Active Induction Loops

Inductive loops are one method of allowing the users of hearing aids or other types of hearing devices to hear sounds they might otherwise miss. The basic principle of the loop system, is a loop of a wire or wires which is laid at floor level, wall, or along the ceiling and is controlled by a constant current amplifier. This creates a magnetic field, which induces a voltage into the T-coil of the hearing aid or inductive receiver. The hearing aid, then converts this voltage into an audio signal.

The design of the loop is quite straightforward and obtaining the required field strength is a simple calculation. The first task is to measure the complete run of the cable, including all detours such as doors and windows. The loop should be laid at the edge of the listening area at a distance of approximately 1 - 2 metres from the hearing device, vertical runs should be avoided. Depending on the size of the loop amplifier to be used, the resistance of the loop must be kept within the manufactures parameters of the amplifier. To achieve the required loop resistance it is matter of choosing the correct gauge wire and the number of "loops" around the listening area.

Strands	Ω/100m
1/0.1mm	219
1/0.2mm	57.6
1/0.6mm	6.4
1/0.8mm	3.6
7/0.2mm	8.2
13/0.2mm	4.4
16/0.2mm	3.6
24/0.2mm	2.4
32/0.2mm	1.78

Cable Resistance Table

If there is a high level of magnetic noise in the listening area. This can be generated by light dimmers, fluorescent fittings, high power mains or data cables, this may impair the operation of the loop. Other consideration to be taken before the loop is installed. Will there be any devices inside the loop which will be effected by the magnetic field, guitar pickups, unbalanced microphones or audio equipment, as this could lead to operating problems with this equipment.