Acoustic attenuation of a curved duct containing a curved axial microperforated panel

This paper is an extension of the previous work by Yang [J. Acoust. Soc. Am. 143, 1102-1105 (2018)], which dealt with achieving acoustic absorption using a microperforated panel (MPP) in the absence of the backing cavity. It was shown experimentally that a MPP can attenuate the acoustic wave without necessarily being accompanied by a backing cavity, provided it is placed in an acoustic environment where the acoustic pressure on the two sides of the MPP is different in terms of the amplitude and/or phase. Such an environment was found in a curved duct with a curved MPP aligned along its axial direction. To further obtain the physics underlying the design, a model is developed in this work to study the property of the acoustic transmission through the duct bend and an eigenvalue analysis is made to study its influence on the decay rate of each mode. It is shown that the treatment results in attenuation for all duct modes including the fundamental mode. A geometrical acoustic approximation is made to assist in interpreting the acoustic absorption effect on each mode and a comparison with the finite element method result is used to validate the proposed model.