Estimated signals for a key depression with F = 3.36 N and M = 0.443 kg on the Stodart’s key playing D₂. From top to bottom: hammer head and key front velocities; key front displacement; acoustic pressure of the produced sound. Vertical lines in the figure indicate the onset time of hammer free rotation (MHV), estimated as the local maximum of hammer velocity, and a representative time of the hammer-string impact phase (HS), estimated as the maximum of the raw hammer acceleration signal (not displayed). The parabolic regression on the displacement signal in the interval (T_On, T_Off), corresponding to displacements (D_On, D_Off), is superimposed on the key displacement signal.