Audio-frequency induction loop systems allow hearing impaired people to hear more clearly.
Most hearing aids have a ‘T’ or ‘MT’ switch which allows them to pick up the electromagnetic field generated by an induction loop system. The hearing aid converts this signal into a sound suited to its user’s specific hearing requirements.
Any person with a hearing aid positioned within or near the loop can hear the loop signal by switching their hearing aid to the correct position, allowing them to participate more effectively in general conversation, ordering goods or services, listening to public performances, etc.
An induction loop system therefore comprises the following main elements:-
In addition to the many routine benefits for hearing aid users, induction loop systems can also be used for other limited area broadcasting applications such as museum ‘walk through’ guided tours and surveillance talkback systems.
Audio-frequency induction loop systems do not use radio frequencies; they operate at audio frequencies.
The signal from an audio source is fed into an induction loop amplifier, which amplifies and sets the signal level in the same way as a conventional amplifier. The amplified signal, instead of going to a loudspeaker, is fed to a closed loop of cable that is normally placed around the perimeter of the room. Employing a constant current amplifier ensures the current is maintained at the set level whilst providing a flat frequency response without the need for equalisation circuitry.
The current flowing through the loop generates a magnetic field that radiates in the space around the loop cable (see diagram below). Any lines of magnetic flux that pass through the telecoil in a receiver, such as a hearing aid, will generate a current in the coil that is then converted back to audio and fed into the listener’s ear.
It is important to remember that the magnetic field will ‘bleed’ outside the perimeter of the loop and therefore a loop system cannot be considered confidential. Ways of reducing this ‘bleed’ or ‘overspill’ are addressed on pages 24 and 25 of this guide.
Magnetic field in a plane through one axis of a square loop
In normal use, hearing aids utilise a microphone for amplifying localised speech. Whilst this is effective for local conversations/quiet environments, it is less effective for listening to speech or music at a distance or in front of a security screen at a ticket counter. This is because the hearing aid’s microphone also picks up any background noise in the room and unwanted speech from other conversations. An induction loop system works by moving the required sound closer to the hearing aid via the hearing aid’s telecoil which is activated by turning it to the ‘T’ or ‘MT’ position.
As telecoils are fitted as standard to most hearing aids (over 90% are said to have the ‘T’ position) induction loop systems can be considered cost-effective compared to other hearing assistance systems. Infrared systems, for example, require special receivers, the cost and maintenance of which must be met by the service provider.
Many modern hearing aids do not just amplify all frequencies equally; they are tailored to suit the user’s hearing problem and amplify different bands by different amounts. This gives maximum intelligibility, so the user has the best chance of understanding what is said.
Induction loop systems are popular because:-
Induction loop system design and installation can be simple provided that a few basic facts are understood.
Maximum area coverage
The approximate coverage provided by an MAJORCOM amplifier is usually quoted in metres squared (m2). For example, the 120m2 quoted for our PA2000 amplifier means that the PA2000 can cover rooms up to 11 x 11m in size. The PAxxxx range has amplifiers that can cover everything from a 1.2m2 ticket counter to a large 900m2 conference hall.
Note that in certain installations, it may not be necessary to cover the whole room, i.e., in a bowling alley the loop may only need to cover the top of the lanes.
Loop amplifier position
Induction loop amplifiers are best sited adjacent to the loop, as the feed cable will generate a magnetic field that may interfere with other areas. However, if a long feed cable cannot beavoided, the cable should be twisted to reduce magnetic radiation.
Loop cable position
Under current IEEE wiring regulations (17th Edition), loop cable is a classified as class 2A and, as such, must be sited at least 600mm away from telephone, mains and control cables.
In most installations, loop cable is usually run around the perimeter (edge) of the room.
The field strength in the plane of the loop (the height at which the cable is positioned) varies greatly so it is best to install the loop above or below the listener at floor or ceiling height. The loop field will not be as strong but it will be much more even and provide better results. Do not mount the loop cable behind dado rails as the loop signal will be uncomfortably strong for the hearing aid user.
Try to locate ceiling loops approximately 1.2m above the listener’s head (listening height with the hearing aid user sitting or standing is normally 1.2 to 1.8m from the floor). Note that when using a ceiling mounted loop, approximately 20% of the amplifier’s power will be lost for every 1.5m increase in height so the cable should never be mounted higher than 7m above listening height.
Structural steel, in particular, large sheets of metal (solid or perforated) such as metal suspended ceilings, can absorb the magnetic field resulting in uneven coverage or dead spots. To avoid the magnetic field being absorbed by structural steel, if possible keep the loop about a metre from large uprights.
If there is a steel-reinforcing grid in the floor, either put the loop in the ceiling or, if it must go in the floor, install the loop in plastic conduit as far above the grid as possible. The amplifier’s drive current may have to be turned up to overcome the effects of the metal, so choose the largest conductor size possible to suit the area
There is nothing electrically special about induction loop cable. Almost any stranded or solid single core cable with tough insulation can be used, provided it is not liable to break (to minimise the chance of it shorting to earth and damaging the amplifier’s output) and is of the appropriate gauge and DC resistance (0.5 to 1 Ohm).
Refer to the loop cable selection chart below.
The chart shows the recommended CSA (cross-sectional area) for different lengths of loop
cable. Simply work out the length of the loop required and choose a cable size that falls into the non-shaded area of the graph.
It is imperative to use unshielded cable