Best option to fix acoustic panels on the wall?!

Hi there! I´ve been reading the forum for some days now, and I´ve notice that some users say that the panels should not be on the wall but instead around 100mm from the wall. Is this correct? If so, what system is best to fix them that way? I´m gonna build around 10 panels with 120x60 cm. The problem is that I can only get panels with 4 cm thick! I know its not much, any way I can put 2 panels inside a wood frame, so I can get 8 cm thick at least? Any help would be great! Thanks :)

That would work fine for general reflection absorbtion. If you want them for bass control, think about 3 layers. The idea of spacing them off the wall is that it helps them reach down into the bottom end a bit farther.