A floating floor should give you good sound insulation, but does it always work?
A floating floor, described simply, is a heavy solid layer suspended on a resilient layer of flexible material above the structural floor of a room. For preference, the floating element should be heavy concrete, although particle board can be used in a less substantial installation. The floor slab can be laid on glass fiber quilting, or even suspended on springs. The floating element should at no point touch the walls of the room, otherwise vibration will be conducted at those points.
The idea behind this is that the heavy floating element should in itself provide good sound insulation. It would do this if it were laid directly on top of the structural floor with no glass fiber or springs. However, isolating the floating element from the structural floor improves the sound insulation, just as a cavity wall is better than a single wall of the same mass.
In the case of a floor however, we have a mass (the floating element) supported by a spring (the glass fiber). This constitutes a resonant system and will have a certain resonant frequency at which it will tend to vibrate given any energy input. If you jump up and down on a floating floor you will be able to feel this.
What is not commonly realized about floating floors is that they only improve sound insulation from a frequency somewhere above the resonant frequency, and at frequencies higher than that.
Floating the floor actually makes the sound insulation worse at the resonant frequency, and in a band of frequencies either side of that.
It is imperative therefore that the resonant frequency be made as low as possible, so the frequencies that suffer are well below 20 Hz. This can be achieved by making the floating element as heavy as possible, and by increasing the compliance (the 'springiness') of the resilient layer.
If low frequency noise is a problem, such as a nearby road or subway, then a lightweight floating floor will not be the solution, and may be worse than not employing a resilient layer at all. At higher frequencies however, the benefits of decoupling are similar to double-leaf construction in walls and other structures.