On the mechanism underlying two distinct types of nonlinear sound absorption trends of porous layers.

Through non-dimensional analysis of the nonlinear sound absorption characteristics of rigid-porous layers, the results indicate that the nonlinear sound absorption trend of a porous layer (PL) at high sound pressure levels (SPLs) mainly depends on the state of flow resistance matching. When a PL is in an under-matched state, the sound absorption coefficient (SAC) will initially rise to a maximum and then gradually decline as SPL increases. Comparatively, when it is in an over-matched state, the SAC will decrease monotonically. In light of this understanding, several guidelines are proposed to improve the sound absorption performance of PLs at high SPLs.