Abstract: A method and an apparatus for compacting newly poured comparatively dry concrete or similar porous plastic raw materials (6) with a direct mechanically coupled 3-mass oscillatory system where the raw material in its confined form is vibrated and compacted from both sides by two vibrating planes with synchronized directional vibrations in phase opposition. Only one vibrating plane (M1) includes synchronized directional vibrators (4). Its other part (M3) is elastically coupled with (M1) by spacer means (8) (M2) arranged between a spring system (7) being so dimensioned that the parts is dynamically balanced whereby the two parts (M1) and (M3) oscillate in phase opposition close to or at the 2nd natural frequency of the system with a dynamic amplification of the amplitudes in relation to the impulses imparted from the vibrators (4) at the same time while (M2) is stationary. During the process (M1) and (M3) are carried towards each other by (M2) as the raw material is gradually compacted.