![]() The MDM characterizes the test specimen for normal- and oblique-incident acoustic impedance and mode scattering coefficients. The modal decomposition method (MDM) presented includes four normal modes in the model of the sound field, thus increasing the bandwidth from 6.7 to 13.5 kHz for a 25.4 mm square waveguide. This research effort increases the frequency range of normal-incidence acoustic impedance testing in square ducts by extending the standard two-microphone method (TMM), which is limited to plane wave propagation, to include higher-order modes. These models ultimately rely on measurements of the acoustic impedance to characterize candidate engine nacelle liners. Accurate duct acoustic propagation models are required to predict and reduce aircraft engine noise.
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