Dan,
You're spot on with the natural frequency of the floated floor.
Having looked at the modes of the room, I cannot say that I'd expect 200 Hz to be a problem. However, there are some overlapping modes in that range. Specifically:
{0,0,3} = 197.9 Hz (Axial)
{4,1,2} = 198.7 Hz (Oblique)
{3,3,0} = 200.0 Hz (Tangential)
{1,0,3} = 200.8 Hz (Tangential)
{3,2,2} = 202.6 Hz (Oblique)
{6,0,0} = 205.8 Hz (Axial)
The modes I'd be most concerned about are the two Axials above. However, some thick treatment on the ceiling should be able to address one and the same type of treatment on the short walls should help with the other.
My main concerns would be either that (a) the floor cavities are resonating, or (b) the plywood layers are resonating.
(a) My calcs for the air cavities show a MSM resonance too low to correspond to 200 Hz. More like 30-50 Hz.
(b) I have measured 3/4" thick plywood, double layer, with SheetBlok between. The measured resonace was around 90 Hz. Unfortunately, I didn't get a chance to measure that same thing without the SheetBlok.
Question for hugo: Is it completely obvious that the 200 Hz problem is the floor? Or could it be the room? I guess, are the rooms with sand-filled floors otherwise completely identical to the non-sand rooms?
