When the sound hits a wall, the sound is reflected - some will be absorbed and a small part will be transmitted.
The sound insulation property of the construction, its reduction power called R, is measured in dB and is a measurement of the power of the construction to prevent the passage of the sound.
High R = good sound insulation = good sound reduction
In porous absorbers like ROCKWOOL materials the sound absorption takes place by the sound energy being converted into frictional heat between the ROCKWOOL fibres. The sound absorption depends on the thickness of the material. The values at the low frequencies are improved with increased thickness. The same tendency can be obtained by moving the absorbers away from the base.
Protection of the absorber by a cover plate does not change the absorption as long as the plate has a sufficient number of openings. A perforation of 15% could be sufficient for a 1 mm thick cover plate of steel. Thicker covers require a large number of small holes.