  This work theoretically analyzes the sound absorption properties of a chirped
multi-layer porous material including transmission, in particular showing the
broadband unidirectional absorption properties of the system. Using the
combination of the impedance matching condition and the balance between the
leakage and the intrinsic losses, the system is designed to have broadband
unidirectional and quasi perfect absorption. The transfer and scattering matrix
formalism, together with numerical simulations based on the finite element
method are used to demonstrate the results showing excellent agreement between
them. The proposed system allows to construct broadband sound absorbers with
improved absorption in the low frequency regime using less amount of material
than the complete bulk porous layer.
