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Audio induction loop

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Audio Induction Loop systems, also called audio-frequency induction loops (abbreviated to AFILs) are an aid for the hard of hearing. They are a loop of cable around a designated area, usually a room or a building, which generates a magnetic field picked up by a hearing aid.

In the United Kingdom, as an aid for disability, their provision where reasonably possible is required by the Disability Discrimination Act of 1995.

AFILs may be provided as stand-alone items, such as at reception desks, or as part of an amplification system in a building.

This article considers the history and theory of induction loops, and some practical considerations.

Contents

History

The pick up coil in a hearing aid is named Telecoil because its early form was to pick up a magnetic field from coils within a telephone. These were included as a part of the method of enabling a 2-way conversation over a single pair of wires. The telecoil enabled hearing aid users to hear the phone conversation more clearly without also picking up background noise around them.

From this, the natural development was to generate audio magnetic fields which the telecoil could receive.

Induction loop theory

The simple form of AFIL is a single wire around a room, driven from a power amplifier as a loudspeaker would be driven. The coupling of magnetic fields is described mathematically by Faraday's law of induction. A summary of the theory necessary for AFILs is included in BSI Specification BS7594, which is a guide to the design and installation of induction loops.

Practical induction loops

The simple form of AFIL using a general purpose amplifier suffers from 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 which breaks the law.

The most important feature is that an amplifier will generate harmonics when driven into distortion, and these will cause radio interference. This must be prevented, both for equipment quality and for legal reasons. It is illegal to cause such interference in these circumstances. In Europe, the EMC Directive applies, and it is also illegal to supply or install unsuitable electronic and electrical equipment.[1]

A second factor is that many forms of hearing impairment mean that sound levels must be kept fairly constant. So a good loop driver will have an automatic level control providing a constant loop signal 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 30dB input range.

A third problem is the inductance 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 reducing inductance with a multi-core cable where the conductors are connected in parallel.

Structural steel, and some other metalwork, in buildings can be a problem in reducing the field strength across the loop area. In some cases, careful positioning of the loop cable can help. In other cases, there is no alternative to large variations in the signal level across the room.

Other equipment within the magnetic field

Audio induction loops create, legitimately, fairly high levels of magnetic fields. Other equipments must be designed and installed to work properly within this field.

The most common cause of problems is earth loops, where equipments are connected together by signal wires, but powered from different mains sockets in different parts of the room or building. The mains earth and signal earth make a receiving loop. this produces an interference signal proportional to the area within the earth loop.[2]

Video equipment, such as projectors, commonly displays a horizontal pattern on the screen. The pattern moves with the sound feeding the AFIL. The cure is to avoid earth loops by powering the projector through a mains extension from the same mains supply as the computer, camera or DVD player which provides the video signal.

Audio equipment, such as a keyboard or guitar connected to the main sound system, may be the source of whistles during silent parts of the programme. Again, the cure is to break any possible earth loops. This is often done by the use of "direct injection boxes" which break the earth. The use of balanced audio cables is advised, and is often essential.

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.[3]

IEC 60118-4 (formerly Britain's BS6083 part 4, also known as EN 60118-4) is now the main specification for international use[4]. This is based on the principle that the long term average of the field strength at a typical listening location must be 100mA/m, +- 3dB. To determine this long term average requires a measurement over 60 seconds or more. The standard therefore sets a more pragmatic requirement for determining the short term peaks of the signal. Short term peaks need to be 12dB (x4) higher than the long term average, based on the fact that peaks of speech are approximately 12dB higher than the long term average level of speech. Therefore an induction loop system or AFILS must be capable of delivering field strength peaks of 400mA/m +- 3dB (280 to 560mA/m). Peaks must be measured using fast RMS measurement (125ms averaging time).

In addition the IEC 60118-4 standard sets limits on acceptable background noise, and requires that the system delivers frequency response of +- 3dB from 100Hz to 5 kHz relative to the field strength at 1 kHz. All measurements must be made with a coil that picks up only the vertical component of the magnetic field, the component that is picked up by the telecoil of a hearing aid. A practical guide to the performance standard can be found at Ampetronic standards guidance.

BS7594 (published by the British Standards Institution (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.

References

  1. The EMC Directive of the European Union, and regulations under "enabling legislation".
  2. Guidance notes in BS7594
  3. Introductory notes in various specifications
  4. Standard available from the International Electrotecnical Commission or IEC www.iec.ch

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