A percussion device (5) for a rock drilling machine (1) including a percussive piston (6; 15) which is reciprocally movable inside a cylinder and an impact receiving element (7;16) with an impact surface (A;A′), against which the percussive piston is arranged to perform strikes in an impact direction for transferring of percussive energy through Shockwaves to a percussive tool. The percussion device is distinguished by the impact receiving element (7; 16) including a Shockwave modifying portion (9; 17) which extends in a direction opposite to the impact direction as seen from a plane (P; P′) through the impact surface (A; A′). The invention also concerns a rock drilling machine and a rock drilling rig.