CLASSIFICATION OF THE MID-FREQUENCY RANGE BASED ON SPATIAL FOURIER DECOMPOSITION OF OPERATIONAL DEFLECTION SHAPES

Aircraft structures are characterized by their lightweight design. As such, they are prone to vi- brations. On numerical side there are several tools suitable for the response analysis of dynamic structures. Each numerical tool is more adequate for different frequency ranges. The Finite El- ement Method, for example, is the state-of-the-art numerical tool for the low-frequency range where modal density is still low and methods based on the modal approach are still appropriate. For the high-frequency range, the Statistical Energy Analysis is more adequate. This method anal- yses the energy transmission between substructures and requires high modal density of the sub- structures. The intermediate section between low- and high-frequency is called the mid-frequency range and is characterized by strong interaction of vibrations and acoustics. The mid-frequency range typically suffers from a lack of tools available. Improvement of tools for the mid-frequency range is highly important e.g. for predicting the acoustic comfort inside an aircraft cabin, where interaction between vibrations and acoustics can be observed. However, as a prerequisite it is nec- essary to determine the beginning and the end of the mid-frequency range. This paper addresses a new method to be applied in acoustic comfort analysis of aircraft fuselage. It utilizes dynamic response measurements conducted on an aircraft fuselage, i.e. a stiffened cylindrical shell which exhibits global and local dynamic behaviour in the mid-frequency range. Existing criteria for the determination of the mid-frequency range are reviewed, but fail in this specific application. Therefore, a new method for the classification of the mid-frequency range for a stiffened cylindri- cal shell test-structure is proposed in this paper. It is based on wavenumber spectra obtained from spatial Fourier decomposition applied to operational deflection shapes extracted from experimen- tal dynamic responses. Analysing the response characteristics of the different components of the aircraft fuselage a classification of frequency ranges is possible.