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An audio induction loop system (also called an audio-frequency induction loop or AFIL, or a hearing loop) is an assistive listening device for individuals with reduced ranges of hearing.


History

The first patented magnetic induction loop communication system was invented by
Joseph Poliakoff Joseph Lazarevich Poliakoff (; 24 April 1873 – 24 November 1959) was a Ukrainian-born British telephone and sound engineer and inventor, particularly of hearing aids. Poliakoff was a Ukrainian who experienced first-hand the communist revoluti ...
(grandfather of Sir Martyn Poliakoff) in Great Britain in 1937. The pickup coil in a hearing aid is known as a '' telecoil'' (or ''T-coil'') because its early form was to pick up a magnetic field from coils within a
telephone A telephone, colloquially referred to as a phone, is a telecommunications device that enables two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most ...
. 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. From this, the natural development was to generate electromagnetic fields representing the audio, which the telecoil could receive.


Design

A hearing loop consists of one or more physical loops of
cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...
which are placed around a designated area, usually a room or a building. The cable generates an
electromagnetic field An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...
throughout the looped space which can be picked up by a telecoil-equipped
hearing aid A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers ...
, a
cochlear implant A cochlear implant (CI) is a surgically implanted Neuroprosthetics, neuroprosthesis that provides a person who has moderate-to-profound sensorineural hearing loss with sound perception. With the help of therapy, cochlear implants may allow for imp ...
(CI) processor, or a specialized hand-held hearing loop receiver for individuals without telecoil-compatible hearing aids. The loops carry
baseband In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable into ...
audio-frequency currents; no
carrier signal In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or frequ ...
is used. The benefit is that it allows the sound source of interestwhether a musical performance or a ticket taker's side of the conversationto be transmitted to the hearing-impaired listener clearly and free of other distracting noise in the environment. Typical installation sites include concert halls, ticket kiosks, high-traffic public buildings (for PA announcements), auditoriums, places of worship, courtrooms, meeting rooms, and homes.


Theory

The simplest form of AFIL is a single wire around a room, driven from a
power amplifier An audio power amplifier (or power amp) amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspeakers or headphones. Audio power a ...
as a
loudspeaker A loudspeaker (commonly referred to as a speaker or, more fully, a speaker system) is a combination of one or more speaker drivers, an enclosure, and electrical connections (possibly including a crossover network). The speaker driver is an ...
would be driven. The coupling of
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
s is described mathematically by Faraday's law of induction.


Implementation

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
harmonics In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st harm ...
when driven into distortion, and these will cause radio interference. This must be prevented, both for sound quality and for legal reasons as it is illegal to cause such
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...
in these circumstances. In Europe, the EMC Directive applies, and it is also illegal to supply or install unsuitable electronic and electrical equipment. A second factor is that many forms of
hearing impairment Hearing loss is a partial or total inability to Hearing, hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to Lang ...
mean that sound levels must be kept fairly constant. An effective loop driver will have an automatic level control to compress 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  dB input range. A third problem is the
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The electric current produces a magnetic field around the conductor. The magnetic field strength depends on the magnitude of the ...
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.
Structural steel Structural steel is steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section (geometry), cross section. Structural steel sha ...
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.


Interference

Audio induction loops create relatively high
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
levels. Other equipment must be designed and installed to work properly within this field. The most common cause of problems is earth loops, 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. 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.


Technical standards

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. IEC 60118-4 (formerly Britain's BS 6083 part 4, also known as EN 60118-4) is now the main specification for international use. This standard specifies long term average field strength with allowance for program peaks, background noise level, and frequency response. 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. BS 7594 (published by the BSI 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.


Legislation

In the United Kingdom, as an aid for disability, their provision, where reasonably possible, is required by the
Equality Act 2010 The Equality Act 2010 (c. 15) is an act of Parliament of the United Kingdom passed during the Brown ministry with the primary purpose of consolidating, updating and supplementing the numerous prior Acts and Regulations, that formed the basis o ...
and previously by the
Disability Discrimination Act 1995 The Disability Discrimination Act 1995 (c. 50) (informally, and hereafter, the DDA) is an Act of the Parliament of the United Kingdom which has now been repealed and replaced by the Equality Act 2010, except in Northern Ireland where the A ...
,Action on Hearing (formerly RNID/Deafness Research UK) 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.


Alternatives

In the United States, an alternative technology using FM 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. Another alternative system, used primarily in theatres, uses invisible
infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
radiation; compatible headsets can pick up the modulated infrared energy to reproduce sound.


References

{{refs Hearing aids Assistive technology Augmentative and alternative communication fi:Induktiosilmukka