Rockwool In Packages Vs Bass Traps

Hello, I was wondering about the low end absorption capabilities of 4lb Rockwool when left in the package. For example, in each of my corners I have a stack of 7 Rockwool packages on their side. These end up taking up a space of 2' x 4' x 8.5'. Can anyone tell me how simply stacking Rockwool in this fashion would compare to using one or two specifically built bass trap such as those by Real Traps or Gik when it comes to absorbing frequencies under 150Hz. Thanks, Brandon

I don't have any references for this, but rockwool in the package works great as a bass trap. Leaving it wrapped in its original plastic will tend to reflect the highs back into the room, but the plastic is transparent to lows, so they'll be absorbed as if the plastic wasn't even there.

in each of my corners I have a stack of 7 Rockwool packages on their side. These end up taking up a space of 2' x 4' x 8.5'.

So what you have is basically a "Superchunk" bass trap design, which works very well. There's a graph on Studiotips somewhere that compares a superchunk to other bass traps, including a couple of commercial ones if I recall correctly, and the superchunk comes out on top. ( I have a link to that graph somewhere, but I can't seem to find it right now. ) - Stuart -

Soundman2020 wrote:

I don't have any references for this, but rockwool in the package works great as a bass trap. Leaving it wrapped in its original plastic will tend to reflect the highs back into the room, but the plastic is transparent to lows, so they'll be absorbed as if the plastic wasn't even there. - Stuart -

While I never saw this measured, I have serious question marks about this. Mostly or often in the packaging I know this is not rarely (rather strong) shrink foil meant to keep these low density packages compressed and strong enough for handling and transport. The analogy which is made here that lightweight foil indeed insulates badly for low frequencies while better for highs resulting in reflected highs at the surface and low frequenties absorbed by the wool in the package. Basically this is just an application of the mass-law. But this assumes that this foil can freely (in a certain degree) can vibrate. Low frequencies call for larger displacement and longer wavelengts. I have made extensive tests in an official univ lab with perforated steel sheets (with all kinds of perforations) on mineral wool with and without very thin foil between them. And here I got complete deviating results. When this foil is clamped between the wool and the perforated sheet, compared to the same tests without the foil, the effect of the foil is the reverse of one should expect, namely the low frequent absorption diminishes much more than the high frequent absorption. What we are talking about here is not just the mass, but the apparent mass. Hence the analogy of just foil, with foil stretched around something is a poetic license which can be true or not depending of how such package is strapped in this foil. I know that such things get their own life on the net and become axioms, but it doesn't mean that they are true or in which degree they are true or not. Often such things establish based on (over)simplified analogies.

So what you have is basically a "Superchunk" bass trap design, which works very well.

Dang it! That stinks! It stinks because I have TONS of unopened packs of Rockwool in my room (about as ugly as it gets without covering in fabric) and I'm entirely not happy with my studio monitoring. Oh well. At least I know I don't need to spend any cash on expensive premade jobs. Thanks! Brandon

Brandon, based on what Eric said, maybe you should try taking the packaging off those things, and see if it makes a difference. It sure is worth trying, and I sure would trust Eric's take on it. He's one of the folks around here who actually does know what they're talking about. (Unlike me: I'm just a good parrot. :) ) Anyway, what is it about your monitoring that you are not happy about? Have you actually done any acoustic measurement of the room, to see where your issues are? Do you have a diagram of your room layout that you could post, and maybe get some suggestions on things that might fix it? - Stuart -

Mostly or often in the packaging I know this is not rarely (rather strong) shrink foil meant to keep these low density packages compressed and strong enough for handling and transport.

How do you define "foil"? :D The reason I ask is I suspect that we may have differences in language. In redneck USA, foil is almost always considered to be metal. On the second reading here, I'm starting to think that the plastic "wrap" the Roxul comes packaged in is what you are talking about. Please verify because once these things get opened, there is no turning back. :shock: Brandon

brandondrury wrote:

The reason I ask is I suspect that we may have differences in language. In redneck USA, foil is almost always considered to be metal. On the second reading here, I'm starting to think that the plastic "wrap" the Roxul comes packaged in is what you are talking about. Please verify because once these things get opened, there is no turning back. :shock:

I meant this plastic shrink foil. A maybe easier way to experiment is cutting big holes in that foil (Stanley knife or something comparable), in such a manner that it still keeps this roll in place/together. Otherwise those blankets fall apart. It's just a thought, experimenting is always a good thing. And you could start with some, rather than doing them all at the same time.

I have to admit that I never considered for a second that the plastic may have an effect on low frequency absorption. I'm going to be in a new house soon and that means my acoustical situation is going to change drastically, obviously. So that's when I'm going to really start slugging it out on the acoustics side. Thanks for the suggestions. Brandon

brandondrury wrote:

I have to admit that I never considered for a second that the plastic may have an effect on low frequency absorption.

Don't worry. Even most acousticians have never considered this. What you find in most books is that's it's never confirmed. I never found my related indisputable and systematic measurement results anywhere described. The idea behind it is the following. Plastic foil is NOT acoustic transparent, it's a sound barrier. Sound brings a barrier into vibration, which in it"s turn brings the air on the other side into vibration. At a first glance one can call this bean counting: whether foil is acoustic transparent or it lets the sound through by an extremely lightweight barrier seems like unimportant. However physically it makes a lot of difference in these circumstances where the difference between transparent and being a barrier becomes defining or dominant. Plastic foil however has a very low mass. By definition and as per mass law he higher frequencies are better insulated than the low frequencies, therefore standard one has more reflection in the highs and less in the lows. In the lows it can even act as a membrane enhancing the low frequent absorption. BUT: This all assumes that that foil CAN vibrate. And in the lows you're confronted with larger displacements and wavelengths. If I give an analogy: round ventilation ducts with traditional thin steel have a significant higher low frequent insulation than square or rectangular ducts with the same section and steel thickness. Reason: these round ducts mechanically suppress the generation of these higher displacements and wavelengths. Hence, here other factors come into play than just mass. We speak here about apparent mass. And before this becomes a new axiom: In how far this is valid for YOUR wrapped blankets depends on the packing and is for you to check or experiment with.

When you explain it like that, Eric, it makes a lot of sense. Just thinking about ("thought experiment"), I bet if you play loud bass notes in the room and gently touch the plastic with your fingertips, you'll most likely feel some vibration in the plastic (either hanging loosely or stretched around the wool). If you can feel it vibrating, then clearly it is not totally transparent to sound, and therefore must have some effect. It would be interesting for someone to do some tests on this, and see what effect it really has, and how much at each frequency. I'm not aware of any published tests, but it would be great to have some, to see if this is something that studio builders should be worried about, or not. - Stuart -

Soundman2020 wrote:

It would be interesting for someone to do some tests on this, and see what effect it really has, and how much at each frequency. I'm not aware of any published tests, but it would be great to have some, to see if this is something that studio builders should be worried about, or not.

And how do you know that such tests are representative for all different types of packing procedures used by different manufacturers? For the packaging I know from here I'm rather sure it has effect.