We all know that IC engine’s Exhaust muffler, aero-liners and the liner in the combustion chambers are application of acoustical dampers. Perforations in the wall of these devices are the reason behind their sound absorbing property. To design a acoustical damper it is necessary to understand that how the perforation affect the sound absorption in the devices. Multi-microphone impedance tube is used to observe the experimented values when grazing -bias flow combination on the wall having the slit perforations. Objective of the experiment is the low Strouhal number i.e. the quasi steady behaviour. For case of biased flow directed from the grazing flow in the direction opposite to the side of wall with the slits analytical models of steady flow low frequency aeroacoustics effect of the two-dimensional wall with the slit openings. Semi-empirical correction is used get the correct limit for pure bias flow and the theoretical values comes in order as that of experimental.