Prediction of interior noise of a small aircraft

The aim was to determine, already at the design stage, the interior noise level in a new small aircraft. The noise predictions were based on full-scale measurements on an existing model. A number of models for the prediction of velocity levels of a fuselage excited by turbulent boundary layers are discussed. Predicted results are compared to the result of in-flight measurements. It is found that the Corcos model gives the best agreement with the measured results. It is found that the velocity level of the plate elements of the fuselage very much depends on the speed of the aircraft. An increase of the speed by 10 % increases the plate velocity level by 3 dB. The velocity level is also strongly dependent on the plate thickness. An increase of the thickness by 50 % will decrease the velocity level by 5 dB. Small variations of other parameters like width and height and curvature of plate have only a very marginal effect on the plate velocity in the frequency range of interest. However, the acoustic power radiated into the aircraft cabin depends on the plate velocity of the fuselage as well the sound radiation properties of the plate structure. In the frequency, range of importance for the A-weighted noise level in the cabin the sound radiation ratio is increased as the plate thickness is increased. The noise level in the cabin is thus reduced much less than the velocity level of the fuselage by increasing the plate thickness. An increase of the width and length of plate elements reduces the sound intensity radiated by a plate. A combination of increased width and length and thickness of plate can result in a reduced noise radiation from the fuselage and thus in a reduced interior noise level.QC 20140102