It is easy to overlook the Calculation Parameters window, but it is a vital part of the calculation set-up.
When you create a new database, it will be given default calculation parameters, but these may not always be suitable.
The cut-off values are crucial to calculation accuracy.
By default NoiseMap sets the Calculation Parameters distance cut-off to 1000 m and the cut-off angle at 1.0 degrees. However, motorways create considerable noise much further away.
If you have very short road segments, then that at a distance, they may subtend less than 1 deg angle of view at the calculation point.
When calculating noise contours for motorways, I would be inclined to use a cut-off distance of at least 2000m and a tile surround of 4 to avoid edge/cutoff effects.
Remember that if you need to include a tile surround of, say, five tiles, so that you include a major noise source, you also need to set the cut-off distance to correspond. Tiles are 500 m square, so across a diagonal the distance could be more than 2500 m if the surround is five tiles.
Plus if segments are very short, I would reduce the cut-off angle to say 0.5 degrees. Then I would run run a ‘segment contribution’ calculation to check that the major roads are properly included.
The downside is that calculation times will increase, although with faster computers this should be less of a problem.
Roger.
