The information below is provided from Ampetronic Ltd
Introduction to Induction Loops - What are they?]
BASIC INFORMATION A4 Leaflet :
Help for the Hard of Hearing
This section introduces Induction Loop technology and how it can be used for assistive listening. Please take some time to read and study this information which has
been accumulated over many years of practical loop design and installation.
Other sections take you through the implications of metalwork in buildings on loop
performance and then how to go about designing and installing an induction loop system to meet published standards.
What is an Audio Induction Loop?
An audio induction loop is a way of transmitting sound through a simple wire loop to a suitable
receiver. They are used most frequently to help hearing aid users listen to a sound source more clearly where there is background noise in a room.
How does an Induction Loop work?
In a very basic form, an induction loop system consists of a loop of wire around the edge of an
area connected to a special amplifier. The input of the amplifier is connected to the sound source that the hard of hearing users of the area want to hear more clearly.
The amplifier drives an audio current (not voltage) through the loop. This current generates a
magnetic field in the area enclosed by the wire that a suitably fitted hearing aid can receive.
Why use an induction loop?
People who suffer from hearing loss - the unseen disability - require more than just increasing the volume of sound into their ears.
The loss of hearing is generally associated with the neurological processing of information in the brain. People with normal hearing require a signal to noise ratio of 6dB for a reasonable
level of intelligibility. This represents quite a noisy background, which might be reverberation, air conditioning, ventilation systems or background noise such as a crowd of people.
When a person loses about 80% of their hearing, they generally need a signal to noise ratio of 15 to 20dB. This can be difficult to achieve unless the wanted signal is taken straight from
the basic source and transmitted directly through the loop system, avoiding any reverberation or additional ambient noise.
Transient situations, such as ticket counters, information and help points, etc., are the worst
areas for listening, though even in churches, theatres and lecture / conference rooms, there is often sufficient degradation of the signal to seriously affect intelligibility.
In most situations it is impractical to issue any form of separate receiver and the use of the
individual' hearing aid is a major step to bringing people with hearing loss back into full contact with their environment. Only induction loop systems are capable of doing this.
Induction loop systems can be configured to reduce spill to surrounding areas and hence confidentiality is not an issue if designed and installed properly.
Where are Induction Loops used?
Induction loops are used in a wide variety of places where an increase in intelligibility of audio
is desired. There are basically two categories of use: transient and extended time.
Transient locations are a very important aspect of induction loop use where no other practical means of assisting the hard of hearing exists.
Transient use may include areas such as:
- Ticket counters
- Reception desks
- Public announcements - Airports, railway stations, shopping malls etc.
- Drive-through sales points
- Elevators, lifts
- Cars, buses, coaches, trams, trains
- Cruise liners
- Museum exhibits
Extended Time Use:
Typical examples are:
- Theatres / concert halls
- Lecture theatres / auditoriums
- Seminar rooms
- Churches, places of worship
- TV lounges (in nursing homes & hospitals)
Are there situations where an induction loop is not suitable?
Yes, there are some.
When the magnetic background noise (environmental noise) is loud, it may be impossible for anyone to receive a clear loop signal until the noise source has been removed. This will
equally affect neckloops connected to a FM radio or IR systems.
There may be practical limitations such as having nowhere to physically fit the loop cabling.
There may not be a sufficiently good audio signal.
Do Induction Loops Interfere With Heart Pacemakers?
Under normal circumstances, a correctly installed induction loop system does not interfere with
heart pacemakers. A minimum separation distance of 50mm (2") should be maintained between loop cable and pacemaker to remove any potential for interference.
What are the alternatives to an Induction Loop System? Top
There are a number of other assistive listening technologies available. All rely on providing a
transmission of the audio signal by some other method to a receiver carried by the listener. The principal systems are "Infra-Red" (IR) and FM carrier systems.
FM works by transmitting a normal radio signal carrying the audio that the user wants to hear. The FM system has to work with limited power and on a narrow unlicensed frequency band. It
is therefore often susceptible to interference from other radio users (e.g. taxis) and general interference.
Infra-Red works by transmitting the audio signal on an Infra Red light beam and requires a line of sight (or reflected light) from the transmitter to the personal receiver unit.
