Sound reflection from a rigid wall; near the wall the particle velocity is zero, and increases with increasing distance from the wall. For absorption in a porous layer in front of the wall, velocity is needed to yield friction, being the mechanism of absorption in porous materials. Low frequency means large wave length, and high frequency meand short wavelength. A certain layer of material will be thin, in relation to large wavelengths- low velocity, low absorption-, but thick, relative to hort wavelengths, hence higher particle velocity, better absorption at these higher frequencies.

Thank you Mr. Ruiter for the valuable description, so I have an information that the easiness of a porous material in absorbing sound depends on mean free path of particles that carry sound wave and the diameter of pores, how do you think..?