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Neither Infrared or Radio systems can replace induction loop systems. Other technologies do have their advantages, however induction loops systems are the most versatile technology available, suitable for the broadest range of applications.
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:
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.
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 a hearing aid to ensure compatibility with induction loop systems.
A permanently installed induction loop system will not interfere with heart pacemakers if correctly designed and installed.
It is possible that interference could occur due to a neck loop as provided with some receivers (for example with Infra-red, FM or similar systems). It is necessary to pin the neck loop away from the location of the pacemaker ensuring 150mm or 6’’ separation between cable and pacemaker.
Interference could also be caused due to portable loops where the loop cable could be held close to the pacemaker.
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Automatic Gain Control is standard in all Ampetronic equipment. AGC automatically adjusts the output level of a loop amplifier to retain a constant level while retaining normal dynamics of speech and music. AGC provides a dynamic range that can be comfortably received by the hearing aid, and provides excellent intelligibility for the hearing aid wearer over a wide range of input levels. AGC is a fundamental part of creating an assistive listening system that is beneficial to the hearing impaired. The international standard for induction loop systems IEC60118-4 does not mandate the use of AGC, however it is practically not possible in normal circumstances to meet the requirements of this standard without gain control.
Current Drive in Induction Loop Amplifiers
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-tun 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.
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induction loop systems
University of Essex
