Effect of geometry and impedance variation on the acoustic performance of porous material lined duct

In this paper, the effect of geometry and impedance on the acoustic behavior of wall and
lined cylindrical ducts is investigated using a numerical model which enables one to compute
the reflection and the transmission coefficients of such ducts using the multimodal scattering
matrix. From this matrix, the acoustic power attenuation is deduced. By using these tools,
the effect of duct diameter increase and duct diameter decrease of the wall or lined duct
section is studied. The numerical results are obtained for two configurations of wall and
lined ducts. Numerical coefficients of transmission and reflection as well as the acoustic
power attenuation show the relative influence of each type of variation.