Upper panel: time evolution of the mass density ϱ(L_D =  {L/2,H/2} ,t). Middle panel: the global wavelet spectrum of the incoming signal (full line) with dominant wave period P, and the 99% significance level (dash-dotted line). Bottom panel: the group speed of the wave v_gr = dω/dk in units of the external Alfvén speed as a function of wave number k_xw (bottom panel). All for the density slab.