With both these alternative technologies, the venue operator has to issue each user with a receiving unit (and get it back from the user afterwards!) The receiver unit often couples to
the hearing aid using a small induction loop worn around the neck.
Note that, because each brand and variety of IR or FM system works differently, a different
receiver is needed each time. Users cannot (and do not) carry around a receiver for FM or IR
systems, although some will have direct audio input (DAI) leads to link the venue's receiver to their hearing aid.
Are Infrared and Radio (FM) systems any real alternatives?
Neither Infrared or Radio systems can replace induction loop systems.
Infrared has the specific advantage that the signal does not cross walls and hence provides a
very high level of confidentiality. It can also be used in multi-channel systems for simultaneous translation, where it is used purely as a communications system.
It suffers quite badly from shadowing, offering many situations in rooms where the signal is lost
. Special receivers have to be issued which draw attention to the hearing disability. There are also very serious concerns about the standards of hygiene; have the receivers really been
cleaned and disinfected? The cost of these processes is a significant expenditure for the operator of the facility.
Radio systems are even less attractive. Apart from the negative user response noted above, there is a major problem with signal loss. Professional radio microphones use diversity
reception to reduce signal loss due to reflection of the radio signals from walls, etc.. This is not
possible with the radio receivers used for assistive listening. Furthermore, there is a major problem with shortage of frequencies and confidentiality is totally non-existent.
In comparison, induction loops have the following advantages:
- Uses built-in T coil in hearing aid
- Utilises internal tonal correction
- No additional receiver needed
- Hygiene problems eliminated
- No loss of special receivers from venues
- Will work in conditions of bright light and outside
Are all hearing aids compatible with Audio Induction Loops? Top
Sadly, not all hearing aids are fitted with the loop facility. In the UK, almost all NHS aids are
equipped with a 'T' position, as are many privately sold aids. In the UK private sector, it is
often the audiologist who decides whether to offer the loop reception facility, but generally
they do offer aids with a 'T' setting. At present, about 95% of hearing aids in the UK are said to have the loop receiving function.
In the USA, audiologists do acknowledge the benefit of the 'T' facility, however up to 50% of aids sold in the USA are without the 'T' coil facility.
The situation may vary in other parts of the world.
What about Digital hearing aids?
Digital hearing aids work in exactly the same way as ordinary analogue aids in terms of
induction loop use but you must make sure that the digital hearing aid has a 'T' switch position
. As far as we are aware, all digital hearing aids supplied by the NHS (National Health Service)
in the UK have a 'T' coil facility. Privately dispensed digital aids may or may not have a 'T' coil.
As policies over 'T' coil provision in hearing aids vary around the world - check with your audiologist about this before you buy, as it may affect what they offer to you.
Many digital hearing aids allow the option of setting the relative levels between microphone
and 'T' coil inputs to be adjusted by the audiologist. If the loop signal is quiet / loud relative to normal microphone use, ask your audiologist to adjust it for you.
The international standard governing the use of induction loops (IEC60118-4) requires that the loop coil be vertically orientated to pick up the magnetic signal. Regretably, IEC60118-1
which applies to hearing aids, does not define any orientation. Some hearing aids are available with a pick up coil adjusted for reception of horizontal magnetic fields and these may
give poor results even when used in a correctly installed loop system unless you bow your head forwards to face the floor. Ampetronic are currently researching this effect and would
welcome your comments if you have experienced this problem. Please let us know the hearing aid manufacturer, model number and date of purchase for our records together with a brief
description of the exact circumstances under which the problem arose. Tell us.
Always check with your audiologist BEFORE purchasing the hearing aid to ensure compatibility with induction loop systems.
Can I have a Digital Loop for use with my Digital Hearing Aid?
No - a digital loop would not be receivable by your hearing aid! Audio induction loops are a purely analogue technology, as is all sound. The way that the loop signal is transmitted to
your aid is an analogue signal defined by international standards so that you can use any good loop system.
Digital audio products have to convert the analogue signal into the digital domain for processing then return the signal to analogue for us to hear it. There is no such thing as a
digital headphone (unless you possess a pair of digital ears!)