Instead of all these special materials and boxes etc, the better and most cost effective way is to use plasterboards and build room inside room...and if its not enough dbi reduction? then put another set of boards, until you get the rating you need...
The only disadvantage of this technique is that it shrinks the space of the room. Especially if you have to place more than 4 boards on top of each other.
But if you want to make it extremely cheap, you always have the egg cartons, which don't seem like it, but even have an interesting soundproofing effect.
You wont need 4 boards ontop of each other... and even still lets say you did, they are 1.25cm thick. so you lose 5cm one wall 5cm the other wall... negligible....
Its a much better idea than overpaying for these materials the companies like to claim "Took 10 years of NASA super conductor research" which then lets them justify the £200 per sqm price tag.
£5 sqm plasterboard beats any of that overhyped rubbish.
Furthermore, acoustics is a very scientific field, different materials effect different pitches and the rest of it.