Several issues here.
Let’s work backwards relative to the questions asked.
“I want to treat the adjacent cabinet surface with a Rockwool absorber, but because it's likely to be mistaken as upholstery, I'm considering the use of a layer of acoustic foam (or any open celled foam) to wrap the Rockwool, which then is wrapped with a suitable fabric. This is to protect the Rockwool from being deformed and guests from possibly picking up any Rockwool fibers through the fabric.”
It’s possible, but not necessarily the most feasible nor the most effective route. Let me try to explain why (gee it would be nice if there were a way for the site to support simply draw tools or insert diagrams!)
Rockwool does not have fibers in the same manner as Fiberglass, and unlike Fiberglass, due to its higher mass it typically simply falls to the ground. Instead it is more ‘crumbly’ and particulate in nature rather than it is like little shards of glass. But for the discussion we will simply limit the discussion to ‘loose material’.
The acoustic foam will be relatively expensive, and it will do little to stop an deformation, and the material, covered in fabric will still look like upholstery. A thin layer of Dacron fiber commonly used in upholstery might be more effective in wrapping the Rockwool to contain any lose particles.
Likewise, another very commonly used technique in industry that addresses all of the issues posed here (but which is less known by the audio and DIY community is the use of perforated metals panel absorbers.
These have the benefit of being very resistant to impact while being resilient to deformation and of releasing little to no particulate debris. This topology would be the optimal topology.
But, moving back to the much more significant ‘what and whys’ of the assumption: reflections.
You are sitting immediately adjacent to a LARGE rear reflecting walls, and yet we are expressing concern, not about the relatively huge perpendicular reflecting surface but rather to a very small (remember, sound has size, and wavelengths larger than the dimensions’ will not see the object, but will instead diffract around it and bounce off the larger more massive planar boundaries.
Meaning, that due to size and orientation relative to both the direct and potential indirect signals, that you MIGHT have some higher frequency reflections, assuming there is sufficient energy arriving at oblique angles to the cabinet surfaces! In other words, the concern is relative to energy moving more from side to side than from front to back in the room. While possible, it is not the first thing one would normally focus upon as a significant source of problems in a basic ray tracing model. Instead there is a greater probability that reflections would be incident off the side cabinet surfaces AFTER energy were incident with the rear wall surface, meaning, when AFTER it was coming back off the wall. (Unless you leave those speakers set on the cabinet there!!! They are both too close and will definitely be a source of destructive reflections as well as diffractive sources due to how they are mounted!)
The MUCH larger concern will be for reflections at near normal (90 degrees relative to the wall, but normalized to 0 degrees for our perspective) or of energy arriving between 0 and ~60 degrees of incidence with the real wall. At such angles, any energy incident off the cabinet side would likely be at least a 2nd order reflection (2 bounces off cabinet and rear wall), excluding earlier incidences sufficient to redirect it at said angles.
(Note, we know little about the complete topological context of the space, so we are ‘ASSuming’ a near centrally located sofa placed against a rear boundary.)
In other words, addressing both size, energy levels of higher order reflections and orientation, we are most likely dealing with a source of problem almost trivial relative to the 100 tone elephant in the room known as the rear wall – and the compact of placing the listening position there without sufficient treatment – which would optimally involve a minimum of either a 4” thick panel of either ~3lb/ft^3 Fiberglas or ~4 lb/ft^3 mineral wool with a 4” gap.
Using less thick panels and only effectively EQing the high frequency energy component while missing the higher energy content lower frequency specular energy from ~250 Hz up by using a truly broadband absorptive panel is of very marginal value!
Otherwise, you will be tossing money away to chase a minor ghost while ignoring the REAL and SIGNIFICANTLY detrimental issue of the dominant cause of problems: the rear wall.
My actionable suggestion: Take a few hours and make very simple ETC response measurements in the listening spot. Resolve the reflections into their directional and incident locations. This will quickly and easily identify the REAL behavior, the actual precise gain and direction of energy in the region and the ACTUAL incident surfaces.
Then spend your money and expend your energy to address the REAL issues that will result in REAL response gains based upon the actual presence of destructive specular issues.
I think you will discover the above bit of napkin analysis won’t be too far off.
Good luck.
(And we will ignore the additional issues created by what appears to be a rear speaker abutting the rear wall above the sofa on the cabinet.... not exactly what the folks who designed the various surround topologies had in mind...) If that is indeed what that is, I would MOVE that speaker (and its partner) and place them further to the side and slightly forward of the rear wall listening position if you are not amenable to optimally placing the listening position.)
