Lnight - do I subtract 2.5 dB to get free-field values as advised in DMRB?
This depends on which Lnight values you are using. In essence, DMRB is saying that the nuisance values are based on L10 18-hour facade values, whilst the Lnight to be used in the impact assessment is a free-field value.
So, if you are using individual receiver noise levels and have set these to include a facade reflection you DO need to subtract 2.5 dB from NoiseMap's Lnight value.
However, the Lnight contours calculated by NoiseMap do not have a facade correction included and so do not need to have 2.5 dB subtracted.
The same point applies to the Lden values - the contours are 'free-field' values, but for individual receiver points, it depends on whether the receiver is designated as being a facade or free-field point.
