Just ran Room Acoustic Analysis using Room EQ Wizard. First one was when I had just installed the 12mm plywood panels. Graphs reported average room audio spectrum response and long sound decay (reverberation).
Now with 1st layer of Acoustic Insulation installed under roof and external walls, a graph report of improved room audio spectrum and much better sound decay is the result. Actually, it has inaudible reverberation effect. Sound is dead.