... 38 Figure E.3 – Field patterns of the vertical component of the magnetic field

of a horizontal loop ...................................................................................................................... 39 Figure E.4 – Field patterns of the vertical component of the magnetic field

of a vertical loop 0,75 m square ............................................................................................................... 40 Figure E.5 – Perspective view of the variation of the vertical field strength level

at an optimum height above a horizontal rectangular loop .............................................................. 41 Figure E.6 – Directional response of the magnetic pick-up coil (telecoil) of a hearing aid ...... 42 Figure F.1 – Magnetic field pattern of a 10 m by 14 m loop, 1,2 m above its plane ................ 45 SIST EN 60118-4:2015

IEC 60118-4:2014 © IEC 2014 – 5 – Figure F.2 – Magnetic field pattern of a 10 m by 14 m loop, 1,2 m above its plane, showing the effect of metal (iron) in the floor ........................................................................ 46 Figure G.1 – Triple Helmholtz coil for calibration of meters ................................................... 47 Figure H.1 – Variation of the current required to produce a specified magnetic field strength at a specific point with the dimensions and aspect ratio of the loop ......................... 49 Figure H.2 – Square and rectangular loops ........................................................................... 50

Table 1 – Application of signals ............................................................................................ 12 Table 2 – Specification of the combi signal ........................................................................... 14 Table 3 – Magnetic field strengths typically produced by different test signals,

with an amplifier having peak-detecting AGC .................................................................................... 17

SIST EN 60118-4:2015

– 6 – IEC 60118-4:2014 © IEC 2014 INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________

ELECTROACOUSTICS –
HEARING AIDS –
Part 4: Induction-loop systems for hearing aid purposes –
System performance requirements

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. International Standard IEC 60118-4 has been prepared by IEC technical committee 29: Electroacoustics. This third edition cancels and replaces the second edition published in 2006. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: Addition of Annexes G, H and I where more information is provided about practical considerations and methods of measurement. SIST EN 60118-4:2015

IEC 60118-4:2014 © IEC 2014 – 7 – The text of this standard is based on the following documents: FDIS Report on voting 29/855/FDIS 29/861/RVD

Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table. This publication has been drafted in accordance with the ISO/IEC Directives, Part 2. A list of all parts of the IEC 60118 series, published under the general title Electroacoustics – Hearing aids, 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.

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.

SIST EN 60118-4:2015

– 8 – IEC 60118-4:2014 © IEC 2014 INTRODUCTION Audio-frequency induction-loop systems are widely used to provide a means for hearing aid users, whose hearing aids are fitted with induction pick-up coils, generally known as 'telecoils', to minimise the problems of listening when at a distance from a source of sound, shielded from the person speaking by a protective window, and/or in a background noise. Background noise and distance are two of the main causes of hearing aid users being unable to hear satisfactorily in other than face-to-face quiet conditions. Induction-loop systems have been widely installed in churches, theatres and cinemas, for the benefit of hearing-impaired people. The use of induction-loop systems has been extended to many transient communication situations such as ticket offices, bank counters, drive-in/drive-through service locations, lifts/elevators etc. The widespread provision of telephone handsets that provide inductive coupling to hearing aids is another significant application, where ITU-T Recommendation P370 [1]1 applies. Transmission of an audio-frequency signal via an induction-loop system can often establish an acceptable signal-to-noise ratio in conditions where a purely acoustical transmission would be significantly degraded by reverberation and background noise. One form of audio frequency induction-loop system comprises a cable installed in the form of a loop usually around the perimeter of a room or area in which a group of hearing impaired persons wish to listen. The cable

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