Defining a step change of aircraft noise in residential areas

Aircraft noise is characterized by intermittent noise events which vary in number and level from day to day, from one season to another, and usually show a gradual increase of the number of flights and a gradual decrease of the LAeq from year to year. Since a real or expected step change in aircraft noise often causes strong change reactions in residential areas, which cannot be predicted by studies in steady-state conditions, it is necessary to distinguish between gradual and step changes in aircraft noise. The paper discusses definitions of step changes from the perspective of residents with respect to (1) the mean LAeq for day or night, (2) statistical considerations, (3) the time history of changes, (4) the total number of aircraft operations, and (5) the distribution of aircraft operations during the day or night. It is concluded that a significant step change in aircraft noise can occur even in situations with no overall change in noise levels simply by changing the distribution of flights between day and night, or within daytime or night time hours.