Abstract: A gradiently slit-perforated double porosity material is proposed by introducing macro-scale periodic gradient slit-perforations into traditional porous materials with single porosity.This material is one kind of multiscale material since it includes two scales of matrix micropore size and slit-perforation size.A theoretical model is developed for the sound absorption of the gradiently slit-perforated double porosity material.In the model,the material is divided into lots of thin layers and each layer is approximated to be straight slit-perforated material.The equivalent density and dynamic compressibility of each thin layer are given by using the low or high permeability contrast double porosity theory.Then the sound pressure and particle velocity relations between adjacent thin layers are obtained by employing the transfer matrix method.Finally,the surface acoustic impedance and the sound absorption of the gradiently slit-perforated double porous material can be calculated.A finite element model is further established to validate the accuracy of the theoretical model.In the considered frequency range of 100-3000 Hz,the simulation results agree well with theoretical results.The influence of multi-scale structural parameters on the sound absorption performance of the porous materials is analyzed theoretically and numerically.It is proved that the multi-scale structure design can significantly improve the sound absorption performance of porous materials.Compared to the slit-perforation gradient,the slit-perforation width plays a more significant influence on sound absorption.The sound absorption enhancement mechanism of the multi-scale structure design is revealed by the analysis of the sound pressure and energy dissipation distributions in the material.This work provides a multi-scale structural design method for improving the sound absorption performance of traditional porous materials at broadband frequency.