BASIC INFORMATION

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 Induction Loop?
How does an Induction Loop work?
Why use an Induction Loop?

Where are Induction Loops used?
Why Use Current Drive?
Why Use AGC?
Are there conditions where an Induction Loop is not suitable?
Do Induction Loops Interfere With Heart Pacemakers?
What are the alternatives to an Induction Loop System?
Are infrared and Radio (FM) systems any real alternatives?
Are all hearing aids compatible with Audio Induction Loops?
What about Digital hearing aids?
Can I have a Digital Loop for use with my Digital Hearing Aid?

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? Top

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 Use:

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
Cinemas
Lecture theatres / auditoriums
Seminar rooms
Churches, places of worship
TV lounges (in nursing homes & hospitals)

As explained elsewhere, the basic principle of an Induction Loop system is that an electrical current through the wire creates a magnetic field which is picked up by the hearing aid. There is an International standard (IEC 60118-4), which establishes the intensity of the magnetic field, and the frequency response needed from the system. This specifies that over the range from 100 Hz to 5 kHz, the signal will be within the limits of ± 3dB relative to the signal at 1 kHz.

A significant amount of research has also shown that speech, in the short term where intelligibility is crucial, requires that the system must handle full power signals up to at least 1600 Hz.

Loop systems have as a fundamental aspect, the fact that a definite length of cable is used. Some designers and contractors ignore the fact that this wire length has a definitive inductance. This component is of such magnitude that audio signals are affected. The use of multiple turn loops has a very serious effect, because the impedance of this inductive component increase by the square of the number of turns in the loop, while the signal strength increase only by the number of turns, for the same current. As the impedance of the loop, due to this inductance, increases with frequency, the serious problem arises that when the loop is driven by an amplifier designed for driving loudspeakers at good quality, the output current when connected to the loop reduced significantly with frequency.

The table below gives the frequency at which the response is down by 3 dB relative to reference, and this problem cannot be resolved easily by simple "Tone Controls" . The table also indicates clearly the immense loss created by multi-turn loops in such a situation. The mathematical basis for the table is simply the ratio between resistance of the loop, giving the base current when driven by an amplifier designed for good loudspeaker damping (low internal impedance), and the magnitude of the loop impedance, made up from both resistance and inductance.

As shown clearly, the frequency response requirements of the standard simply cannot be met.

Cable section (mm²)	Single Turn Loop	2-Turn Loop	3-Turn Loop
0.50	2864	1432	954
0.75	1910	955	637
1.00	1432	716	477
1.50	955	477	318
2.50	573	286	191
Cable Section AWG	Single Turn Loop	2-Turn Loop	3-Turn Loop
22	4052	2026	1350
20	2548	1274	1350
18	1772	884	591
16	1091	545	364
14	697	346	231
12	436	218	145
10	275	137	92

Using Constant Current feed removes the problem, as the special loop drivers as designed by Ampetronic control the current into the loop independent of the loop impedance. While there are limitations of cable length etc., as referred to elsewhere, the Ampetronic drivers easily offer the required frequency response.

Automatic Gain Control

All Ampetronic induction loop driver amplifiers include a high performance Automatic Gain Control (AGC) circuit. This, when combined with wideband audio circuitry, maximises both speech and music reproduction for hearing aid users. Whilst retaining normal speech dynamics to maximise expression and naturalness, the total dynamic range is reduced to a level that can be accepted by hearing aids without overload or distortion.

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 some 40 to 60% of aids sold in the USA are without the loop facility.

The situation may vary in other parts of the world.

What about Digital hearing aids? Top

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.

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? Top

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!).