An Extended Operational Modal Analysis Method Enabling the Separation of Whirling Direction Information in Rotating Machinery (Full Operational Modal Analysis Method)

Low-frequency broadband sound absorption with minimal dimensions and material cost is an ongoing research challenge in engineering acoustics. Common acoustic structures, such as microperforated panels (MPPs) and porous structures, are ineffective in alleviating low-frequency noise. In this context, a sound-absorbing panel consisting of two axially coiled-up tubes and MPP is proposed for effective low-frequency noise abatement. Initially, an electro-acoustic analogy-based analytical approach is developed to predict the acoustic absorption performance of series and parallel configurations of MPP and coiled-up tubes, and the findings are corroborated by full-field finite element simulations. The parametric analyses revealed that by carefully choosing the geometric features of the coiled-up tubes, the absorption spectra of each tube can be coupled with that of MPP. Thus the bandwidth of absorption can be broadened. Furthermore, it is observed that the parallel configuration of MPP and coiled-up tubes significantly lowered the thickness of the absorber without affecting the absorption bandwidth. Importantly, the parallel configuration of MPP and coiled-up space demonstrated more than 80% absorption in the frequency range of 250-350 Hz.