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<p>An <b>audio induction loop</b> system (also called an <b>audio-frequency induction loop</b> or <b>AFIL</b>, or a <b>hearing loop</b><sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>) is an <a href="Assistive_listening_device" title="Assistive listening device">assistive listening device</a> for individuals with reduced ranges of hearing.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>

<p>The first patented magnetic induction loop communication system was invented by <a href="Joseph_Poliakoff" title="Joseph Poliakoff">Joseph Poliakoff</a> (grandfather of Sir <a href="Martyn_Poliakoff" title="Martyn Poliakoff">Martyn Poliakoff</a>) in Great Britain in 1937.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

</p><p>The pickup coil in a hearing aid is known as a <i><a href="Telecoil" class="mw-redirect" title="Telecoil">telecoil</a></i> (or <i>T-coil</i>) because its early form was to pick up a magnetic field from coils within a <a href="Telephone" title="Telephone">telephone</a>. These were included as a part of the method of enabling a two-way conversation over a single pair of wires. The telecoil enabled the hearing aid user to hear the phone conversation clearly without picking up background noise.

</p><p>From this, the natural development was to generate electromagnetic fields representing the audio, which the telecoil could receive.

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<div class="mw-heading mw-heading2"><h2 id="Design">Design</h2></div>

<p>A hearing loop consists of one or more physical loops of <a href="Electrical_cable" title="Electrical cable">cable</a> which are placed around a designated area, usually a room or a building. The cable generates an <a href="Electromagnetic_field" title="Electromagnetic field">electromagnetic field</a> throughout the looped space which can be picked up by a <a href="Hearing_aid#Telecoil" title="Hearing aid">telecoil</a>-equipped <a href="Hearing_aid" title="Hearing aid">hearing aid</a>, a <a href="Cochlear_implant" title="Cochlear implant">cochlear implant</a> (CI) processor, or a specialized hand-held hearing loop receiver for individuals without telecoil-compatible hearing aids.

</p><p>The loops carry <a href="Baseband" title="Baseband">baseband</a> audio-frequency currents; no <a href="Carrier_signal" class="mw-redirect" title="Carrier signal">carrier signal</a> is used. The benefit is that it allows the sound source of interest – whether a musical performance or a ticket taker's side of the conversation – to be transmitted to the hearing-impaired listener clearly and free of other distracting noise in the environment.<sup id="cite_ref-ny_times_6-0" class="reference"><a href="#cite_note-ny_times-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> Typical installation sites include concert halls, ticket kiosks, high-traffic public buildings (for <a href="Public_address" class="mw-redirect" title="Public address">PA</a> announcements), auditoriums, places of worship, courtrooms, meeting rooms, and homes.<sup id="cite_ref-npr_7-0" class="reference"><a href="#cite_note-npr-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Theory">Theory</h2></div>

<p>The simplest form of AFIL is a single wire around a room, driven from a <a href="Audio_power_amplifier" title="Audio power amplifier">power amplifier</a> as a <a href="Loudspeaker" title="Loudspeaker">loudspeaker</a> would be driven. The coupling of <a href="Magnetic_field" title="Magnetic field">magnetic fields</a> is described mathematically by <a href="Faraday's_law_of_induction" title="Faraday's law of induction">Faraday's law of induction</a>.

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<div class="mw-heading mw-heading2"><h2 id="Implementation">Implementation</h2></div>

<p>A basic AFIL using a general purpose amplifier has some disadvantages. The loop driver amplifier requires some additional circuits to overcome these. Using anything other than a correctly designed loop driver amplifier is not only unsatisfactory, but may result in a loop installation that can generate <a href="Harmonics" class="mw-redirect" title="Harmonics">harmonics</a> when driven into distortion, and these will cause <a href="Radio_interference" class="mw-redirect" title="Radio interference">radio interference</a>. This must be prevented, both for sound quality and for legal reasons as it is illegal to cause such <a href="Electromagnetic_interference" title="Electromagnetic interference">interference</a> in these circumstances. In Europe, the EMC Directive applies, and it is also illegal to supply or install unsuitable electronic and electrical equipment.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup>

