Solid blue line: ratio of the total absorbed to total emitted fluxes for all sources within voxels defined by channel width of 1 MHz placed at various redshifts using the O9 simulated sky, identical to the result presented in the right panel of Fig. 5. The grey lines show the results due to the uncertainty of the relation between integrated optical depth and HI column density, i.e. the grey shaded region in Fig. 4. The scale of the plot is identical to Fig. 8.