Let’s work backwards relative to the questions asked.
“I want to treat the adjacent cabinet surface with a Rockwool absorber, but because it's likely to be mistaken as upholstery, I'm considering the use of a layer of acoustic foam (or any open celled foam) to wrap the Rockwool, which then is wrapped with a suitable fabric. This is to protect the Rockwool from being deformed and guests from possibly picking up any Rockwool fibers through the fabric.”
It’s possible, but not necessarily the most feasible nor the most effective route. Let me try to explain why (gee it would be nice if there were a way for the site to support simply draw tools or insert diagrams!)
Rockwool does not have fibers in the same manner as Fiberglass, and unlike Fiberglass, due to its higher mass it typically simply falls to the ground. Instead it is more ‘crumbly’ and particulate in nature rather than it is like little shards of glass. But for the discussion we will simply limit the discussion to ‘loose material’.
The acoustic foam will be relatively expensive, and it will do little to stop an deformation, and the material, covered in fabric will still look like upholstery. A thin layer of Dacron fiber commonly used in upholstery might be more effective in wrapping the Rockwool to contain any lose particles.
Likewise, another very commonly used technique in industry that addresses all of the issues posed here (but which is less known by the audio and DIY community is the use of perforated metals panel absorbers.
These have the benefit of being very resistant to impact while being resilient to deformation and of releasing little to no particulate debris. This topology would be the optimal topology.
But, moving back to the much more significant ‘what and whys’ of the assumption: reflections.
You are sitting immediately adjacent to a LARGE rear reflecting walls, and yet we are expressing concern, not about the relatively huge perpendicular reflecting surface but rather to a very small (remember, sound has size, and wavelengths larger than the dimensions’ will not see the object, but will instead diffract around it and bounce off the larger more massive planar boundaries.
Meaning, that due to size and orientation relative to both the direct and potential indirect signals, that you MIGHT have some higher frequency reflections, assuming there is sufficient energy arriving at oblique angles to the cabinet surfaces! In other words, the concern is relative to energy moving more from side to side than from front to back in the room. While possible, it is not the first thing one would normally focus upon as a significant source of problems in a basic ray tracing model. Instead there is a greater probability that reflections would be incident off the side cabinet surfaces AFTER energy were incident with the rear wall surface, meaning, when AFTER it was coming back off the wall. (Unless you leave those speakers set on the cabinet there!!! They are both too close and will definitely be a source of destructive reflections as well as diffractive sources due to how they are mounted!)
The MUCH larger concern will be for reflections at near normal (90 degrees relative to the wall, but normalized to 0 degrees for our perspective) or of energy arriving between 0 and ~60 degrees of incidence with the real wall. At such angles, any energy incident off the cabinet side would likely be at least a 2nd order reflection (2 bounces off cabinet and rear wall), excluding earlier incidences sufficient to redirect it at said angles.
(Note, we know little about the complete topological context of the space, so we are ‘ASSuming’ a near centrally located sofa placed against a rear boundary.)
In other words, addressing both size, energy levels of higher order reflections and orientation, we are most likely dealing with a source of problem almost trivial relative to the 100 tone elephant in the room known as the rear wall – and the compact of placing the listening position there without sufficient treatment – which would optimally involve a minimum of either a 4” thick panel of either ~3lb/ft^3 Fiberglas or ~4 lb/ft^3 mineral wool with a 4” gap.
Using less thick panels and only effectively EQing the high frequency energy component while missing the higher energy content lower frequency specular energy from ~250 Hz up by using a truly broadband absorptive panel is of very marginal value!
Otherwise, you will be tossing money away to chase a minor ghost while ignoring the REAL and SIGNIFICANTLY detrimental issue of the dominant cause of problems: the rear wall.
My actionable suggestion: Take a few hours and make very simple ETC response measurements in the listening spot. Resolve the reflections into their directional and incident locations. This will quickly and easily identify the REAL behavior, the actual precise gain and direction of energy in the region and the ACTUAL incident surfaces.
Then spend your money and expend your energy to address the REAL issues that will result in REAL response gains based upon the actual presence of destructive specular issues.
I think you will discover the above bit of napkin analysis won’t be too far off.
Good luck.
(And we will ignore the additional issues created by what appears to be a rear speaker abutting the rear wall above the sofa on the cabinet.... not exactly what the folks who designed the various surround topologies had in mind...) If that is indeed what that is, I would MOVE that speaker (and its partner) and place them further to the side and slightly forward of the rear wall listening position if you are not amenable to optimally placing the listening position.)
