The acoustic behaviour of a circular aperture backed by sound-absorbing materials under grazing flow is investigated through a linearized model. The backed material is formulated by its bulk effective parameters, and the grazing flow is assumed uniform. Across the vortex sheet, the 'particle velocity match' condition is applied as a compromise of the relatively thick shear layer under the vortex-sheet simplification. Then both the dissipative effect and the influence on the stability of the vortex sheet caused by the material can be taken into account. It is revealed that the effects of grazing flow and sound-absorbing materials on the aperture impedance are complexly coupled with each other. The deployment of sound-absorbing materials results in a significant increase on the magnitude of both the resistance and the reactance of the aperture. The impedance predicted by the present model shows reasonable agreement with the experimental measurements available in the literature.
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