CATEGORY 2 DESIGN

VERTICAL DISPLACEMENT

This refers to the distance vertically between the loop cable and the listening device (hearing aid). You might assume that best results would be obtained if both were at exactly the same height but this is NOT true. Best results are obtained when the loop cable is displaced by some distance above or below the level of the hearing aid. The very best position is a point 12 - 16% of the smallest dimension of the loop above or below LPL!

Suppose our room is 10m x 8m. Let's choose a mid value of VD say 14%.
14% of the smallest dimension (8m) would be 0.14*8 = 1.12m.

Now we need to find the height at which the hearing aid will operate most frequently. Suppose our listener is sat in a meeting hall on a standard chair. The height from the floor to the hearing aid (head height) will be about 1.2m for the majority of the time. We can either add, or subtract the vertical displacement figure (1.12m) to / from this figure (1.2m) to give the ideal loop cable height relative to the floor. Our two locations are:

1.2 + 1.12 = 2.32m or 1.2-1.12 = 0.08m. In other words 2/3rds up the wall or alternatively, on the floor.

If either of these two positions are unavailable, then you can go higher or lower (NOT closer to head height) BUT, you need extra power to drive the loop. The amount of extra power needed is related in a non-linear way to the extra displacement and implies a larger amplifier with perhaps different cable.

With a category 2 building the choice of loop position is determined by the metalwork. If you have a suspended ceiling, the loop cable will be fitted at the lower level.

If the floor is reinforced concrete, the loop will be at the higher level. If this proves to be impractical for aesthetic / cosmetic reasons, then you will need to install the cable higher, perhaps putting it above the ceiling. BUT, this may break the 12 - 16% Vertical Dislacement rule.

SUMMARY - Work out the vertical displacement and add / subtract from the listening plane dependant on where the metalwork is situated.
A VD greater than 25% needs additional design input - treat as category 4.
Cat 1 and Cat 2 VD is different to Cat 3.

ASPECT RATIO

Now we need to work out the Aspect Ratio (a) which is simply the shortest side ÷ by longest side. Suppose your room is 10m x 8m as above:

Aspect ratio = 8/10 = 0.8

We can use this value together with the room dimensions to choose the correct amplifier. Note that long thin rooms have a much lower aspect ratio and need greater loop currents to drive them.

So we need to take three things on to the next section:

1. Size of area to be covered.
2. Loop wire height relative to the floor.
3. Aspect Ratio figure.

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