Flanking transmission - how sound gets around soundproof walls
Flanking transmission occurs where are partition is built up to the height of a suspended ceiling, or down to the level of a raised floor, but not all the way to the solid structure of the building.
No matter how good the soundproofing qualities of the partition, sound will take the flanking path over or under the obstacle. In many buildings, the floor - even at ground level - does not sit directly on concrete but is suspended on wooden joists. The same applies to the ceiling - the visible ceiling may block light and drafts, but it is almost transparent to sound.
So if a good solid partition is built between the suspended floor and the ceiling, sound will take one look at it and simply hop over and under it.
The answer is to build the partition all the way down to the ground, or to a solid concrete floor. Likewise, the partition must extend above the thin drywall (plasterboard) ceiling to solid concrete. Unfortunately, if there are no solid concrete structures, then flanking transmission is unavoidable.
In this case, one alternative is a 'box within a box' construction, where an entire room consisting of massive floor, ceiling and walls is built within the structure. Needless to say, this is expensive. But it is better to be aware of the correct solution in advance rather than undertaking a lot of work and then finding that it doesn't solve the problem.
In some cases, this interior structure might not have to be physically large - perhaps just a vocal booth would be enough.