Illustration of the (slow) divergence of the displacement signal for a centered target Eulerian field. The centered displacement signal (top-left) generates an Eulerian field (top-right) with a negative mean value. Therefore, a centered target Eulerian field (down-right) requires a corrected piston velocity thas has a positive mean value (down-middle) and the corresponding displacement constantly increases with time (down-left). For the sake of illustration, calculations have been carried out with exaggerated displacement amplitude of 1 m at 10 Hz.