sm_jamieson wrote:
I am making a soundproofing partition just inside a bathroom brick
wall, not attached to it, and to save space, the partition is to be
solid 18mm plywood with no battens. Wall area is only about 2.5 x
2.5metres.

Hm. Not at all sure about that. What's on the other side of
the wall? Won't a 2.5m square (how do you join it?) of ply
act like a giant wobble-board?