However, although the principle of this system is simple, the practical conditions for implementing BIM in public rooms must meet a number of requirements; Too often BIMs installed in places do not function well, because they are installed by technicians (not practicing conformity tests) who are not qualified in this field or who do not work because the technical staff of the room does not have the necessary information.
The magnetic loop is the subject of an Afnor standard, which focuses little on its constituent elements but especially on the result (bandwidth, uniformity of the frequency response curve, measurement of the amplitude of the field) obtained, the obligation is of result. For the boucle d’induction magnétique this is important.
The term “loop” can be misleading because belting a room with a “perimeter” loop does not always make it possible to have a uniform field. In these cases the loops will have different paths. In reality we speak of loop because the ends of the wire constituting “the antenna” are brought back to the outputs of the amplifier, forming a loop.
It is often difficult to choose a loop amplifier on the sole basis of the surface to be covered because, for the same surface, depending on the size of the loop, its size, and its environment, the characteristics of the amplifier could be very different.
A little bit of technique
The system consists of the loop itself, made of an insulated conductor, and a dedicated amplifier because a conventional amplifier, even provided with an impedance adapter transformer, PA is not suitable. The low frequency audio signal that travels through the loop creates a magnetic field directly captured by hearing aids equipped with a T position.
The magnetic field generated by the loop is not affected by insulating materials such as glass, stone, wood, etc. but it is however disturbed by the presence of metal. The metal causes losses, distorts the magnetic field and modifies the audio response.
Two scenarios can therefore arise:
- the metallic disturbances are non-existent or very small (old buildings)
- the metallic disturbances are no longer negligible (modern structures with reinforced concrete, metal steps, suspended ceilings on metal frame, metal walls).
When the metallic disturbances are negligible, it is often possible to install simple perimetric loops that can reach quite large dimensions.
It is however imperative to respect a suitable height difference of the loop with respect to the listening plane in order to obtain a regular magnetic field. When the dimensions of the area to be covered are suitable, the loop can be placed at ground level. A simple loop can possibly surround a building from the outside or be buried.