I’ve seen them even used as stands for speakers as well - open face of tube to the bottom, speaker on cushioned pads on timber plates on the closed top of the tube, with 3 strong L braced angle shelf brackets as feet bolted to the tube periphery, with the bottom of the L facing out for stability - this was in a room very tight for space and the speakers almost in the corners of the room front.
If the panels are to go up against a wall they can be hard backed as long as the gap between is small, if angled in a corner they should have absorbent material on both sides with a centre hard plate. For against the wall if hard fixed I use 12mm or 16mm MDF (particleboard) as the wall provides mass but if not hard fixed and say rubber mounted I’ll use 19mm flooring particleboard - for free standing double sided I use 19mm (or even 22mm for drum isolators) flooring particleboard. I’ve found that flooring particleboard is the cheapest - here we have yellow tongue which is 19mm and red tongue is 22mm, blue tongue is 25mm - different thicknesses are for different floor span joist spacing (the tongue is a plastic strip that fits into slots on the edges to flush join the boards and colour indicates thickness of board).
The 83Hz is due to the length down the room. The angling of the monitors is critical as well - if they point inward too far you get a false image the same as if they point straight down the room. If I recall correctly your monitors are designed for 60°.
Put the oscillator on and walk around the room at all heights and places and see if you can hear where the frequencies are loudest. If you can't get your head behind the corner traps I would run a room response test with your mic behind them and see what that tells you. If you have a build-up of those freq's in the corners then your corner traps are not absorbing enough.
You have this due to across the room standing waves. The ceiling top corners running down the side edges may be causing your issues as may the floor ones as well as the flat walls as corners concentrate the energy. It seems your panels are net giving the resistance needed - by putting the wool hard against the wall it would give it something to work with - if there’s a gap then it’s all down to only the mass and rigidity of the wool which is very low. Inside a sealed wall with absorbent material the standing waves have to pass back and forth between the hard surfaces through the absorbent material and air thus damping the sound.
the chart you’ve shown there is for a 2000mm (2 metre) thick absorber. It’s the opposite for a 200mm absorber
That will be the Jet lag kicking in have deleted my mis-information. It's a good calculator to play around with to give you an idea of different materials.
So around 300mmm + is where you might hear a real benefit at 180Hz over rockwool.
Good idea especially if you know your problem frequencies. Also pays to have a look at the response curve of the mic being used but in this case it’s pretty obvious the room dimensions are the cause - it’s just finding where the problem peaks are located.
