The airframe itself can strongly influence the noise radiation from aircraft. Not only can the airframe through diffraction, refraction and reflection modify the noise radiated by the engines but it also acts as a strong source of additional noise radiation either directly from its components (for example the undercarriage and wing with high lift devices deployed) or indirectly by modifying or coupling with certain engine flow features. To date such sources of noise whilst important have had limited impact on the overall optimisation of the aircraft. However, the trend towards very high by-pass ratio engines with their differing source breakdown coupled with the trend to aircraft of larger size offers the prospect that future new aircraft designs may well be limited in realising their full operational efficiency and hence competitiveness by such acoustic effects.