Abstract: The invention is intended for sizing separation of difficult-to-screen powder materials and slurries. A sieve assembly includes a rigid frame, attached inside the separator housing, and upper and bottom sieves, stretched on the frame. The interface apparatuses are connected to the upper sieve, while actuator means—to the bottom sieve. The forced frequency of the vibrator is close to one of the natural frequencies of the bottom sieve. When the vibrator is operated, the actuator means interact with interface apparatuses, thus generating repeated collisions therebetween and mechanical impulses, which provide multifrequency excitation of the interface apparatuses and upper sieve. This ensures continuous self-cleaning of the upper sieve, intense loosening of the material bed and disintegration of the particle agglomerates. Separation capacity and products' quality substantially rises. In other embodiments presented sieve assembly includes buffer elements and inertial apparatuses, connected to the upper sieve.