Influence of sound reflection on aircraft directivity and lateral attenuation on the ground

The calculation of the lateral attenuation correction for computing aircraft noise footprints on the ground, as proposed by Doc29 and AIR5662, is based on semiempirical methods not suitable for accounting for the particular characteristics of some of the novel aircraft designs under study; involving new configurations and propulsion and power systems. The lateral attenuation correction is known to depend on three factors: engine installation e_ects, ground absorption, and refraction and scattering of the sound. This study aims to numerically calculate the e_ects of the sound produced by the aircraft main sources (fan, jet) when reflected and scattered o_ of the wing, tail-plane surfaces and the fuselage of the aircraft. A simple 3D model geometry was used, approximating that of an A320, a MD11 and a MD83. Subsequently modelling the main sound sources as acoustic monopoles and using ray theory, a sound directivity field around the aircraft was calculated, which in turn was propagated to the ground. Then employing typical constant altitude flyovers, the Sound Exposure Level (SEL) were calculated and compared to an equivalent spherically symmetric source. It is shown that aircraft directivity is highly dependant on the nature of noise sources, their position relative to reflective surfaces and the geometry of those surfaces.