The noise barrier problem is simulated by a still, single frequency, three dimensional sound source located within two parallel porous medium layers with rigid back walls. The solution of the reflection sound field is represented by an asymptotic solution which is derived by the method of steepest descent. We find that there is a viscous damping factor α for the porous medium layer of the noise barrier which affects the trapping and absorption of sound waves. Because there exists a layer similar to Stokes boundary layer in fluid mechanics due to the variation of thickness of the porous medium, we also find that it will be a waste if the porous medium used is too thick. The optimum choice of noise barrier which minimizes reflection within parallel noise barriers, according to the numerical results, is α = 1, while the layer thickness‐to‐wave length ratiod/λ= 1/4. We also find that increasing the height of the noise barrier in order to improve its noise control effect is not economical.