The effect of mean grazing flow on the acoustical response of a single rectangular slot in a wall to imposed sound is investigated experimentally. For this purpose an accurate multi-microphone impedance tube set-up is employed. In particular the influence of the mean grazing flow characteristics as well as the orifice's (edge) geometry is examined. For this purpose different boundary layer flows and slot geometries are used. Hot-wire boundary layer- and shear layer measurements are carried out. Care has been taken as to remain in the regime of linear acoustic perturbations. The change of the orifice's acoustic impedance due to the mean grazing flow shows an oscillating behaviour as function of the Strouhal number. It is found that, when the Strouhal number is based on the phase velocity of the spatial hydrodynamic instability of the shear layer, these oscillations coincide for different boundary layer flows. The amplitudes of the oscillations increase with decreasing laminar boundary layer thickness. The oscillations disappear when the shear layer, according to linear theory, becomes stable. Furthermore, especially the downstream edge—rather than the upstream edge—of the slot is of influence: oscillations in impedance increase up to a factor five in amplitude when the edges are sharp.