Indirect Acoustic Impedance Determination in Flow Ducts Using a Two-Port Formulation

A passive acoustic two-port formulation is commonly used to describe the acoustic transmission characteristics and hence the acoustic performance of duct systems. In this paper, the acoustic two-port representation of flow ducts is used for the indirect determination of the acoustic impedance of acoustic absorbent materials under grazing flow conditions. Since the two-port parameters can be easily obtained experimentally, the methodology offers a possible alternative for the low-frequency estimation of the grazing flow impedance in comparison to the direct determination of the acoustic impedance. This paper discusses the theoretical model that is used for the indirect deduction of the grazing flow impedance. This theory is based on the analytical description of the sound field in flow ducts. A numerical validation on a geometry which is similar as the proposed experimental configuration is performed and the preliminary validation of the theory illustrates the capability of the indirect impedance determination technique to obtain a good estimation of the grazing flow impedance of acoustic absorbent materials.status: publishe