Predictive tools for acoustic performance of perforated tube mufflers rely on empirical perforate impedance models derived from experiments. These experiments have usually been carried out in branched ducts, with a flat perforated plate in the main duct carrying the mean flow, communicating with an acoustically excited branch duct. The normalized impedance values show an order of magnitude variation between proposed models in the literature. This paper explores the use of Computational Fluid Dynamics (CFD) tools to calculate the in-situ perforate impedance of a straight through perforated muffler. An unsteady three-dimensional Navier-Stokes equation based formulation is used. A turbulent velocity profile input having time-periodic fluctuations typical of an engine is provided and the exit of the muffler is assumed to be at atmospheric pressure. The spatial and temporal scales are chosen fine enough to capture acoustic perturbations. The impedance estimates from CFD, obtained by a transformation to frequency domain, is compared with various empirical models and some of the issues related to the simulation are discussed.