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Data presented in this AIR are in terms of differences between one-third-octave-band free-field sound pressure levels and are intended for use in estimating far-field aircraft noise. Allowance may be made for the acoustic impedance of the ground surface and sound pressure levels at any height above the ground plane may be estimated. In estimating the long-term time averaged sound level at several locations around an airport served by a mix of airplane types under a variety of atmospheric conditions, airplane operational weights, and flight procedures, it would be more appropriate to use the method of prediction for lateral attenuation given in reference 1.