# Acoustic foam over rockwool



## Kaisergrendel (Jul 19, 2011)

Hi, I have a cabinet next to my listening position (couch), which is placed up against the rear wall. Without getting into a discussion about how bad a position this is, basically the cabinet forms a 90 degree angle with the rear wall right at the corner of the couch and causes some pretty bad reflections. 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.

Is this feasible?


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## SAC (Dec 3, 2009)

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.)


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## stump (Sep 14, 2011)

Hey Kaisergrendel
I was asking myself the same question over the weekend.I just lined the wall behind the speakers with acoustic foam and wondered the affect the foam would have wrapping up rockwool for bass traps.The foam wont affect the way the rockwool will work.Given the fibre issue I ended up using Polymax Absorb XHD 100.Its made in Sydney and does a good job without the irritation of rockwool or batts.
SAC makes some good points regards if doing the corner section would make much differance.
Cheers Stump


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## robbo266317 (Sep 22, 2008)

I totally agree Stump. If you are worried at all about the product then finding a potentially safer alternative is always desirable. 
Personally, considering the low cost involved, I would definitely try it and see if the sound improves.

Cheers,
Bill


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## Kaisergrendel (Jul 19, 2011)

SAC: Thank you for that wonderfully comprehensive (napkin) analysis. I spent the last day or so reading and re-reading your post to try to absorb as much as I could.

I apologize for bringing an issue of compromise to a field where compromise is the devil, but as I indicated (somewhat vaguely) in my first post, my hands are tied with regards to treating that back wall due to objection from parents. That said, I hope to sneak in an absorber at some point and let the results convince them. :heehee: Any thoughts on how to double up that 4+4" panel as a headrest? I'm thinking of maybe encasing the rockwool in some stiff wire mesh, overlaying that with some open-celled upholstery foam or dacron, then covering in fabric.

Back to the original topic, I'm thinking of treating that particular surface on the cabinet just to tame some brightness, especially for the listener sitting next to it. I know it isn't going to address more major problems. I just got the idea after testing it with a couch pillow, and I have lots of leftover rockwool to throw around after making two traps. I don't require the panel to look unlike upholstery, rather if it could function as if it were, it would be a bonus. I took a couple classes in acoustics while studying for my interior design degree, so my ideas may tend to be a bit more pragmatic than you're used to.

Stump, robbo266317: Thanks for the suggestion, I was thinking of getting the polyester-based acoustisorb 3 but decided on the rockwool after time/cost considerations, and I have way too much leftover to justify ordering more material! Robbo, your attitude is the same as mine regarding cost. I made this thread specifically to ask about the feasibility of combining different forms of acoustic insulation for acoustic and practical reasons.

Since this post has raised further questions about the nature of the room, I've attached some pictures to help you understand the asymmetrical layout. The broadband absorbers in the left corners are there to help address the asymmetry. I'm in the process of setting up REW, so in the meantime, some questions:

1. What's the recommended way to treat those blinds? An upright piano sits on the opposite side of that window, at a greater distance from the center of the listening position than the window is.
2. I've moved the surround speakers off the wall as you suggested, but the room prevents me from pushing them further off to the sides. My idea was to use the rear wall to diffuse the surround channels. What are the problems associated with speakers abutting the wall in this manner?
3. The layout of the room is basically fixed unless I can find a way to rearrange it in a visually and spatially acceptable way, or if I move furniture in only minor increments. Is there any way the room can be improved with these caveats in mind?
4. Sorry for asking, but what is an ETC response measurement? I haven't been able to find this out on my own.


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## robbo266317 (Sep 22, 2008)

That is going to be a difficult area to get great sound in. However, as you already realise, it is a compromise and you simply try various arrangements and treatments till you have something useable. 
In your case it is even more difficult as your parents may not see it as an issue. 

Cheers,
Bill.


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## Kaisergrendel (Jul 19, 2011)

robbo266317 said:


> That is going to be a difficult area to get great sound in. However, as you already realise, it is a compromise and you simply try various arrangements and treatments till you have something useable.
> In your case it is even more difficult as your parents may not see it as an issue.
> 
> Cheers,
> Bill.


You are right, although this does not preclude the use of removable elements when higher quality listening is desired. Some kind of panel that latches onto the window blinds to absorb first reflections, and a towel draped on the piano opposite, for example.


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