Abstract: A developed theory dealing with fibrous sound absorbents is proposed. It is supposed that filaments of the material are distributed randomly. The macroscopic quantities, such as the effective density and the effective bulk modulus are derived from the microscopic scattering process of the filaments according to rigorous statistical consideration. The experimental examination is discussed briefly. It is shown that the theoretical results are in good agreement with the experimental data. No empirical constant is to be fitted.