Effective soundproofing can only be provided by materials which reflect sound energy.
Such materials would be massive and non-porous, such as concrete, or a well-made brick or blockwork wall.
Here is a list of suitable materials:
The two characteristics that all of these have in common is mass and non-porosity.
The last item, 'proprietary flexible soundproofing materials' covers an immense range of potential solutions, some of which - when you look at their advertising material - seem to work by magic rather than physics.
They will only work if they are massive and non-porous - simple as that.Come on the FREE COURSE TOUR