How To Design An Induction Loop System (ILS) This page shows a typical design process (and hence some of the processes used in the design software on this site) for any of the medium sized venues above, seating around 50 - 150 people. It goes through the design method and gives a fully worked solution including a wiring layout diagram and equipment list. The key to getting a correct solution is to understand fully the implications of metalwork in the structure and then install the loop wiring in the right place. Small variations in building geometry, installed loop height and the listening plane height make quite significant changes to the design and no single element can be excluded if you are to reach the correct solution. Note also that there can be a number of ways to achieve the desired result. Induction Loop Systems Design - Typical Church Hall, Meeting Hall, Kingdom Hall Installation (L1E & Varients) For the purposes of this illustration, we will assume a modern single story standard construction of building with a reinforced concrete floor, brick or any other type of walls and a metal grid office type suspended ceiling with drop in tiles at a height of 3m. The room to be covered has an area of 11m x 15m and has a carpet floorcovering stuck directly onto the concrete base. Seating will be arranged in rows though the seating layout may change from time to time. There are no "low spill" issues associated with a need to confine the loop magnetic field. At the outset it is most important to understand that the physical construction of the room has a dramatic effect upon loop performance and that ignoring this aspect of design will lead to a poor or reduced quality installation that will not meet standards. Hence the emphasis is on doing some background research into the amount of metalwork BEFORE designing your loop system - this cannot be over emphasised. Having determined how much metalwork is present, we can then move on to place the room design into one of three categories. For categories 1-3 we can then specify the correct loop amplifier and wire layout to install. All you need to do then is order the parts and install the system - easy!
The Design
First we need to analyse the structure to determine how much metalwork if any is present and then which category the hall falls into. After that, we can work out the losses due to Aspect Ratio and/or Vertical Displacement, wiring inductance and feeder cable. These losses are cumulative and some are also frequency selective. All losses imply extra current (larger amplifier) and frequency selective issues addressed with MLC (Metal Loss Correction).
The next section is broken into a series of simple questions - if unsure or if you want to err on the side of caution, answer Yes to the question and follow it through. All the terms are referenced out if you need further help.
Our example is an obvious Cat3 installation from following the questions above. If you ignored this design process and fitted a Cat1 or Cat2 single perimeter loop to our example you would get a very uneven, weak magnetic field strength across the dimensions of the hall together with a large loss of high frequencies. In other words the sound would vary from inaudible to quiet, would be muffled to anyone listening and would be worse in some places than others - exactly what we do NOT want! Now we need to work out how many sections the two arrays need and what the spacing will be. How to do this is shown here.
Having worked out the number of sections and a wiring plan we can move on to select the right equipment to use.
To do this we need to workout the total wire length for one array. For our example with two sections this will just be a case of adding up all the sides and ends together with the length between array and amplifier. Always err slightly on the longer side if in doubt. For our example the length will be: (4*longest side) + (2 * (loop width + gap)) + (2 * gap) + tails (connection between array and amplifier). This is: 60 + (2 * (3.29+1.84)) + (2 * 1.84) + 5 = 78.94m (80m). Note that we can install the wiring in one of two places (see VD), either at the lower level (floor) or higher level (ceiling) and that this will influence the choice of cable we specify. Floor level implies flat copper foil but ceiling level can use ordinary stranded cable. The choice is entirely down to local conditions, but the preference would be for a floor array - but be careful of carpet fitters who need warning not to cut the wiring when fitting the carpet. A thin layer of hardboard stuck on top of the array can be very effective.
The area of our room is 11 * 15 = 165m², the wire length 80m and the aspect ratio = 11/15 = 0.73. The table here shows us we need to use an ILD252 amplifier and the table here shows we can use any cable between 0.75mm² and 4mm² or the flat foil. Of course this is an array design so we double up on cable and use two amplifiers. To complete the specification we need a SP5 phase shifter.
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