Numerical simulation, experimental comparison with noise measurements and sound synthesis of airframe noise

In this paper the airframe noise of civil transport aircraft is numerically evaluated and compared with measurements taken in some European airports with the target of separating the airframe contribution from other noise contributions that can be identified on a typical aircraft. Then the numerical results have been used for generating the typical sound track due to the aircraft flyover: in this case, only the airframe noise has been taken in account and the resulting sound would be the one perceived by an observer if the engines were not running. The attention has been focused on airframe noise since it sets a lower limit below whichever reduction of noise generated by engine have no significant effect on the overall noise level perceived by an observer and due to the aircraft flyover. The intensity of airframe noise depends on the aircraft configuration: an aerodynamically ‘clean’ configuration produces less noise than the configuration assumed by the aircraft during landing and take-off, that usually is referred as ‘dirty’ one, with slats extended, flaps down and undercarriage lowered. The main result of this work has confirmed that during landing the slats and flaps give the most important contribution to airframe noise. A computer program has been developed for obtaining the free field noise levels emitted by an aircraft during a flyover. These emissions have been then corrected taking in account the atmospheric absorption and the ground reflection. Then LMS International has generated the airframe sound using third octave spectra provided by the University of Naples, through a sound synthesis approach already used in other applications (vehicle road noise, helicopter noise and so on). The obtained sounds have been perceived as highly realistic and can be widely used in psychometric tests.