Induction loop systems

Induction loop systems are all around us in the modern world. The chances are you may not have heard of them, but you will probably have seen the T symbol in many public buildings including banks, shops, churches and meeting rooms.

The symbol is used throughout the UK to indicate that the building is fitted with induction loop technology to aid customers/employees with hearing difficulties.

What is an induction loop?
It is a coil of wire through which an electrical signal is passed from an induction loop amplifier. This may be connected to a public announcement system or simply a microphone.

This coil gives off a magnetic signal that regular hearing aids can pick up and amplify for the user, when they switch their hearing aids to the T position provided.

Why would you need one?
In 1995, the UK Government passed the Disability Discrimination Act banning the discrimination of persons based upon their disabilities.

Under the law, businesses/companies are required to make reasonable changes to their buildings to provide adequate facilities for hearing impaired employees and customers. Organisations have had five years to make the changes and the deadline is October 2004. Any company failing to meet the requirements will face prosecution.

In public buildings there is often a lot of background noise from people and machines. Hearing aid wearers usually have their aids set to M (microphone), which amplifies the noise around them. However, if they need to hear someone speaking over the background noise,

it can be very difficult. With induction loop systems, for instance in a bank, the teller/presenter can speak into a microphone which transmits the sound directly to the user's hearing aid, allowing them to hear, even through glass screens.

A common question is: Do loop systems affect computers and other electrical equipment?

The answer is that induction loop systems generally do not cause interference with electrical equipment. However, if there is high electrical activity in the vicinity ie power cupboards it can create a growling/buzzing noise on the loop.

Standards
All induction loop systems must be tested and comply with British Standard BS 7594. There is also the international standard IEC 60118-4.

These two standards describe the magnetic field strength, frequency response, area cover, etc and apply to any size loop, from a loop pad to a large theatre loop system.

The magnetic field must be high enough for an acceptable signal to noise ratio (400 mA). The value set in the standard has been chosen so that these requirements are met.

The driving loop amplifier is not specified in the standard. However, a recommendation of the frequency dependence of the magnetic field strength is included. The stated values for magnetic field strength may also be applicable to transmitting coils intended for a very short range ie close contact, for example neck loops, loop pads and loop cushions. In the case of large areas or magnetic disturbances from mains or lighting regulation systems, it may be necessary to deviate from the standard and enforce a stronger magnetic field.

The recommended average value for magnetic field strength is 100 mA/m.

Maximum magnetic field strength
The magnetic field strength obtainable within a specified area corresponds to the maximum short-time average of the speech signal (approximately 0·125 s, referring to the integration time used during the averaging process) applied to the system.

Specified magnetic field area
The area within which the hearing aid induction coil will be located under normal use of the hearing instrument and within the magnetic field strength is required to meet the recommended specifications. Note that the specified magnetic field area is not necessarily the same as the geometrical area of the plane of the induction loop.

When the hearing aid user is sitting or standing in an upright position, the specified vector component relates to the vertical component. When sitting, the magnetic field is correct, even if the hearing aid user is leaning forwards or backwards.

Planning an induction loop

Fulfil the requirements of the British Standard.

Ensure the induction loop amplifiers generate the necessary peaks (12 dB and 400 mA/m).

Use the cable suggested for the chosen amplifier; use a twin cable (it is then possible to chose between one/two or parallel turn loop configuration).

Type of wire: there are no special isolation demands, ordinary speaker cable is excellent. Check the cable area according to the specification.

Select the amplifier depending on the size of the room and observe the amplifier's maximum coverage area measured in a free field situation.

In reality, coverage may be reduced by 20-50% in the vicinity of metallic objects such as steel reinforced concrete.

Follow the installation manual, in particular look at how to set the input sensitivity level with the automatic gain control (AGC) system.

Do not use dynamic microphones, as it increases the risk of magnetic self-oscillations.

The service provider
When buying and working with a loop system it is important to:

• Ask for a Certificate of Conformity or a measuring protocol showing that the system can produce the necessary peaks 12 dB (400 mA/m) or the hearing aid user will be disappointed.
  
• Obtain a loop amplifier that fulfils the standard – a lack of current/power is the most common problem with loop systems.
  
• Try to understand the basics of the system eg which connectors and potentiometers are needed.
  
• Make a note of the potentiometers' settings.
  
• Listen to the system frequency response.
  
• Do a basic check of the system, listen and check levels according to the British Standard.
  
• Not use a hearing aid; the gain and frequency response is individual.

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