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The purposes of this AIR are to provide technical background associated with the problem of correcting measured noise spectra to the "free field" wherein the measurements are taken in the proximity of a ground plane; to provide practical examples of correction methods; and to present some techniques for minimizing the errors associated with the measurement of noise in the presence of ground planes. The application of this AIR should assist the acoustician in understanding the spectral effects of the reflection phenomena and suggests some methods for obtaining more accurate "free field" acoustic data; however, the AIR does not purport to provide unique, exact solutions to all measurement problems, for each measurement situation is unique in itself. Of paramount importance to the correction procedures is a good knowledge of the acoustic impedance of the ground plane and the geometry associated with the measurement setup. A method of measuring these ground plane characteristics is described.