</p><p>A second factor is that many forms of <a href="Hearing_impairment" class="mw-redirect" title="Hearing impairment">hearing impairment</a> mean that <a href="DB_SPL" class="mw-redirect" title="DB SPL">sound levels</a> must be kept fairly constant. An effective loop driver will have an <a href="Automatic_gain_control" title="Automatic gain control">automatic level control</a> to <a href="Audio_level_compression" class="mw-redirect" title="Audio level compression">compress</a> the signal, providing a constant loop amplitude for a wide range of source levels. Meeting this requirement is likely to meet the interference requirement at the same time. To do this, the loop driver should give constant output for at least 30 <a href="Decibel" title="Decibel">dB</a> input range.

</p><p>A third problem is the <a href="Inductance" title="Inductance">inductance</a> of the loop cable, and its effect upon the higher frequencies of sound. To overcome this, many loop drivers operate as current mode amplifiers instead of voltage mode. By setting the amplifier characteristic between voltage and current mode, the overall performance is optimised for good bandwidth with minimum distortion. There are other options for reducing the effect of cable inductance, including the use of a multi-core cable where the conductors are connected in parallel.

</p><p><a href="Structural_steel" title="Structural steel">Structural steel</a> and other metalwork in buildings can cause problems by reducing the field strength unevenly across the loop area, causing frequency distortions. In most cases, a solution can be found using combinations of loops with phase shift between them, combined with frequency correction and increased signal strength.

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<div class="mw-heading mw-heading2"><h2 id="Interference">Interference</h2></div>

<p>Audio induction loops create relatively high <a href="Magnetic_field" title="Magnetic field">magnetic field</a> levels. Other equipment must be designed and installed to work properly within this field.

</p><p>The most common cause of problems is <a href="Ground_loop_(electricity)" title="Ground loop (electricity)">earth loops</a>, where different pieces of equipment are connected together by signal wires, but powered from different power sockets in different parts of the room or building. The combination of the mains earth and signal earth creates a receiving loop that produces an interference signal proportional to the area within the earth loop.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> Various steps are used to prevent interference on audio and video equipment. Powering signal sources and output devices from the same mains circuit to prevent formation of an earth loop; shielded cables or signal isolators may be used.

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<div class="mw-heading mw-heading2"><h2 id="Technical_standards">Technical standards</h2></div>

<p>An objective of the field strength requirements of standards for AFILs is to make the perceived loudness of sound from the loop the same as from the microphone in the hearing aid. This is the basis of the average field strength of 100mA/m used to generate today's performance standards around the world.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup>

</p><p>IEC 60118-4 (formerly Britain's BS 6083 part 4, also known as EN 60118-4) is now the main specification for international use.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> This standard specifies long term average field strength with allowance for program peaks, background noise level, and frequency response.

</p><p>IEC 62489-1 specifies how to measure the performance of the system components in a hearing loop system. The standard recommends fixed and portable monitoring devices to allow verification of the proper operation of the hearing loop.

</p><p>BS 7594 (published by the <a href="British_Standards_Institution" class="mw-redirect" title="British Standards Institution">BSI</a> and widely used in Britain) is a non-mandatory guideline for the design and installation of induction loops. It has a comprehensive guide to theory, as well as guidance for those considering the installation of AFILs in buildings for which they may be responsible. It also contains some valuable guidance relating to other equipment within the loop area. The calibration of field strength measuring devices is also included.

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<div class="mw-heading mw-heading2"><h2 id="Legislation">Legislation</h2></div>

<p>In the United Kingdom, as an aid for disability, their provision, where reasonably possible, is required by the <a href="Equality_Act_2010" title="Equality Act 2010">Equality Act 2010</a> and previously by the <a href="Disability_Discrimination_Act_1995" title="Disability Discrimination Act 1995">Disability Discrimination Act 1995</a>,<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> and they are available in "the back seats of all London taxis, which have a little microphone embedded in the dashboard in front of the driver; at 18,000 post offices in the U.K.; at most churches and cathedrals", according to Prof. David G. Myers.<sup id="cite_ref-npr_7-1" class="reference"><a href="#cite_note-npr-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Alternatives">Alternatives</h2></div>

