Audio induction loop systems, also called audio-frequency induction loops (AFILs) or hearing loops, are an assistive listening technology for individuals with reduced ranges of hearing.
A hearing loop consists of one or more physical loop of cable which are placed around a designated area, usually a room or a building. The cable generates an electromagnetic field throughout the looped space which can be picked up by a telecoil-equipped hearing aid, a cochlear implant (CI) processor, or a specialized hand-held hearing loop receiver for individuals without telecoil-compatible hearing aids.
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 include concert halls, ticket kiosks, high-traffic public buildings (for PA announcements), auditoriums, places of worship, courtrooms, meeting rooms, 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. Myers.
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 radiation; compatible headsets can pick up the modulated infrared energy to reproduce sound.
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. 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.
The simplest 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 British Standard BS 7594, which is a guide to the design and installation of induction loops.
A basic form of AFIL using a general purpose amplifier, as described above, 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 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 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 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.
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.