Noise contours, or statements of the noise level heard at various positions on the ground around an airport, are computed rather than measured because of the large areas of ground covered and the length of time over which noise data have to be averaged.

The noise at any point on the ground in the vicinity of an aircraft operation will depend upon a number of factors. Uppermost amongst these are the types of aircraft and their power-plant; the power, flap setting and airspeed conditions throughout the operation; the distances from the points on the ground to the aircraft; and the effect of local topography and weather on propagation of the noise. Most airport operations will include a variety of aircraft and flight procedures and a wide range of operational weights. Because of the large quantity of data (which can be regarded as aircraft or airport specific) required to compute the noise of each individual operation, it is normal to make certain simplifications to reflect average noise exposure over long time periods. These time periods may range from as little as one day to several months.

The normal process of computation embraces three main steps:

1. the calculation of noise level from individual aircraft movements at a matrix of observation points around the airport;

2. the integration of all the individual noise levels over a defined period of time; and

3. the interpolation and plotting of the information in the form of noise contours.

The simplifying assumptions that are most frequently made include the noise levels of “groups” of similar aircraft types, average climatic conditions and the average operational pattern over the time period in question.