Angular and distance dependence of the standard deviation of maximum sound level for aircraft noise

摘要

The current method to calculate number above threshold (NAT) in Sweden does not take into account the natural variation of sound level arising from variations in the sound emission and sound propagation. This leads to unrealistic discontinuities in the calculated noise contours as all movements of a specific aircraft type with a specific procedure are assumed to generate the same maximum sound level. Using long term measurements, the standard deviations of maximum sound levels are analyzed after dividing the airspace around the microphones into sections depending on the elevation angle and distance from aircraft to microphone. The standard deviation in each section is determined by calculating the standard deviation of each aircraft type that passes through that section and combining the aircraft type specific results into one figure. The standard deviation varies from about 1 dB to 3 dB with generally higher values the greater the slant distance and the smaller the elevation angle. The found standard deviations are presented in polar diagrams and are implemented in NAT calculations. The resulting noise contours are not only smoother and without discontinuities, but they are also a better representation of what NAT stands for.