Uncertainty of Standardized Aircraft Noise Descriptors Evaluated from Measurements

The standards ISO 20906 (2009) and DIN 45643 (2010) define a set of descriptors for aircraft noise impact which are to be evaluated on basis of measured sound pressure levels. As boundary conditions fluctuate, in general randomly, the magnitudes of those descriptors also vary correspondingly. Hence they reveal an inevitable uncertainty whose common basic measure is the variance. The specific variance algorithms are outlined and - in a further step - the confidence intervals. To evaluate the uncertainty of the averages the variance of a sum is performed explicitly, not as usual the sum of the single variances assuming a priori statistical independence. This straightforward leads to the covariance matrix whose n•(n-1)/2 distinct elements quite comfortable can be calculated numerically by computer. By this way an eventually existing autocorrelation can be uncovered for sake of a realistic presentation of the uncertainty. A further aspect to be accounted of in the correct description of variance when measuring over finite time intervals is the variation of the event number. This aspect is outlined for some types of time series of the single events. Examples for the evaluation of short- and long-term measurements in the vicinity of an airport in operation are presented.