Review of a Method for Local Damping Identification in the Low- and Mid-Frequency Range Based on a Finite Element Model

Kurzfassung

Passenger demand for enhanced cabin comfort have made cabin noise to an important factor. The damping behaviour of the cabin must be understood in order to predict noise emission. Due to the high modal density and overlap, state-of-the-art methods are not able to accurately identify damping in the mid- and high-frequency range. However, these frequency ranges are relevant in the field of acoustics. 2016 a method was proposed for local damping identification, which was applied to an aircraft fuselage. A spatial distribution of damping was calculated in the low-frequency range. This calculated damping distribution supports damping modelling in numerical models in order to achieve more accurate results in the low-frequency range. This method needs to be extended to the mid-frequency range for accurate predictions of cabin noise. The present work reviews and tests this method with numerical examples. A plate representing a simplified aircraft fuselage skin field is taken as model. Three test cases were defined to test the method. Homogeneous structural damping is applied to the first case. In the second case a patch with higher structural damping is added. The patch is replaced with viscoelastic material in the third case. The applied damping distribution in the first case is homogeneous and in the second and third case the patch should show a different damping behaviour than the rest of the plate. The calculated damping distribution in each case, however, does not reflect the applied damping distribution. It is shown that the calculated damping distributions relate to the mode shapes. Especially the nodal lines and closely spaced eigenfrequencies yield a calculated distribution of damping, which is not driven by physics. The interaction between closely spaced eigenfrequencies disturb the decay of the vibration. Thus the damping ratio is identified wrongly. Finally the damping identification in the mid-frequency range is tested. Instead of a local damping identification per frequency, a global equivalent identification in a frequency band is investigated. Therefore, the calculated damping ratio is averaged in frequency and in space, so the measurement of several points is necessary. The calculated damping ratios are close to the values applied.