Call me i am an acoustic specialist working and sell and design things like this everyday and have some thin solutions. Ian 07836267144

The most important thing to know what the existing wall is. Is it brickwork or blockwork and is it skimmed.

Basically you need to add mass and disconnect the new wall from old which can be done with an independant wall or by using res bars. Then you need some mass either soundbloc plasterboard or we do a product that filled with silica sand call easypanel but you need to go over this with plasterboard pref soundbloc. Dont have to use res bars but you gain 5/7db which dont sound alot excuse the pun but sound works in a lothartyhmic scale so the difference between 40db reduction and 45/47 is vast

Cheers

Ian