At considerable distances from a noise source, the specific noise is masked by ambient activities and the calculated noise level (which ignores the ambient activities) is no longer relevant. This would mean that when subtracting two scenarios, any apparent change in noise level is simply a calculation artefact. In order to avoid this, you can set a noise floor in the compare contour dialog box. Where calculated noise levels are below this noise floor, the change in noise level will be set to zero, irrespective of calculated levels.
