Framework for Modal Based Assessment of Liner Structures

A framework for the detailed investigation of the acoustic damping of higher order modes in an annular duct was set up and a first series of experiments was conducted investigating a noise absorbing section with homogeneous impedance and good sound absorbing performance, and a reference hard-wall section. The new setup enables the comparison of plane wave sound attenuation as obtained in small duct setups with measurements in the annular setup and in addition the acoustic damping of individual modes up to azimuthal mode order m=8 and radial mode order n=1. The paper addresses the investigation of the capabilities and limitations of excitation of individual dominant modes, of uncertainties of the setup and associated methods of data processing as well as the importance of losses in the acoustic boundary layer for the propagation of higher order modes.