In internal concrete and wood floors the resistance to sound transmission is achieved by several ways. First is the mass of the floor, applying sound-insulating material at some point of manufacture, and the formation of voids between a heavy element of construction (e.g., concrete slab) and a thin and rigid plate, which usually forms a raised floor or concrete floors. Offering a good sound insulation is important because of their mass and because it does not allow the creation of discontinuities in the structure. Wooden flooring insulation is considered necessary. Also the perimeter of the walls must be relatively thick (> 112 mm bricks) to prevent the passage of sound there through. The schemes are showing this visually and the link will explain more.

Source.

...