Acoustic liners play an important role in aircraft gas turbine engine noise reduction by converting acoustic fluctuations into nonradiating vortical disturbances through small openings, or apertures. In recent years, numerical investigations of acoustic liners have become useful. In this paper, we use direct numerical simulations to investigate the unsteady flow induced by sound normally incident on a comprised of a circular aperture with a hexagonal backing cavity. A series of simulations were performed by varying both the intensity and frequency of the incident sound. Through the numerical simulation data, a detailed flow visualization and quantification of the acoustic energy dissipation at different sound pressure levels and frequencies are presented. Impedance values were predicted using the traditional two microphone method. We also develop a time domain model to predict the impedance values predicted by DNS data. The reduced-order model takes several factors into account that may influence the liner impedance and provides a reasonable match up to 150 dB. Several candidates for improved predictions are proposed and evaluated.