Don Phillipson wrote:

Even if you suspend the speakers from the ceiling, the low
frequencies coming from the speakers will still couple with
the room acoustics and your floor and walls will still
probably vibrate almost as much as if the speakers were
in contact with the floor.

This suggests a non-standard physics.

No it doesn't.

The basic complaint is that the floor vibrates,

No - the basic complaint is that low frequency sound is heard either
above or below the room with the speakers. The theory is that this can
be reduced if the speakers are removed from direct contact with the
floor.

and we know the speaker cases are in direct contact with the
floor. HG now suggests that if the cases were separated from
the floor by a foot or two of (compressible) air the "floor
and walls will still probably vibrate almost as much."
We should be interested to learn how this could possibly occur.

Car speakers are not directly connected to the exterior surfaces of a
car, yet low-frequencies are easily transmitted by those surfaces to a
listener dozens or even hundreds of yards away.

If the room's walls, floors and ceiling is made from compliant materials
(ie wood, sheetrock, etc) then it's quite likely that low frequency
sound from a suspended speaker will create standing waves that will
cause the walls, floors to resonate.

In this case, if indeed we are talking about a poured cement structure
(ie - modern multi-story apartment building) then that will be far less
likely.

But I still can't believe that a speaker sitting directly on a concrete
floor that is presumably 3 or more inches thick can possibly experience
enough of a vibration amplitude to be transmitted through the floor.
Presumably there is carpeting on this floor, which would decouple the
speaker to a great extent from the concrete floor.