Audio induction loop
Encyclopedia
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 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 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 older technology using FM transmission to "neck loops" is more established. In comparison, hearing loops require a greater initial investment, but offer greater convenience and avoid the social stigma entailed by the FM system's paraphernalia.
This article considers the history and theory of induction loops, and some practical considerations.
. 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.
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.
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 (Ampetronic Overview of typical loop configurations) to give different patterns of magnetic fields and solve different technical problems such as the presence of metal structures.
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.
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.
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 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.
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.
Public address
A public address system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building.Simple PA systems are often used in...
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 Disability Discrimination Act 1995
Disability Discrimination Act 1995
The Disability Discrimination Act 1995 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 Act still applies...
, 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 older technology using FM transmission to "neck loops" is more established. In comparison, hearing loops require a greater initial investment, but offer greater convenience and avoid the social stigma entailed by the FM system's paraphernalia.
This article considers the history and theory of induction loops, and some practical considerations.
History
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 telephoneTelephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
. 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 loudspeakerLoudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
would be driven. The coupling of magnetic fields is described mathematically by Faraday's law of induction
Faraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
. 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 interferenceElectromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
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 impairment
-Definition:Deafness is the inability for the ear to interpret certain or all frequencies of sound.-Environmental Situations:Deafness can be caused by environmental situations such as noise, trauma, or other ear defections...
mean that sound levels must be kept fairly constant. An effective loop driver will have an automatic level control
Automatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
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
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
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 (Ampetronic Overview of typical loop configurations) 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.
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.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 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.
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.