<p>In the United States, an alternative technology using <a href="Frequency_modulation" title="Frequency modulation">FM</a> transmission to "neck loop" receivers was more widely adopted due to economic advantages. In comparison, hearing loop systems require a greater initial investment by the facility operator, but offer greater convenience and avoid the social stigma and hygienic concerns entailed by the FM system's paraphernalia for those who have hearing aids.<sup id="cite_ref-npr_7-2" class="reference"><a href="#cite_note-npr-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>

</p><p>Another alternative system, used primarily in theatres, uses invisible <a href="Infrared" title="Infrared">infrared</a> radiation; compatible headsets can pick up the modulated infrared energy to reproduce sound.

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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.audiology.org/resources/consumer/Pages/Get_in_theHearingLoop.aspx">"Get in the Hearing Loop"</a>. <i>American Academy of Audiology</i><span class="reference-accessdate">. Retrieved <span class="nowrap">25 October</span> 2011</span>.</cite></span>

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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.hearingloss.org/content/get-hearing-loop">""Get in the Hearing Loop" Campaign Promotes Doubling Functionality of Hearing Aids"</a>. <i>Hearing Loss Association of America</i><span class="reference-accessdate">. Retrieved <span class="nowrap">25 October</span> 2011</span>.</cite></span>

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<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://hearinghealthmatters.org/hearinginternational/2011/induction-loops-around-the-world-where-are-we-part-i/">"Induction Loops Around the World……Where are we? – Part I"</a>. 2011-11-30.</cite></span>

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<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://patents.google.com/patent/US2252641A/en">"Method of and apparatus for the transmission of speech and other sounds"</a>.</cite></span>

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<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.youtube.com/watch?v=NYvEnAvouVA&t=84">"Sir Martyn Poliakoff - Geissler Tubes - Periodic Table of Videos"</a>. <i><a href="YouTube" title="YouTube">YouTube</a></i>. 27 October 2016. <a rel="nofollow" class="external text" href="https://ghostarchive.org/varchive/youtube/20211215/NYvEnAvouVA">Archived</a> from the original on 2021-12-15.</cite></span>

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<li id="cite_note-ny_times-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-ny_times_6-0">^</a></b></span> <span class="reference-text"><cite id="CITEREFTierney2011" class="citation news cs1">Tierney, John (2011-10-23). <a rel="nofollow" class="external text" href="https://www.nytimes.com/2011/10/24/science/24loops.html">"A Hearing Aid That Cuts Out All the Clatter"</a>. <i><a href="The_New_York_Times" title="The New York Times">The New York Times</a></i><span class="reference-accessdate">. Retrieved <span class="nowrap">25 October</span> 2011</span>.</cite></span>

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<li id="cite_note-npr-7"><span class="mw-cite-backlink">^ <a href="#cite_ref-npr_7-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-npr_7-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-npr_7-2"><sup><i><b>c</b></i></sup></a></span> <span class="reference-text"><cite class="citation news cs1"><a rel="nofollow" class="external text" href="https://www.npr.org/templates/story/story.php?storyId=128268480">"Helping Those With Hearing Loss Get In The Loop"</a>. <i><a href="NPR" title="NPR">NPR</a></i>. 2010-07-02<span class="reference-accessdate">. Retrieved <span class="nowrap">25 October</span> 2011</span>.</cite></span>

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<li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text">The EMC Directive of the European Union, and regulations under "enabling legislation".</span>

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<li id="cite_note-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-9">^</a></b></span> <span class="reference-text">Guidance notes in BS7594</span>

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<li id="cite_note-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-10">^</a></b></span> <span class="reference-text">Introductory notes in various specifications</span>

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<li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text">Standard available from the International Electrotecnical Commission or IEC www.iec.ch</span>

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<li id="cite_note-12"><span class="mw-cite-backlink"><b><a href="#cite_ref-12">^</a></b></span> <span class="reference-text">Action on Hearing (formerly RNID/Deafness Research UK)</span>

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