The interaction between Tile Surround and Cut-off distance is important and mistakes in setting these can lead to calculation error.
Firstly, it should be remembered that only noise sources within the loaded tiles are considered during calculation. If a tile is not loaded for a calculation, then any noise source within that tile will be ignored, even if it is very close (eg just over the tile boundary from the calculation point.
Secondly, it should be remembered that the cut-off distance is the perpendicular distance from the source line. It is possible for a source line several tiles away from the calculation point to be aligned towards the calculation point and thus at a very small perpendicular distance. However, if its tile is not loaded, it will not be included in the calculation.
Thirdly, very noisy sources may contribute a significant amount of noise at a calculation point even when the separation distance is large. This is because, for line sources, the rate of decay is very slow once the distances become large - for example, there may only be a 3 dB reduction between 1 km and 2 km.
The effects will differ from case to case. But they have become more critical - especially for roads with very high traffic flows where noise changes can occur at large distances - since the latest edition of DMRB requires the number of properties with a 1 dB change in noise level to be assessed.
To set these values correctly, firstly you should decide the appropriate value for the cut-off distance. This will depend on the traffic flows and density of the road network. For a single major motorway, then a cut-off of 2,000 m may be needed. In this case, you will need a surround of 4 tiles. This will mean that an area 9 tiles square (81 tiles) will be loaded, with calculations taking a relatively long time.
However, suppose you are limiting your study area to 600 m each side of the road in question. Can you use a smaller cut-off distance or surround? In this situation, you will only need to calculate for two tiles each side of the road, although probably these will fall in a staircase pattern so you will need to choose them carefully. In this case, I would be inclined to limit the calculations to a 2-tile surround, and 1 km cut-off distance. However, I would then look carefully at the contours to see if there are obvious edge-effects, especially at the tile edges. (note that there will be a clear edge effect at the 1 km boundary, but this is not important as it is beyond your study area).
Note that it is still possible to get tile-surround edge effects even when adopting this strategy. These are not always easy to see. However, if you load the contours and then select to 'calculate database contours' you will be presented with the 'check edges' dialogue. This enables you to check for adjacent tiles where the noise levels differ by more than a certain amount on the touching edges. You can then increase the surround and submit these tiles for recalculation.
A downside of this approach is that tiles surrounds can keep creeping up, with the calculation time getting considerably longer each time, so this is not particuarly efficient. It's better to choose a big enough surround from the off. It will save time overall if you make a few tests on this before commencing the production run.
You should note that the same considerations apply to individual receivers as well as contours. To ensure consistency, you must use the same tile surround and cut-off distances for receivers as you did for contours. Note that the tile surround is not remembered by NoiseMap between runs, so take care that you set this to the right value each time.
