The propagation of noise between a source and receiver close to the ground is affected by a number of phenomena. These propagation effects are extremely important in the evaluation of aircraft engine noise in two areas: first, in evaluating the change in acoustic characteristics produced by a source configuration change, and second, in extrapolating measured data to distances remote from the original measurement point.

An extensive body of literature has resulted from investigations of noise propagation phenomena. The Bibliography in Section 9 contains many of these reports, particularly those related to ground plane interference. These propagation effects include atmospheric attentuation of sound, scattering of noise in a turbulent atmosphere, refraction of noise due to wind and temperature gradients, scattering by the ground plane surface, and reflection/absorption by the ground. This latter phenomenon is the subject of this AIR. The basic analysis is straightforward for the ideal case of an infinitely hard surface, a point source of broadband noise, and a stable homogeneous medium.

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 situtation 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 in Section 4.2.2.

Section 5.2 presents a brief discussion of reflection effects in flight noise measurement, and provides some recommendations for ground test techniques aimed toward improving the ability to predict in flight noise.

Section 6 of this AIR provides an insight into the limitations of the current state-of-the-art of measurements and corrections as they pertain to this AIR, and in addition provides recommendations as to additional work that might be carried out to further improve the methods outlined in this AIR.