Effective soundproofing can only be provided by materials which reflect sound
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:
- Bricks or non-porous blocks
- Plasterboard (also known as drywall, sheetrock, wallboard, & gypsum
- Plywood and dense particle board
- Proprietary flexible soundproofing 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.