I considered using rockwool in some diy absorbers a while ago but was wary of the safety side. It’s not supposed to be harmful if you breath in the fibres but installation requires overalls and a good mask. So it seems that it’d need to be sealed in some way, a non porous material like plastic sheeting taped up would work. This would be reflective for higher frequencies so only good for bass. I think foam would be better for high frequencies behind a canvas, but the canvas itself has to be porous enough to let those frequencies through to the foam instead of bouncing off!
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There’s a myth in our Hi fi world — egg crate … people using them for sound panels
If you google , you can see they experiment them in a room w hifi systems
When i tried them many moons ago, i concluded that they killed the depth of the soundstage behind the speakers, amongst over inconsistencies, such as tonal confusion. Plus they were an eyesore and an alarm bell for SWMBO to call the men in white coats to come and take me away. Needless to say, it was fun while it lasted 
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Here’s a link to my experiment I did a while ago … they have subsequently been painted and it’s quite thick acrylic so don’t think they’re doing much for high frequency absorption 
Didn’t work out well ?
There are some comments further up the thread saying that they may still transmit some energy to the 50mm acoustic foam behind helping with midrange. The room measures surprisingly well for decay so maybe they are having an effect. Never got round to measuring with and without.
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Morning Mark appreciate your advice regarding the mineral fibre, I think the type of material on the picture frame will probably dictate the outcome.
I will place the panel in front of a speaker just to get an idea of how the material behaves, I did notice on the GIK wall panel the material was very fine guessing it maybe acoustically transparent, similar to a projector screen.
What ever happens it will be worth experimenting with a couple of ideas.
Tried the cushion behind the head and yes it does work but inevitably it leads to a power 
.
It’s all good fun .
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Looking forward to hearing how it turns out, should be great 
Distance .
Erm, you need reflection to form a standing wave to create a node…
The mids and highs can be viewed like a beam of light reflecting off a surface, that’s why we find the reflection points and absorb, scatter, diffuse etc in those areas. The bass is completely different. It’s linked to pressure. The different frequency “nodes” which cause lumps and anomalies are at certain DISTANCES. Bass does not “reflect” in real terms. Fitting bass traps in corners is not linked to reflection is it! I would recommend the free Amroc software to anyone interested.
I historically learned a great deal from the PeterR and certain You Tube broadcasters and dramatically improved the way my room sounds.
Happy listening
If there weren’t reflections there’d be no standing waves, no nodes and no cancellations. That is the case at the bass end, and at higher frequencies. You declared that bass performance is linked to pressure and nodes and nothing to do with reflections whatsoever - I have simply been correcting that last claim. This can readily be verified by a bit of reading on acoustics.
With bass, absorbers on a flat wall can absorb bass and reduce their reflection, but because of the longer wavelengths and higher energy it takes very much deeper absorbers to be effective. At the same time, in a room and at distances from source that are short relative to wavelengths sound directionality is lost, and the effect of room walls bouncing them around results in higher bass pressure in corners, therefore positioning bass absorbers in corners is more effective at reducing the overall level of reflections in the room than positioning in the middle of a wall.
However, this is all a distraction from the OP’s query. The early reflections that are of particular significance in muddling the sound of music in a room are primarily those of higher frequencies, for which absorbing the sound at the wall behind the listening position to stop or reduce the reflections will be beneficial. The only question is how best for the OP to do that.
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Looking into this a bit more, I think it doesn’t come in until the very high density slabs, but I haven’t yet found anything definitive confirming from what point (from vague memory maybe 100kg/m3 upwards), and also likely depends on the front facing - and some rockwool has one face firmer than the other. At lower densities the available data suggests that the main difference is that for a given thickness the higher the density the lower it will go in effective absorbing frequency range. If only 25-50mm of available depth then certainly I’d suggest a minimum of 45kg/m3. The difficulty if only a small amount wanted is that the slabs come in packs - but of course excess can be used to make other panels for other parts of the room…
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The game with acoustic treatment is to reduce long delay times (RT60’s) and avoid too many direct reflections, in reality experimentation is in my view reqiuired as you can take some of the energy out of the room and close things in if you take it too far. Diffusion is good but again you need to be careful. Diffusion needs some distance between the panel and you.
I have ended up with RT60’s in the 350ms region from 150 Hz > the bass is around 800ms and reducing further isnt possible, but with EQ you can reduce the delay as the room nodes often have very long reverberation
Actually corner bass traps are not the best place to start. Yes they do work there but you can get much better results in treating the front and rear walls with bass traps first.
As for higher frequencies its generally recommended to treat the first reflection points on the side walls and ceiling first.
The number of times i read about too much treatment killing the room , which is false. What will kill the sound is – 1-the wrong type of treatment, 2- the wrong position of the treatment, 3- the wrong quantity of treatment.
Dont blame the treatment , blame the application.
I’m using 44 panels in my room and no way does it sound dead. In fact its the best ‘non’ sounding room ive heard.
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Indeed. But the OP’s question was regarding the wall inches behind the listening position, and, depending on distances, reflections from there could arrive before reflections from side walls.
As for bass traps, everything I’ve read suggests that they are most effective (at absorbing bass) placed in corners, however indeed they can indeed be beneficial in other positions, and behind speakers, and behind listener, can be desirable, particularly with wide range absorbers. One practical problem with bass absorption is the depth necessary to make a significant difference - a 2 inch thick absorber doesn’t do much at the bass end! Studios often have the entire rear wall absorbing, with absorbent material depth of many feet (I seem to recall one I read about done tears ago having about 12ft depth). In a domestic situation active absorbers may be the best solution, though at a cost (but not that expensive relative to many decent systems).
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