Passive and active low frequency sound absorption performance of the finite and large sized micro-perforated panel absorber on oblique incidence condition

This paper presents a comprehensive investigation on the passive and active low frequency sound absorption performance of the finite and large sized micro-perforated panel absorber (FLS-MPPA) for oblique incidence excitation (OIE), which is of great significance for the engineering implementation of this technic. The theoretical model of the FLS-MPPA is firstly established by applying the modal analysis approach. It is also validated by experimental test. Then, the passive sound absorption performance is explored and the underlying physical mechanisms of these peculiar findings are investigated thoroughly. Finally, the active control performance (applying point source to actively control the sound field) and physical mechanism of the sound absorption improvement of the FLS-MPPA are analyzed. Results obtained demonstrate some key findings. The high order cavity modes excited by OIE reflect most of the incident sound power and have little contribution to the sound absorption in the low frequency range owing to their strong mutual coupling effects and being greatly excited with high modal amplitudes. They also cannot produce additional sound absorption peaks due to their wide resonant frequency band. Weakening such mutual coupling effects can improve the sound absorption and produce additional absorption peaks. The low frequency sound absorption of the FLS-MPPA is significantly improved with control and the achievable maximum sound absorption coefficient is slightly greater than the difference between 1 and the sound power reflection coefficient of the MPP, which validates the feasibility of the active control method for OIE. The reflected sound power induced by the cavity modes is highly reduced and part of the reflected sound power produced by the MPP can even be absorbed by these modes when their amplitudes are reduced to the optimal value, which is the physical nature of the active control. Sensing and controlling the cavity sound field in the corner achieves almost equal sound absorption improvement as that of the optimal control state, based on which the error sensing strategy is constructed.