Perimeter Loops

A perimeter induction loop is the simplest loop configuration and as the name suggests, it is a loop of wire marking the perimeter of the area to be covered. Sometimes, the wire is wrapped around the perimeter 2 or more times to make a multi-turn perimeter loop. Each turn has an equivalent affect on the field strength generated to doubling the current. Whilst in principle this seems like a good idea, the induction loop amplifier will run out of voltage headroom and will not be able to maintain the current output.

The impedance of the cable is the sum of the dc (ohmic) resistance/metre of cable, the inductance/metre of cable and for a multi-turn loop, the mutual inductance /metre of cable .The longer the cable, the higher the impedance. The last 2 parameters are frequency dependent.

Often the first sign you have that the amplifier is struggling to maintain a constant current is the clipping of the high frequencies, the resulting sound is muffled and intelligibility is lost.

From the field strength plot it is clear that the signal from a perimeter loop is still present outside the looped area (room) This signal, often called the spill field, can exist at a distance of 2 to 3 x the width of the loop in both the horizontal and vertical planes . As a result, a perimeter loop will not provide confidentiality and can not be used in adjacent rooms where induction loop sytems are to be used simultaneously.

The signal strength at the listening height must be 0dB +-3dB and according to the standard (IEC 60118-4:2006) must remain within these limits throughout the listening plane. So whether you are in the middle of the room or close to the wall, the signal strength is within this range and as a user, you do not need to adjust the sensitivity of your hearing aid.

A perimeter loop must never be installed at the listening plane.

From curve A on the graph, we can see that the induction loop must not be installed at ear height if the system is to meet this standard. Whilst the signal strength in the centre of the room may meet standard, the signal near the wall will be too strong (much greater than the 6dB allowed in the standard, 60118-4:2006 )

If the Loop is to be installed in the floor for seated use (Listening plane of 1.2m, the maximum width of the loop is 20 metres.

If the Loop is to be installed in the floorfor standing use (Listening plane of 1.7m, the maximum width of the loopis 22 metres.

The further away the loop is from the listening plane, the more even the field distribution is

and the more power needed to drive the loop. We can take advantage of this in some buildings such as churches.  By installing the loop at height, the simple perimeter loop can be used to cover a very large space.

Metal in the same plane as the loop can absorb the signal from the loop.

This absorption or loss of signal is dependent on its proximity to the loop, the amount of metal and its conductivity. Aluminium is more lossy than mild steel and stainless steel with the highest resistivity causes little or no loss. The phenomenon is also frequency dependent.

To overcome this issue, the first option is to drive more current through the loop and correct the frequency response. With the Power of Univox® and the MLC or tone control featured on all our amplifiers, this can work well.

The second choice is to move the loop further away from the metal. For example, installing the loop at ceiling level instead of around the floor. (Remember, there may be construction metal in the ceiling. A false ceiling using tiles on an aluminium frame which may be very lossy depending on its age)

Where the loss is too high and the signal too variable ( signal variation across the listening plane is more than 6db), smaller multiple loops must be used if the system is to comply with IEC 60118-4:2006.

A perimeter loop has a spill field 2 - 3 times its width

Spill field is the name given to the signal generated by the system that exists outside of the induction loop. Magnetic fields fall off very rapidly with distance, however, 2 - 3 times the width of the loop away, a signal level greater than -32dB may still be present. A room 5m wide may still be radiating above the acceptable background noise level 10m - 15m away.

(-32dB is the maximum level of background magnetic noise that the standard IEC 60118-4:2006 deems acceptable)

INFORMATION

Perimeter loops are clearly the easisest and cheepest induction loop type to install, but due to their constraints:- poor spill performance, restricted size dependent on proximity, type and fabrication of metal structures, field strength variation in horizontal plane etc, they are rarely the best solution and too often, they will not be capable of delivering standard compliant performance.

An induction loop system that does not and is not capable of delivering standard compliant performance may mean that the facility is still in breech of its duty to provide an equal level of access to everyone as set out in the Equalities Act 2010.

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