Audio induction loop
Audio induction loop systems, also called audio-frequency induction loops (AFILs) or hearing loops, 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. The loops carry baseband audio-frequency currents; no carrier signal 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. Typical installation sites would include concert halls, ticket kiosks, high-traffic public buildings (for PA announcements), auditoriums, places of worship, and homes.
In the United Kingdom, as an aid for disability, their provision where reasonably possible is required by the Equality Act 2010 and previously by the Disability Discrimination Act 1995, 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. Meyers.
In the United States, an newer technology using FM transmission to "neck loops" is more established. In comparison, hearing loops require a greater initial investment and are of lower quality in transmission for the casual hard of hearing patron, but offer greater convenience and avoid the social stigma entailed by the FM system's paraphernalia for those who have hearing aids.
An alternative system used in theatres uses invisible infrared radiation; compatible headsets can pick up the modulated infrared energy to produce sound.
This article considers the history and theory of induction loops, and some practical considerations.
The pickup 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 two-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 that can generate harmonics 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 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 mean that sound levels must be kept fairly constant. An effective 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 other metalwork in buildings can cause a problem by reducing the field strength unevenly across the loop area and 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.
There are many different ways to configure conductive loops to give different patterns of magnetic fields and solve different technical problems such as the presence of metal structures.
Other equipment within the magnetic field
Audio induction loops create, legitimately, fairly high magnetic field 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.
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 BS6083 part 4, also known as EN 60118-4) is now the main specification for international use. 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 100 Hz 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.
BS7594 (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.
- "Get in the Hearing Loop". American Academy of Audiology. Retrieved 25 October 2011.
- ""Get in the Hearing Loop" Campaign Promotes Doubling Functionality of Hearing Aids". Hearing Loss Association of America. Retrieved 25 October 2011.
- Tierney, John (2011-10-23). "A Hearing Aid That Cuts Out All the Clatter". The New York Times. Retrieved 25 October 2011.
- "Helping Those With Hearing Loss Get In The Loop". NPR. 2010-07-02. Retrieved 25 October 2011.
- Action on Hearing (formerly RNID/Deafness Research UK)
- The EMC Directive of the European Union, and regulations under "enabling legislation".
- "Induction loop systems in metal structures". Ampetronic.com. 2012-03-24. Retrieved 2012-07-25.
- "Audio induction loop system layouts". Ampetronic.com. 2012-03-24. Retrieved 2012-07-25.
- Guidance notes in BS7594
- Introductory notes in various specifications
- Standard available from the International Electrotecnical Commission or IEC www.iec.ch