Inductive loop systems: Inductive transmission of audio signals directly to personal hearing devices – acoustic accessibility and inclusion in public areas
What is an induction loop system?
Using an inductive loop system, wearers of hearing-aids can pick up the signals emitted by electric/electronic audio sources (microphone or hi-fi systems) directly in their hearing aids or CI systems. As a result, they hear the sound without any annoying background noise, echo or reverberation. An outstanding advantage of using induction loop systems: no special receivers are required. All that’s needed is the hearing aid.
Most modern hearing aids are now equipped with adjustable »M« and »T« functions. »M« stands for microphone, i.e. hearing the sound in the room via the integrated microphone, and »T« (telecoil) means that the audio signals are directly transmitted by induction via a receiver coil integrated in the hearing aid. Some hearing aids are also equipped with an »MT« function, which combines both types of reception - microphone and telecoil - so that the listeners are able to hear the sound in the room as well as the inducted signals.
HUMANTECHNIK is at your side - with full service.
The planning, design and installation of acoustic transmission systems often require special knowledge. For this reason we offer a comprehensive service package for planners and operators in this sector. From basic information up to qualified expert design assistance to installation and commissioning, we accompany you with our consultation service for the effective setup and use of professional audio transmission equipment.
How does an inductive loop system work?
Alternating current creates a magnetic field in a wire/a loop. If a second wire is introduced into this magnetic field (here: the telecoil in a hearing aid), a correspondingly alternating current will also be created in it: the electrical impulses are »induced« into the second wire without any actual direct electrical connection.
An inductive loop system consists of a loop and an amplifier. The loop - basically an insulated wire - runs around the outside of the listening area. The amplifier, connected with an audio source (hi-fi system, speaker’s microphone, etc.), transmits the acoustic signals, converted into alternating current, into the loop.
The magnetic field generated inside the loop allows the listeners to move freely within the room and pick up the audio signals, which are fed directly into their hearing aids. The user needs no further equipment.
Planning induction loop systems
In order to install a inductive loop system correctly, it is necessary to take into account many different factors – with respect to the architecture as well as the area in which they are to be used. Some areas with special requirements include:
- Rooms without equally rectangular or square floor plans
- Walls with strong steel reinforcement or electromagnetic fields from other sources
- Theatres / cinemas / lecture halls
- Rooms in which the loop is positioned far away from the listening level
- Buildings in which several independent loops have to be installed close to one another without interfering with one another.
Installation tips
Loop position: In order to create an even magnetic field, the distance between the loop level to the normal listening level should be between approx. 1.2 and 2 metres.
The position and size of the loop can also be determined by the position of the listening area (often much smaller than the size of the room) or the position of the microphones as well as other technical equipment that affects the magnetic field.
If possible, avoid using dynamic microphones in combination with induction loop systems. If they cannot be avoided, they must be positioned outside the loops. The distance between the loop and the dynamic microphone should be at least half the diameter of the loop.
Using a condenser or electret microphone usually produces better results with respect to interference or acoustic feedback. A microphone with a high-quality, insulated connection cord also generally reduces magnetic feedback.
Reduce the area of the loop! The connection wire to the loop must be firmly twisted or closely parallel.
Make a test installation. Always test the loop system to make sure that it is powerful enough, that the overspill does not cause any problems, etc.
Adjust the height of the loop (1.2 to 2.0 m) to provide for the greatest possible range with the smallest amount of overspill.
Conducting materials, such as reinforced concrete, can increase or decrease the area covered by the loop. The magnetic field strength is often increased outside the loop, while it is decreased inside. This fact considerably increases the risk of acoustic feedback. In a new building, you should try to install an LOS system. The LOS system makes it more difficult for the current to connect with large metal objects.
Pay particular attention to metal frames. Do not fasten the loop to structural elements made of metal or to similar objects.
Selecting an amplifier:
| Area used | Amplifier |
|---|---|
| Reception, counter: | LA-90 »Cross-the-Counter« |
| TV rooms, nursing homes, living rooms: | Pro Loop C |
| Conference rooms, theatres: | Pro Loop C Pro Loop DCCplus |
| Very large induction loop systems: | Pro Loop DCCplus Pro Loop LOS |
| Systems with little overspill: | Pro Loop DCCplus Pro Loop LOS |
| Portable induction loop system: | loop system kit LA-90 |
This table serves only as general orientation for selecting the amplifier. Please take advantage of our consulting services before you make your purchase.
Low-overspill systems (LOS): Inductive loop systems with low overspill
Standard induction loop systems produce a magnetic field that usually exceeds the »core service area«. This »overspill«, which travels horizontally and vertically, might still be audible three to four loop widths away. This limits the use of this technology, especially if multiple loops are to be installed in a building or if confidentiality is to be considered.
Low-overspill systems expand the options for the application of inductive loop systems.
Low-overspill systems (LOS) reduce this »overspill« considerably. They use special loop patterns to reduce the signal strength outside the loop. For these patterns, two loops have to be operated out of phase with each other to achieve an equal distribution of the desired signal without creating any undesirable underfed areas.
An LOS system consists basically of two induction loop amplifiers that are operated out of phase. This configuration prevents overspill and makes it possible to operate several induction loop systems in adjacent rooms without having their respective signals interfere with each other:
- No field strength reduction in the centre of the loop due to reinforced concrete,
- Greater reproducibility of the results,
- Reduced danger of feedback due to lower power consumption,
- Extremely low overspill at reduced directional sensitivity.
HUMANTECHNIK service for planning low-overspill systems
Each individual optimal customisation and configuration depends on the conditions prevailing in the intended area of application, in other words, upon the room's shape and the sizes of the coverage areas, the number of internal loops and the demands towards low overspill.
Determining the corresponding values for the »customised« low-overspill layout and the optimal level adjustments in each case are based on a complex method of calculation and installation. That’s why we strongly recommend that you take advantage of the services offered by HUMANTECHNIK. We provide qualified support to architects, technicians and installers in planning LOS installations.
The overspill (Crosstalk) of standard induction loop systems works in a horizontal and vertical direction. The resulting »eavesdropping effect« is often still measurable over distances of up to three induction loop widths.
Low-overspill systems (LOS), that are competently customized and configured reduce overspill considerably to allow the operation of inductive loop systems in directly adjacent rooms.