Accurate duct acoustic propagation models are required to characterize and reduce aircraft engine noise. These models ultimately rely on measurements of acoustic impedance for candidate materials used in engine nacelle liners. This paper seeks to increase the frequency range of acoustic impedance testing by extending the standard two-microphone method (TMM), which is limited in bandwidth to ranges where only plane waves propagate, to include higher-order modes. The modal decomposition method (MDM) presented includes the first four normal modes in the model of the sound field and thus increases the frequency range from 6.7 to 13.5 kHz for a 2.54- by 2.54-cm waveguide. To resolve both the incident and reflected amplitudes of the first four modes, eight microphone measurements are required. This paper formulates and simulates the MDM and applies it to measurement of the reflection coefficient for various specimens over the tested frequency range of 0.2 to 13 kHz. Mode scattering, predominately from the plane-wave mode into other modes, is revealed. The experimental uncertainty of the method is discussed.