question about insulation

Hello, I am building some clouds and wall units and I was wondering about the difference between some rockwool, I have bought some semi-rigid rockwool, they are not that rigid actually and they have some paper for condensation on one side of the panel. I recently saw some rigid one with no paper at all. Those panels are quite tough and can stand by themselves if left against a wall. What acoustic difference is there between those to two types ? On the wall units DIY link from Jls index page, it is said that rigid wool should be used for a better absorption of lows, should I used them for clouds and side wall absorbers as well ? Should I sell what I have already bought ? Thanks for your advise.

All this information can be relatively easily found on this forum, but I'll try and summarize it anyway. Should others detect any nonsense (which is very possible), feel free to debate or correct. As far as I know: when it comes to porous absorbers, it's all about density, thickness and spacing. All absorbing or reflecting qualities are strongly frequency dependent. Higher density (+-80kg/m3 or more) will contain lower frequencies better, but will reflect more high frequencies. Lower density (+- 40kg/m3) still has effect in the lows, but has its absorbing peak at mid/high frequencies Thickness is easy to understand. Considering the direction of a ray or wavefront of sound, the more material it encounters, the more it will loose it's energy trying to pass through it. Spacing is mainly a money saver. Spacing 10cm of insulation about 10cm from a reflecting surface will have similar effect as should you attach 20 cm of insulation to the wall itself without any air gap. When talking bout in-room treatment, paper backing is generally avoided due to its reflecting qualities in the higher frequencies.

If you know the model number of the type of non-rigid rockwool you have, you can google it and maybe find a data sheet with the acoustic properties. Roxul is pretty good about including that info, it's one of the first things on the data sheet.