Problems with the INM: Part 3 - Derivation of NPD Curves

The standard procedure used in the US for derivation of aircraft noise power distance (NPD) curves is to conduct a measurement at one position and extrapolate those results by general acoustic formula with corrections for atmospheric attenuation at standard atmospheres. Initial work undertaken for helicopters operating from military bases in Australia found different rates of attenuation for slant distances less than 1,000ft versus that for slant distances greater than 2,000ft. The position of the aircraft when creating the maximum level at a receiver location is not physically at the slant distance used on an NPD curve for that same position. SAE/FAA procedures for conducting/calibrating NPD curves from field measurements do not exist and from FAA advice, no such work has been carried out. Examination of military jet aircraft and military helicopter operations in Australia, with DGPS positioning and atmospheric attenuation corrections identified in the previous papers has resolved this issue. The NPD derivation investigations are discussed. INM & NPD CURVES The Integrated Noise Model (“INM”) is a computer program used throughout the world for producing noise exposure contours around an airport. To generate an INM contour it is necessary to identify the aircraft types to be operating at the subject airport, the flight tracks and power settings of aircraft utilising those tracks, the direction of aircraft operations (along such flight paths), and the number of movements.